Frequently Asked Questions
Many pages on this site feature FAQs – here are all the FAQs together in one place, categorized by topic.
ADU 101
What is the difference between a site-built and a prefabricated or manufactured ADU?
Site-built/Traditional: A traditionally constructed ADU is designed and built specifically to your preferences and property and built on site (“stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.
Prefabricated/panelized/modular: These ADUs are partially or mostly built in a factory, then shipped to your site to be put together. Sometimes the company will include all services in their fee (“turn-key”), including help with permitting and all on-site construction tasks (e.g., laying the foundation, utility hookups, etc.). Other times you’ll need to hire additional professionals to help.
See more in our webinar about prefab and manufactured ADUs.
What is the difference between an ADU and a JADU?
Accessory Dwelling Units (ADUs) come in many shapes and sizes but are always a self-contained home that is smaller than the main house and legally part of the same property. They must have a kitchen, bathroom, and place to sleep, and typically range from efficiency-size studios to 1,000-square-foot homes with multiple bedrooms.
Junior Accessory Dwelling Units (JADUS) are smaller units of up to 500 square feet of space within or attached to a single-family home (including within an attached garage). JADUs typically include an efficiency kitchen (sink, cooking appliance, fridge, and small counter). Some JADUs have their own bathroom and some share with the main house. Construction costs for JADUs are typically much lower than a regular ADU. Unlike regular ADUs, the property owner must live in the primary unit or the JADU.
State law now allows homeowners to have both a JADU and a regular ADU on their property.
What do I have to do to build an ADU?
This site walks you through each part of the ADU process, from gathering initial inspiration and learning what you can build through construction and becoming a landlord or moving in.
You can also use our Process Map resource for an overview of the process and some initial issues to consider as you get started.
What if I am outside of Napa or Sonoma counties?
Unfortunately, our nonprofit is unable to provide support with ADU projects outside of Napa and Sonoma Counties. We recommend that you check out the Casita Coalition and reach out to your local planning department for assistance with next steps on your ADU project. Thank you for reaching out, and we wish you the best of luck with your ADU build!
I am a vendor interested in being added to your vendor registry – what do I do?
We’re so glad that you’re interested in being added to our nonprofit’s vendor registry!
We are currently accepting submissions from builders/general contractors, architects/designers, prefab companies, and engineers via our page here. Please review the comprehensive instructions on the webpage, and if you have any questions don’t hesitate to let us know. If you are another vendor type, please feel free to share your company name, website, and contact information with us as well as your vendor type and we will add you to our list to circle back to when we are accepting your vendor type for the registry.
If you are a designer, architect or prefab company interested in being added to our Napa Sonoma ADU Standard Plans Program please sign up for our wait list here.
What do I do first?
The best place to start is with a customized ADU Feasibility Consult to understand your specific property and local ADU rules. At the same time, you can work on thinking about what you want and looking at other ADUs to get a sense of what’s possible. Once you have some ideas in mind, you can move on to Learning the Rules to figure out what you can build on your property (the feasibility study will help with this too).
You can also use our Process Map resource for an overview of the process and some initial issues to consider as you get started.
How long does it take to build an ADU?
Building an ADU is an investment of time as well as money. Most projects take one to two years to complete. Typically, it takes homeowners one to three months to get started and assemble their team, then one to six months to develop plans, meet with the city, and submit the application. Depending on what permits are required, how many rounds of review are required and how quickly a homeowner and their project team can respond to comments, it will take one to three months to get permits. Construction usually takes six to twelve months.
Getting Started
What is the difference between an ADU and a JADU?
Accessory Dwelling Units (ADUs) come in many shapes and sizes but are always a self-contained home that is smaller than the main house and legally part of the same property. They must have a kitchen, bathroom, and place to sleep, and typically range from efficiency-size studios to 1,000-square-foot homes with multiple bedrooms.
