A project of the Solano County Planning Collaborative


Many pages on this site feature FAQs – here are all the FAQs together in one place.

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 (“turnkey”), 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 square feet to large 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 (sink, cooking appliance, fridge, and small 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 room behind or next to your main home? You can build it in your front yard instead.

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 the Renting & Move-In steps for resources on understanding rental laws, tenants’ rights, and more, and our Worksheets 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 the Renting & Move-in guide 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.

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:

  • 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. Ask your designer and community for recommendations of good local professionals and make a list of candidates. It’s great if they have ADU experience, but it’s not necessary.

First, you’ll solicit bids. You will want to get at least three bids for comparison. When you have bids, you can begin selecting your contractor.

Before you hire a contractor, make sure to check their license and insurance and when they present you with a contract, review everything carefully.

Permits typically expire 1-2 years from date of issue but this will vary based on your location. Talk to City or County staff to find out.

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.

State law says an ADU permit cannot be denied due to nonconforming zoning, building code violations or unpermitted structures unless there is a threat to public health or safety, and they are not affected by building the ADU.

An unpermitted ADU can make it difficult to sell or refinance your property. If an unpermitted unit is discovered and is under construction, the county will issue a stop work order. If the building is complete, it will need an as-built permit, which has extra fees and requires substantial physical work on the building to assess the condition and details.

For unpermitted ADUs built before January 1, 2018, state law says a permit to legalize cannot be denied even if there is a violation of ADU laws or building standards, unless it is a “health and safety concern” or if the building is deemed “substandard” by state Health and Safety Code.

All ADUs and JADUs require building permits in order to start construction. Other permits are required based on location, special zones, ADU type, and other conditions.

Once you have a design established with your architect/designer, it’s a great idea to discuss it with City or County staff by visiting their permit counter, so they can point out any issues before you prepare your application. See our Worksheets for questions you might want to ask.

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.

The American Institute of Architects provides helpful information for homeowners and maintains a local East Bay chapter website where you can find professionals accepting new work.

See our Worksheets for a list of questions to ask a potential architect or designer.

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. Remember you can’t do short-term rentals with your ADU.

Make sure to consider potential income when planning the finances for your project.

JADUs: You’ll need to record in a deed restriction for the property that the JADU cannot be sold separately from the primary home.
ADUs: New state laws will allow for the separate sale of a detached ADU.

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 County Assessor & Recorder’s 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.

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 the California Housing Finance Agency (Cal HFA) ADU Program has grants available of up to $40,000 to qualified homeowners – check their website for the most up-to-date information.

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 Casita Coalition’s ADU Finance Guide for Homeowners for more details on financing options and our Worksheets for questions to help you decide a financing strategy.

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 $300 per square foot for construction (“hard costs”) and design and fees (“soft costs”), depending on your design and the materials you chose.

Wildfires are a reality throughout our state, which is why it is important to understand the risk in your area. If your property is in a Fire Hazard Severity Zone or Fire Protection District there may be additional requirements or reviews. Find out about your location (use this Fire Department and Protection Districts resource list) and talk to City or County staff early on to learn how where you live might impact your ADU.

State agencies have developed several resources and guidelines to help. View the Fire Severity Zone Map  and plug your address into the Fire Hazard Severity Zone Tool to look up your property and identify your zone. Use the Homeowners Checklist and review the Disaster Ready Guide and Board of Forestry Code to make sure your ADU and property are fire safe.

Hazard requirements depend on property elevation and location and may factor into ADU plans. Contact City or County staff early on to see what additional requirements your team will need to include.

Properties in Historic Districts may require additional review for ADU projects and are very likely subject to design review for consistency with historic criteria. Contact City or County staff to confirm the process and submissions required for ADUs in historic districts.

Conversion ADUs: New connections and fees not required.
Attached/detached ADUs: Separate utility connections and connection fee/capacity charges are required for new ADUs and are due when building permit is issued.

Parking is much less of a concern than it used to be.

One off-street parking space is required per unit, unless:

  • JADU
  • Within ½ mile to public transit
  • In an architecturally or historically significant district
  • Replacement parking for a garage demolition or conversion
  • On-street parking permits are required but not offered to ADU occupant
  • Within 1 block of car share
  • Built as part of a new home

The required space may be tandem or in the side yard/setback (ask City or County staff to verify local rules). Garage conversions do not require replacement of the lost parking space.

No. ADUs and JADUs cannot be rented for fewer than 30 days at a time.

Under state law, owner occupancy is no longer required for properties with ADUs. However, JADU owners must live in either the JADU or the main home – and this will need to be recorded in a deed restriction for the property.

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

See the Learning the Rules guide for more details.

Homeowners can build both an ADU and JADU on their property. Multifamily properties like duplexes, triplexes and apartment buildings can have at least two detached ADUs. Talk to your City or County staff for more information if interested in building ADUs on a multifamily property.

Homeowners can convert legally built structures (garage, barn, art studio) into an ADU and do not have to replace the parking space. JADUs can be converted from an attached garage (but not detached) but you may be required to replace the parking space.

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.

Demolition permits for an existing detached garage can be processed at the same time as the ADU permits. Note that garage conversion ADUs may require significant moisture barriers and other design elements in order to meet building codes.

ADUs often need separate waste treatment, and if you have a well, you may need to provide a well report. In either case, your system may need to be upgraded. Talk to your City or County staff or your Environmental Health Department early in the process.

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.

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

The best place to start is by 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.

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). Confirm with local Planning staff – see our Contact page for contact information.

Not sure where to start?

We can help!

Check out our guide to Getting Started to take the first steps. 

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