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.

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.

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.

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.

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.)

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

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.