Let's start at the beginning
ADU 101
What is an ADU?
Accessory dwelling units (ADUs), also known as “granny flats,” “in-law units,” or “backyard cottages,” come in many shapes and sizes but are always a self-contained home that is usually smaller than the main house and legally part of the same property.
They must have a kitchen, bathroom, and place to sleep, and can range from studios small studios under 500 square feet to larger homes with multiple bedrooms.
ADU Types
Detached ADU
New freestanding structure separate from the main home, like a backyard cottage.
Pros
- Provides the most privacy between the main home and ADU
- Lots of flexibility in design
- Lower construction impact on the main home
- Higher potential rental income
Cons
- Usually the most expensive option
- Highest cost and more difficult to add new utility connections
- Takes up more space in your yard
- Solar panels are usually required
Attached ADU
Pros
- Lower cost for utility connections and upgrades
- Takes up less space in your yard
- May blend in better with the main home
- Easy access for a caregiver or loved one
- May have lower construction impact on neighbors
Cons
- Probably requires a custom design (can’t use a pre-designed plan or prefab unit)
- Less privacy between the main home and ADU (sounds and smells may be more noticeable and you may have to share outdoor space)
- Can be challenging to find a good place to connect to the main home
- Construction will likely impact the main home
Conversion ADU
Converted existing space in the main home or on the property (bedroom, basement, detached garage). Napa Sonoma ADU developed a helpful handout on garage conversions in partnership with the Casita Coalition and others, which is located right here – plus a very helpful webinar recorded and available here. Please check out our ADU Calculator tool for cost information for a garage conversion.
Pros
- Usually the lowest cost option, especially if converting existing living space
- Usually lower cost for utility connections and upgrades
- May have fewer or lower overall fees
- Takes up less yard space
- May blend in better with the main home
- May have lower construction impact on neighbors
Cons
- Can be challenging and costly if significant upgrades are necessary, such as bringing a garage up to code
- Less flexibility in design
- Potential loss of existing living space (bedrooms, etc.) or storage space (garage, basement, attic)
- Construction can impact the main home, especially if you convert existing living space
- May require upgrades to the whole house (talk to local staff if you want to convert more than 50% of the main home)
Junior ADU (JADU)
A type of conversion up to 500 square feet within the main home (existing or proposed). Size and other requirements may be different where you live. Check with local staff to confirm JADU rules and see our JADU blog post.
Pros
- Usually the lowest cost option - design, construction, and maintenance
- Lowest cost for utility connections and upgrades
- Easy access for a caregiver or loved one
- Blends in better with the main home
- Lower construction impact on neighbors
- Doesn't require a full kitchen
Cons
- Requires the property owner to live in either the main home or the JADU and file a deed restriction
- Less privacy between the main home and ADU (sounds and smells may be more noticeable)
- Can be challenging to fit the necessities and design for comfort
- Lower potential rental income
- Construction will probably impact the main home
Multi-Family ADU
Any of the other types (except JADU) that is part of a multi-family building like a duplex, triplex, or apartments (existing or proposed).
Rules around Multi-family ADUs can be more complicated than ADUs for single-family homes. For a super helpful and digestible written guide to ADUs and multi-family properties, check out the written guidelines from the Casita Coalition right here. And if you’re the type of learner who does better hearing it and seeing it, you can check out the accompanying on-demand webinar from the Casita Coalition on YouTube right here!
There are some important considerations to keep in mind for multi-family ADUs:
- Provides new housing options for community members usually at a more affordable rent
- Good option for underutilized parking, commercial or storage spaces
- Increased rental income and property value
- Larger construction loans may be available
- Some properties may be allowed to build multiple ADUs
- May increase property taxes
- Construction may impact existing tenants
- May reduce commercial space, shared outdoor space, parking, or storage
Not ADUs
RVs, tiny homes on wheels, yurts, and storage structures are usually not considered ADUs. Check with local staff to confirm what structures on your property may be permitted as a ADU.
Project Timeline
Building an ADU is an investment of time as well as money. Most projects take one to two years to complete, with interior conversions being the fastest.
Project Cost
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. See more about costs in our ADU Workbook and on the Budgeting & Finance page. Our ADU Calculator is a great place to start when developing a budget.
Where to start?
The best place to start is with an ADU Feasibility Consult. You’ll meet with an ADU expert and receive a customized ADU Feasibility Report about your property and your local ADU rules.
Related Blog posts
FAQs
Find answers to the most frequently asked questions about ADU basics.
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.
“
When you need an ADU, it’s already too late.
It’s better to plan ahead of time and rent it out until a family member needs it.”
Sonoma Resident
Benefits for Owners and the Community
Building and renting an ADU is truly a community service — ADUs often provide homes for the local workforce and small families who have difficulty finding local housing that meets their needs.
For homeowners, there are many reasons to build an ADU — rental income, safe and convenient housing for relatives, flexibility over time, and many others. Here are some of the reasons to build an ADU:
Housing Friends & Relatives
Space for adult children, aging relatives, or loved ones with special needs.
Financial Security
A steady source of rental income and increased property value.
A home as you age
A home for a caregiver or an accessible and comfortable option for aging adults.
Safety
An extra set of eyes on the property for safety, travel, and more.
Environmental benefits
Small-scale living and reduced commutes for local workers.