Junior Accessory Dwelling Units (JADUS) are smaller units of up to 500 square feet of space within or attached to a single-family home (including within an attached garage). JADUs typically include an efficiency kitchen (sink, cooking appliance, fridge, and small counter). Some JADUs have their own bathroom and some share with the main house. Construction costs for JADUs are typically much lower than a regular ADU. Unlike regular ADUs, the property owner must live in the primary unit or the JADU.
State law now allows homeowners to have both a JADU and a regular ADU on their property.
What do I have to do to build an ADU?
This site walks you through each part of the ADU process, from gathering initial inspiration and learning what you can build through construction and becoming a landlord or moving in.
You can also use our Process Map resource for an overview of the process and some initial issues to consider as you get started.
What if I am outside of Napa or Sonoma counties?
Unfortunately, our nonprofit is unable to provide support with ADU projects outside of Napa and Sonoma Counties. We recommend that you check out the Casita Coalition and reach out to your local planning department for assistance with next steps on your ADU project. Thank you for reaching out, and we wish you the best of luck with your ADU build!
What do I do first?
The best place to start is with a customized ADU Feasibility Consult to understand your specific property and local ADU rules. At the same time, you can work on thinking about what you want and looking at other ADUs to get a sense of what’s possible. Once you have some ideas in mind, you can move on to Learning the Rules to figure out what you can build on your property (the feasibility study will help with this too).
You can also use our Process Map resource for an overview of the process and some initial issues to consider as you get started.
Do I need to tell my neighbors?
You’re not required to tell your neighbors about your ADU, but it’s always a good idea to communicate with them early in the process. Your project will run more smoothly if they are kept informed, and they may have great ideas for your project!
If you live in a Neighborhood or Homeowners Association, talk with your representative or board early in the process. They can’t prevent you from building or renting an ADU, but they may have guidelines you’ll need to know for design and construction. Depending on where you live, written approval from your HOA may be required before your location will permit your ADU.
I’m looking to put an ADU on new property – what should I be looking for?
This is a great thing to be thinking about before you purchase! We’ve written a guide to what you should be thinking about and looking for.
How long does it take to build an ADU?
Building an ADU is an investment of time as well as money. Most projects take one to two years to complete. Typically, it takes homeowners one to three months to get started and assemble their team, then one to six months to develop plans, meet with the city, and submit the application. Depending on what permits are required, how many rounds of review are required and how quickly a homeowner and their project team can respond to comments, it will take one to three months to get permits. Construction usually takes six to twelve months.
When do I need to think about utility connection?
Utility connections can have significant design and scheduling impacts, so it’s good to think about them early on.
In general, it is less expensive to hook the ADU utilities to the main house, as opposed to building new water and sewer laterals to the mains in the street, or a second set of electrical lines from the pole. In some cases, particularly if your ADU is larger, over 750 sf, you may be required to construct new connections. It is best to discuss this issue with your jurisdiction early in the design process. This conversation may require talking to multiple departments to get a full understanding of the requirements. Start with the Planning Department, but make sure to also check with the Building Department, Public Works and/or your local water and sewer district offices.
Most homes will need to upgrade the main electrical panel and possibly electrical service to the property. This will involve both an electrician as well as coordinating with PG&E. Due to the demands on PG&E services, the upgrade process can take significant time and homeowners are often charged a $1,500 engineering fee for PG&E to begin the process. Additional fees may be charged depending on the condition and size of the electrical components running to the property. If your ADU will be detached or in a separate, converted building (such as a converted detached garage), PG&E requires the installation of a second electric meter.
Typical utility upgrades for an ADU include a main electrical panel of at least 200 Amps and a sewer lateral of 4″ for the whole property. If the existing lateral is 3″ you may have to replace it with a 4″ lateral to the street to handle the additional demand created by the ADU. Please consult with your building department for more information.
