A project of the Santa Clara County Planning Collaborative

A project of the Santa Clara County Planning Collaborative

6 Build

PERMIT IN HAND, YOU’RE READY TO BREAK GROUND and see your project become a reality! From site preparation to passing your final inspection, you’ll stay in touch with your contractor and make final decisions about project details.


Here are a few of the most frequently asked questions about ADU construction. See the content below and our ADU Guidebook for more guidance, resources, and tips for all steps of the process.

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 Guidebook 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.

When you have bids, you can begin selecting your contractor. See our Guidebook for more details on how to compare bids and choose the best option for you.

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 Guidebook for more details.

Construction costs for your ADU will vary significantly depending on personal preferences, site conditions, location, and many other factors. 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:

  • 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
  • Whether doors and windows meet emergency exit standards
  • 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.

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.


Building Your ADU


Hire your contractor

If you are not using a design/build firm, you will need to hire a contractor for the construction phase of your ADU. You will use drawings from your designer to get bids from contractors and make sure you are specific about what you want each bid to include (license details, insurance information, examples of past work, etc.). Your design team may be able to help you with this. 

It’s a good idea to select between at least three bids, and it’s okay to ask contractors clarifying questions about their bid. Your designer may be able to help you compare the bids. It’s a good idea to reach out to references and to consider your personal interactions with the contractor – you’ll want to feel like you work well together and that they understand your goals. 

See our Guidebook for many more details on getting bids and hiring a contractor.

Helpful Tools

Monitor construction

Once you have building permits, the contractor you hire will lead the construction of your ADU. Be sure you have all funding in place before you start construction. 

Communicate frequently with your contractor to make decisions about fixtures and finishes as needed and verify progress before making payments. Timelines for construction vary, but 6-12 months is fairly common. See more details about managing construction in our Guidebook.

Get inspections

During construction, your ADU will be inspected multiple times to ensure it is being built according to the permitted plans (typical inspections include foundations/footings, framing, electrical/plumbing, and exterior finishes). 

Many cities have online portals or direct phone numbers for scheduling inspections. Generally, it is your and your contractor’s responsibility to schedule all required inspections. For a list of inspections and how to schedule, contact your Building Department. After your final inspection is approved, some cities will issue a certificate of occupancy. Your ADU is ready for move-in!

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