A project of the Santa Clara County Planning Collaborative

A project of the Santa Clara County Planning Collaborative

All FAQs

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-At-A-Glance resource for an overview of the process and some initial issues to consider as you get started.

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.

Accessory Dwelling Units (ADUs) 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 typically range from studios under 500 to 1,000-square-foot homes with multiple bedrooms.

Junior Accessory Dwelling Units (JADUs) are within the footprint of your home (or attached garage) and less than 500 square feet. They can share a bathroom with the main home and/or have an efficiency kitchen (generally a sink, smaller appliances, and counter) Construction costs for JADUs are typically much lower. In most cases, the property owner must live on site in either the main home or the JADU.

State law now allows homeowners to have both a JADU and a regular ADU on their property.

No. Generally, J/ADUs are not allowed to be rented for 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. Some cities may require you to file a deed restriction agreeing that the unit will not be used for short-term rentals.

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.

See our Guidebook for resources on understanding rental laws, tenants’ rights, and more, and our Exercises for help with your lease terms.

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 below for more responsibilities of being a landlord.

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.

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

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.)

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.

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.

Some cities may require homeowners to upsize service and or meters to meet capacity requirements. Check Local ADU Rules and talk with staff to learn more about local requirements for utilities.

Submitting an application is different in each city. Some have online portals while others ask for multiple sets of paper copies on various sizes of paper. Some cities require one application package, while otherwise require separate processes from the Planning and Building departments. Check with your City to confirm the application process and requirements and for details about permit materials.

Once you have a design established with your architect/designer, it’s a great idea to discuss it with City staff so they can point out any issues before you prepare your application.

Depending on the city, you may be able to schedule an appointment to speak with a planner or walk in to the Planning Counter or Permit Center. For contact information, see the Local ADU Rules and Contacts.

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.

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 may need to be stamped by a licensed engineer. Check with your City early on.

See our Exercises for a list of questions to ask a potential architect or designer, our Glossary to be clear on terms, and our Guidebook for more details.

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. Generally, you cannot rent your ADU for less than 30 days at a time (e.g., AirBnB, Vrbo). The Santa Clara County ADU Calculator can help you estimate how much rental income could be generated by your new unit.

Generally no. You may need to record in a deed restriction for the property that the ADU cannot be sold separately from the primary home. Check Local ADU Rules and contact your City to find out what local restrictions apply.

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.

Each property will require a one-on-one analysis to determine the added value of an ADU, so contact the Santa Clara County Assessor’s Office once you have an idea of your plan. They may 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.

If you have equity in your home, a cash-out refinance or home equity loan/line of credit (HELOC) might work for you. Financing is typically unavailable for homeowners with lower income and insufficient home equity, but as of January 2023, the California Housing Finance Agency (Cal HFA) ADU Program provides a grant of up to $40,000 to qualified homeowners for the reimbursement of ADU pre-development costs, including but not limited to impact fees. To qualify, a homeowner must be low or moderate income. Make sure to check if funds are available and if you qualify.

Many homeowners use a mix of options to finance their ADU, including savings, funds from family, and/or loans. It is strongly recommended that your financing is in place before construction starts. Be sure to factor in potential rental income since that will help you repay loans. See our Guidebook and Exercises for more details on financing options.

The Santa Clara County 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 vary — a very rough placeholder for you to use is $400-550 per square foot for construction (“hard costs”) and design and fees (“soft costs”), depending on your design and the materials you chose.

See more details about costs – including design, permitting, and construction – in our ADU Guidebook.

\Homeowners can build both an ADU, and JADU on their property. Some cities allow you to build more. Multifamily properties can have multiple ADUs, depending on the type and other details of the project.Contact staff for more information if interested in building ADUs on a multifamily property.

Parking is much less of a concern than it used to be. JADUs do not require a parking spot. Check Local ADU Rules and talk with your staff to see if ADUs require parking. No new parking is required if ADU is:

  1. Within ½ mile walking distance to transit (including a ferry);
  2. Within an architecturally or historically significant district;
  3. On-street parking permits are required and not provided to the occupant of the ADU;
  4. Located within one block of car-share access, or
  5. Built as part of a new home.

Check the Local ADU Rules to see what parking may be required. 

Generally, ADUs and JADUs cannot be rented for fewer than 30 days at a time.

JADU owners need to live in the primary unit or the JADU – and this may need to be recorded in a deed restriction for the property. You may also be required to live on the property if it includes an ADU. Check Local ADU Rules and talk with staff early to find out. 

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.

According to state law, you can build up to an 800 square foot ADU, as long as it is not over 16 feet tall and rear and side setbacks are 4 feet or more. Otherwise, size limits depend on your property and local rules. No room behind or next to your main home? You can build it in your front yard instead.

ADUs and JADUs are allowed in all residential and mixed-use zones, with limited exceptions for safety, traffic, and water.

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). If you demolish your garage or other enclosed structure and build an ADU in its place, the ADU can be in the same footprint if it’s the same size and height of the structure it’s replacing. You may need to provide replacement parking; check Local ADU Rules or more details.

If you plan on replacing a detached garage with an ADU, demolition permits, and public notice cannot be required if you have your ADU permit (unless it is in an architecturally and historically significant district). Check with staff for other garage-related policies.

Note that garage conversion ADUs may require significant moisture barriers and other design elements in order to meet building codes.

In almost all cases, yes! ADUs and JADUs are allowed in all single-family and mixed-use zones. If residential buildings are allowed, ADUs are almost always allowed too (with limited exceptions for safety, traffic, and water).

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 (HOA), 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, staff may ask to see written review from your HOA.

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 staff, 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 six months to get permits. Construction usually takes six to twelve months.

The best place to start is with thinking about what you want, understanding your goals and concerns, and looking at other ADUs for inspiration. Once you have some ideas in mind, you can consider your budget and move on to Learning the Rules to figure out what you can build on your property.

You can also use our Process-At-A-Glance resource for an overview of the steps and some initial issues to consider as you get started.

Skip to content