Water service may need to be upgraded as well, particularly when sprinklers are required such as when an ADU is built above a garage. The scale and cost of this work depends greatly on the age of the main house and the infrastructure provided by jurisdiction. It is important to determine if sprinklers will need to be planned for and get information on the water supply to your property early in the design and bidding process. This information is typically available from your Public Works or Engineering Department.
At what stage of the ADU process should I get an ADU Feasibility Consult?
As early as possible! The goal of the consult is to make sure you have a deep understanding of the best ADU options for you given your goals and budget objectives. Doing so will save you time and resources in the long term. If you’re already far along in the process with a designer or architect, you may no longer need an ADU Feasibility Consult – instead, just reach out to us if you have questions along your ADU journey.
Learning the rules
Do I need to tell my neighbors?
You’re not required to tell your neighbors about your ADU, but it’s always a good idea to communicate with them early in the process. Your project will run more smoothly if they are kept informed, and they may have great ideas for your project!
If you live in a Neighborhood or Homeowners Association, talk with your representative or board early in the process. They can’t prevent you from building or renting an ADU, but they may have guidelines you’ll need to know for design and construction. Depending on where you live, written approval from your HOA may be required before your location will permit your ADU.
Am I allowed to build an ADU? How many?
In almost all cases, yes! ADUs and JADUs are allowed in any single-family or multi-family zone. If residential buildings are allowed, ADUs are almost always allowed too (with limited exceptions for safety, traffic, and water availability).
State law allows a single-family property to have a JADU, and either an internal conversion or attached ADU or a detached ADU. Some places allow for more. Multifamily properties – like duplexes, triplexes, and apartment buildings – can have two detached ADUs or up to 25 percent of the number of units, depending on your property. Talk to your local planning staff for more information if interested in building ADUs on a multifamily property. Developers building new homes or multi-family housing can often include ADUs at the same time – confirm with planning staff.
Can I put an ADU in my garage?
Homeowners can convert legally built structures (garage, barn, art studio, etc.) into an ADU. JADUs can be converted from an attached garage (but not detached). Check out this extensive guide to garage conversion ADUs from the Casita Coalition.
How large can my ADU be?
According to state law, you can at least build up to an 800 square feet ADU, as long as rear and side setbacks are at least 4 feet and it is not above 16 feet tall. Otherwise, size limits vary depending on your jurisdiction. Typically the maximum size ranges from 850 sf to 1,200 sf.. No room behind or next to your main home? You can build it in your front yard instead.
Generally, ADUs can range from 150-square-foot efficiency units up to 1,200 square feet. Junior ADUs (JADUs) can be up to 500 square feet. See Local ADU Rules for more details on height, setbacks, and other rules and our Workbook for more detailed information about important ADU laws.
I don’t think I can fit an ADU on my property – what can I do?
According to state law, rules about setbacks, lot coverage, and open space requirements cannot restrict you from building an 800 square foot ADU, as long as the ADU has setbacks of at least 4 feet and is not above 16 feet tall. Front setbacks also cannot restrict you from building an 800 square foot ADU, which means an ADU can be in a front yard – but only if rear or side placement isn’t possible.
What about ADUs on multifamily properties?
Multi-unit properties can have multiple ADUs, depending on the type and other details of the project, but no JADUs.
Do I need to live in the main house to build an ADU or JADU?
JADU owners need to live in the primary unit or the JADU – and this will need to be recorded in a deed restriction for the property.
Will I need to add parking?
Parking is much less of a concern than it used to be. JADUs do not require a new parking spot. Check Local ADU Rules to see what parking may be required in your location.
Typically, no parking is required if it is:
- Within ½ mile walking distance to transit (including a ferry);
- Within an architecturally or historically significant district;
- On-street parking permits are required and not provided to the occupant of the ADU;
- Located within one block of car-share access, or
- Built as part of a new home project.
Will my ADU need fire sprinklers?
In most cases, if the main house doesn’t have fire sprinklers, then the ADU won’t be required to install sprinklers. However, there are some exceptions to that rule where sprinklers may be required:
- If you build an ADU larger than 50% of the main house
- If you build over a garage
- If the ADU is farther than a certain distance from the street (typically 150′) or closest fire hydrant (typically 400′)
Check with your local Planning Department and Fire Marshall for exact details once you’ve developed your project further.
When do I need to think about utility connection?
Utility connections can have significant design and scheduling impacts, so it’s good to think about them early on.
In general, it is less expensive to hook the ADU utilities to the main house, as opposed to building new water and sewer laterals to the mains in the street, or a second set of electrical lines from the pole. In some cases, particularly if your ADU is larger, over 750 sf, you may be required to construct new connections. It is best to discuss this issue with your jurisdiction early in the design process. This conversation may require talking to multiple departments to get a full understanding of the requirements. Start with the Planning Department, but make sure to also check with the Building Department, Public Works and/or your local water and sewer district offices.
Most homes will need to upgrade the main electrical panel and possibly electrical service to the property. This will involve both an electrician as well as coordinating with PG&E. Due to the demands on PG&E services, the upgrade process can take significant time and homeowners are often charged a $1,500 engineering fee for PG&E to begin the process. Additional fees may be charged depending on the condition and size of the electrical components running to the property. If your ADU will be detached or in a separate, converted building (such as a converted detached garage), PG&E requires the installation of a second electric meter.
Typical utility upgrades for an ADU include a main electrical panel of at least 200 Amps and a sewer lateral of 4″ for the whole property. If the existing lateral is 3″ you may have to replace it with a 4″ lateral to the street to handle the additional demand created by the ADU. Please consult with your building department for more information.
Water service may need to be upgraded as well, particularly when sprinklers are required such as when an ADU is built above a garage. The scale and cost of this work depends greatly on the age of the main house and the infrastructure provided by jurisdiction. It is important to determine if sprinklers will need to be planned for and get information on the water supply to your property early in the design and bidding process. This information is typically available from your Public Works or Engineering Department.
Budgeting & Finance
What will it cost to build an ADU?
Our ADU Calculator is a great place to start when developing a budget. It provides a rough estimate of costs and income and will help you understand how choices can impact your budget over time. In general, it is helpful to avoid having a fixed budget total in your head as you explore your options.The cost to build an ADU typically ranges from $30,000 for a simple interior conversion JADU, to $400,000+ for a large detached ADU with high-end finishes on a hillside lot. Cost per square foot is a good way to estimate, though this too can range — a very rough placeholder for you to use is $400-700 per square foot for construction and other costs (design, fees, etc.), depending on your design and the materials you chose.
See more details about costs – including design, permitting, and construction – in our ADU Workbook. Also see our blog post on 5 hidden costs.
How am I going to pay for an ADU?
There are five main options available to homeowners to finance their ADU. Many homeowners use a mix of these options.
- Cash savings or other liquid assets
- Cash-out refinance
- Loans from friends or family
- Home equity loan or home equity line of credit (HELOC)
- Renovation loans
As you’re doing financial planning for your ADU, remember to include the rental income you’ll receive in your calculations. See more details about financing options in the steps on the Budgeting and Finance page and in our Workbook and Exercises.
What if I don’t have a lot of money available right now to build an ADU?
If you have equity in your home, a cash-out refinance or home equity loan/line of credit (HELOC) might work for you. Some banks and credit unions have begun offering specialized ADU financing that can help people with lower incomes or equity qualify for an ADU construction loan.
How will building an ADU affect my taxes and property value?
Adding an ADU will likely affect your property taxes and the resale value of your home. However, your primary house will not be reassessed, and your property taxes will only increase based on the added value of your ADU. For example, if you build an ADU that adds $150,000 to your property value, and your tax rate is 1%, your taxes will increase by 1% x $150,000, or $1,500 per year.
Building a JADU will have a significantly smaller impact on assessed value. In some cases, your taxes will not increase at all. Home sharing will also not increase the assessed value of your home. Generally, garage conversions will not raise your tax bill as much as new construction, but they will also not add as much value.
Check out our blog post on the subject to learn more. You can also watch this helpful 4 minute video from ADU company Maxable on property tax implications!
Each property will require a one-on-one analysis to determine the added value of an ADU, so contact your County’s Assessor Office once you have an idea of your plan. They will be able to provide you with a rough estimate of tax implications.
Adding an ADU may impact your income taxes as well. This can be rather complicated, and it’s best to discuss these with a tax advisor.
Can I eventually sell my ADU separately from the main building?
No. You will need to record in a deed restriction for the property that the ADU cannot be sold separately from the primary home. California passed a law that does allow for ADUs to be sold separately but it requires the jurisdiction to “opt-in” to allow it, and as of the time of this FAQ (Summer 2024) only San Jose has allowed for the separate sale of an ADU.
What about the money I could make renting the ADU?
Rental income is a major benefit of having an ADU or JADU on your property – for many people, it provides flexibility in their budget or an opportunity to grow their savings. Our ADU Calculator can help you estimate how much rental income could be generated by your new unit. Note that many locations do not allow ADUs to be rented for less than 30 days at a time – confirm with local Planning staff in your community.
How much does it cost to design an ADU?
Design costs cover the work of your architect or designer and may or may not include engineering, surveying, and other professional services necessary to prepare for permitting. Together, these services may total around 10% of your total project budget – the exact amount depends on the details of your property and project. See the Budgeting and Finance page for more details.
How much does it cost to get permits for an ADU?
ADUs can be charged a variety of fees from your local government, the State, and other agencies. Some fees are based on details of the project while others are fixed. The total amount depends highly on your property and plans, but local staff can most likely provide an estimate once you have a good idea of your project details. See the Budgeting and Finance page, our ADU Calculator, and our ADU Workbook for more details.
Design
What is the difference between a site-built and a prefabricated or manufactured ADU?
Site-built/Traditional: A traditionally constructed ADU is designed and built specifically to your preferences and property and built on site (“stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.
Prefabricated/panelized/modular: These ADUs are partially or mostly built in a factory, then shipped to your site to be put together. Sometimes the company will include all services in their fee (“turn-key”), including help with permitting and all on-site construction tasks (e.g., laying the foundation, utility hookups, etc.). Other times you’ll need to hire additional professionals to help.
See more in our webinar about prefab and manufactured ADUs.
How do I find an architect, designer, or prefab company?
Most homeowners choose to work with some type of design professional to plan their ADU and help throughout the process. Bringing on a professional early in the process is often key to getting your ADU approved quickly, managed efficiently, and built cost-effectively. Relevant experience and fit will be critical.
There are a variety of types of designer, and they may be an architect, builder, “designer,” design/build, or a modular/prefab company. If you’re hiring a local individual or team, they’ll likely start the process by visiting your home and talking to you about your ideas and goals. If it seems like a good match, they will prepare a proposal detailing their services and fee. Professionals typically charge for an initial consultation or proposal.
Note that if you’re not using a licensed architect to design your ADU, your plans will need to be stamped by a licensed engineer.
Check our Vendor List and our Standard Plans Gallery for some design professional options. See our Exercises for a list of questions to ask a potential architect or designer, our Glossary to be clear on terms, and our Workbook for more details.
When do I show local staff my designs?
Once you have a basic design established with your architect/designer, it’s a great idea to discuss it with Planning & Building staff so they can point out any issues before you prepare your application.
For contact information, see our Contact page.
This is also a good time to reach out to utility agencies (water, sewer, gas, etc.) to inquire about their infrastructural requirements and confirm connection and service fees.
How much does it cost to design an ADU?
Design costs cover the work of your architect or designer and may or may not include engineering, surveying, and other professional services necessary to prepare for permitting. Together, these services may total around 10% of your total project budget – the exact amount depends on the details of your property and project. See the Budgeting and Finance page for more details.
Are there pre-approved plans I can use?
Yes! We help run ADU Plans Galleries for many of our participating communities. These galleries feature a browsable, filterable gallery of 50 ADU plans, many of which have been pre-approved by your City or County. Your City or County may also have a listing of pre-approved plans on their website. Search for your local gallery or browse the overall statewide gallery at the California Plans Gallery search page.
Do you have any resources on eco-friendly ADU design?
Yes! Check out this blog post on Green Building Strategies for ADUs in California.
Permitting
Are there pre-approved plans I can use?
Yes! We help run ADU Plans Galleries for many of our participating communities. These galleries feature a browsable, filterable gallery of 50 ADU plans, many of which have been pre-approved by your City or County. Your City or County may also have a listing of pre-approved plans on their website. Search for your local gallery or browse the overall statewide gallery at the California Plans Gallery search page.
What permits are required for ADUs and JADUs?
Application requirements vary by where you live, but applications for a new ADU typically include:
- Site plan (depicting existing and proposed structures)
- Building/architectural plans (floor plans, elevation plans, and details)
- Structural plans (foundation plan, framing plans, and associated details)
- Other items (Title 24 energy calculations, deed restriction, address assignment form, or other materials required by your location)
- Deed Restriction filed with the County
- Supplementary Materials
Some local Planning Departments have a specific ADU permit application or submittal requirements checklist that outlines all the required documentation and materials that must be submitted. Conversions of existing living space often require less documentation. Contact local staff to learn about your specific application process and requirements.
How much does it cost to get permits for an ADU?
ADUs can be charged a variety of fees from your local government, the State, and other agencies. Some fees are based on details of the project while others are fixed. The total amount depends highly on your property and plans, but local staff can most likely provide an estimate once you have a good idea of your project details. See the Budgeting and Finance page, our ADU Calculator, and our ADU Workbook for more details.
Are there water or sewer issues I should consider?
Some cities may require homeowners to upsize service and or meters to meet capacity requirements. Check with local Planning & Building staff to learn more about local requirements for utilities.
Can my ADU be stopped because of other noncompliance issues on my property?
In most cases, state law no longer allows cities and counties to comment on pre-existing zoning issues unrelated to the ADU. For example, you should not receive comments about correcting the main house or a fence unrelated to the ADU, unless there is an obvious public safety issue.
What are the issues I should be aware of if my property uses a septic system?
If you are on a septic system, it is important to ensure with your local agency that you have septic capacity to add an ADU or Junior ADU with your desired number of bedrooms before you move forward on plans for your project. Upgrading a septic system can be costly or challenging, and it is good to assess realistic needs upfront. Learn a ton more about septic systems from our detailed blog post on the topic right here. You can do so by reaching out to your local agency:
- For Sonoma County please contact Permit Sonoma Well & Septic at PermitSonoma-Well-Septic@sonoma-county.org or at (707) 565-2849.
- For Napa County please contact the Napa County Environmental Health Division at Environmental@countyofnapa.org or at (707) 253-4417.
If you don’t currently have septic capacity for your ADU project and do not want to upgrade the system, ask your local planning department about the possibility of a “bedroom swap,” which allows you to make construction changes to an existing bedroom in the primary residence to convert it to non-bedroom space (e.g., an office) and then use the newly freed up septic capacity for your ADU. Again, whether or not this is a possibility for you depends on your local jurisdiction and the specific details of your property.
What is Title 24 and how does it affect my ADU project?
Title 24 regulates minimum energy efficiency for new construction in California. For example, these standards dictate how much insulation is required and the types of light fixtures that can be used. All new ADUs will need to demonstrate compliance with Title 24, which usually requires hiring a consultant and typically costs less than $500. For a detailed blog post on Title 24 read here. In some cases, like newly constructed freestanding ADUs, you might need to add solar panels to meet the rules. For a list of Title 24 consultants, check out our Vendor Registry.
What are the solar requirements for ADUs?
Newly constructed ADUs are subject to the Energy Code requirement to provide solar panels if the unit(s) is a newly constructed, non-manufactured, detached ADU. Per the California Energy Commission (CEC), the panels can be installed on the ADU or on the primary dwelling unit. ADUs that are constructed within existing space, or as an addition to existing homes, including detached additions where an existing detached building is converted from non-residential to residential space, are not subject to the Energy Code requirement to provide solar panels.
Please refer to the CEC on this matter. For more information, see the CEC’s website. You may email your questions to: title24@energy.ca.gov, or contact the Energy Standards Hotline at 800-772-3300. CEC memos can also be found on HCD’s website.
For important information for homeowners thinking of going solar, reference the California Public Utilities Commission (CPUC)’s California Solar Consumer Protection Guide.
How does SB-9 impact my ability to build an ADU?
Use of SB-9 to split a lot and build additional housing in dependent on specific conditions of the property and proposed project. Below is some background information on SB-9 to review, and from here we advise you to speak directly with the city to get more information specific to your property and how the city is applying SB-9 as of now. Some information can be found at this Maxable blog post. You can also look at the California Department of Housing and Community Development SB-9 Fact Sheet. Finally, the Casita Coalition webinar on SB-9 is available to watch.
I have an unpermitted ADU on my property, what should I do?
A: Each local jurisdiction has a different Code Enforcement process and protocol with regard to unpermitted ADUs. Many jurisdictions are focused on a collaborative approach to help homeowners bring their ADU up to code. State law requires that local jurisdictions should provide homeowners up to 5 years to bring a unit up to code if the local jurisdiction does not determine that there is a health or safety issue with the unpermitted unit. As of now, in our experience most jurisdictions determine that there is a health or safety issue, so we have not yet seen this delay in enforcement applied. For more detailed information, we recommend watching this on-demand webinar from the Casita Coalition on unpermitted ADUs.
Construction
What is the difference between a site-built and a prefabricated or manufactured ADU?
Site-built/Traditional: A traditionally constructed ADU is designed and built specifically to your preferences and property and built on site (“stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.
Prefabricated/panelized/modular: These ADUs are partially or mostly built in a factory, then shipped to your site to be put together. Sometimes the company will include all services in their fee (“turn-key”), including help with permitting and all on-site construction tasks (e.g., laying the foundation, utility hookups, etc.). Other times you’ll need to hire additional professionals to help.
See more in our webinar about prefab and manufactured ADUs.
I am a vendor interested in being added to your vendor registry – what do I do?
We’re so glad that you’re interested in being added to our nonprofit’s vendor registry!
We are currently accepting submissions from builders/general contractors, architects/designers, prefab companies, and engineers via our page here. Please review the comprehensive instructions on the webpage, and if you have any questions don’t hesitate to let us know. If you are another vendor type, please feel free to share your company name, website, and contact information with us as well as your vendor type and we will add you to our list to circle back to when we are accepting your vendor type for the registry.
If you are a designer, architect or prefab company interested in being added to our Napa Sonoma ADU Standard Plans Program please sign up for our wait list here.
What will it cost to build an ADU?
Our ADU Calculator is a great place to start when developing a budget. It provides a rough estimate of costs and income and will help you understand how choices can impact your budget over time. In general, it is helpful to avoid having a fixed budget total in your head as you explore your options.The cost to build an ADU typically ranges from $30,000 for a simple interior conversion JADU, to $400,000+ for a large detached ADU with high-end finishes on a hillside lot. Cost per square foot is a good way to estimate, though this too can range — a very rough placeholder for you to use is $400-700 per square foot for construction and other costs (design, fees, etc.), depending on your design and the materials you chose.
See more details about costs – including design, permitting, and construction – in our ADU Workbook. Also see our blog post on 5 hidden costs.
How do I find a contractor?
If you are not using a design/build firm, you will need to find a contractor to take over for the construction phase of your ADU.
First, you’ll solicit bids. See our Workbook for more details on what you want to see in a bid, what other documentation to collect from potential contractors, and what to look for in your bidding candidates. You will want to get at least three bids for comparison. Typical construction costs for new, detached ADUs are between $400-$700 per square-foot as of July 2024.
When you have bids, you can begin selecting your contractor. See our Workbook for more details on how to compare bids and choose the best option for you.
See our Vendor Registry.
Before you hire a contractor, make sure to check their license and insurance and when they present you with a contract, review everything carefully. See our Workbook for more details.
How can I keep construction costs down?
Construction costs for your second unit will vary significantly depending on personal preferences, site conditions, location, and many other factors.
Size: Despite what many think, smaller ADUs may cost almost the same as larger ones. Many costs like foundation, kitchen and bathroom work only increase slightly for larger ADUs. Kitchen costs will range from $25,000–$50,000 with each bathroom ranging from $15,000–$25,000.
Type: New construction, both detached and attached, tend to be the most expensive. Garage conversions are not much cheaper than new construction if at all. Conversions of interior space (basement or otherwise) are often the cheapest.
Other factors that affect cost:
- Quality of interior finish work and amenities
- Architectural form and details
- Extent of utility, structural, mechanical, electrical, and plumbing upgrades required
- Required site upgrades (sidewalks, sewer and water)
- Whether sprinklers are required
- Lot complexity (slope, trees, fault lines, etc.)
How long does construction take?
Traditional construction will take 6-12 months, though this will vary depending on the specifics of the project. Stages of construction include:
- Site preparation: 1-2 months
- Foundation: 1 month
- Walls, roof, doors: 1-2 months
- Plumbing & electrical: 1-2 months
- Insulation & drywall: ½-1 month
- Fixtures & finishes: 1-2 months
- Final touches: ½-2 months
What are my responsibilities during construction?
While your contractor will lead the construction process, you will have the following responsibilities:
- Keep in touch with your contractor and set up a schedule for checking in.
- Regularly walk through the construction area to monitor the quality of the work and make sure the work is progressing the way you expect.
- Be prepared to make decisions about the details—light fixtures, appliances, and other materials—in a timely manner so your contractor can stay on schedule.
- Follow the contract you agreed to, including any changes as described specifically in a change order form.
- Although your contractor will usually arrange the required city or utility inspections, it is your responsibility as the property owner to make sure that the inspections are conducted as required.
Renting & Move-in
What about the money I could make renting the ADU?
Rental income is a major benefit of having an ADU or JADU on your property – for many people, it provides flexibility in their budget or an opportunity to grow their savings. Our ADU Calculator can help you estimate how much rental income could be generated by your new unit. Note that many locations do not allow ADUs to be rented for less than 30 days at a time – confirm with local Planning staff in your community.
When is my ADU ready for move-in?
As soon as the final inspection is complete, your ADU is ready for move-in! Make sure utility services are set up, an address is established, and other preparations are in place. See the Renting & Move-In page for more responsibilities of being a landlord.
What do I need to know about becoming a landlord?
Renting an ADU comes with many responsibilities, including understanding local and state housing laws, executing a lease, finding and managing a tenant, and maintaining a rental property. It’s important to understand the laws as they may affect things like future rent increases, changing use over time, evicting tenants, and moving family into the unit. Fair Housing groups in Napa and Sonoma have recommended several Nolo Press books as a good source for understanding the rights and responsibilities. At a minimum, they suggest both First Time Landlord and The California Landlord’s Law Book: Rights & Responsibilities.
Can an ADU or JADU be used as a short-term/vacation rental?
No. Napa and Sonoma County properties with ADUs and JADUs must file a deed restriction agreeing that the unit will not be used for short-term rentals (less than 30 days). This discourages the listing of ADUs on popular websites like Airbnb and VRBO and promotes them as a means to increase housing stock for the diverse needs of county residents.
