Building an ADU in the City of Napa

What you can build in Napa

The City of Napa allows ADUs and junior ADUs on lots in zones that allow homes, with an existing or proposed single-family or multifamily home. You need an ADU or JADU building permit, and the city's Community Development Director approves it ministerially if it meets the rules. The rules are in Napa Municipal Code 17.52.015, last amended in September 2025.

The number of ADUs you can build follows state law (Gov. Code 66323). For a single-family lot, HCD's handbook lists one ADU converted from existing or proposed space, one JADU, and one new detached ADU of up to 800 sq ft with 4-foot side and rear setbacks. HCD's 2024 letter to Napa says these types can be combined on one lot. On a multifamily lot, state law allows conversions of non-living space (at least one, up to 25% of the units) and up to eight detached ADUs with an existing building, or two with a proposed one. The code says a detached 800 sq ft ADU is always allowed on a lot with a home. There is no minimum lot size.

Napa also offers a bonus: one extra ADU for each ADU on the lot that you keep affordable to very low or low income households for at least 10 years, or moderate income households for at least 15 years. You sign an affordable housing agreement with the city before the building permit is issued.

If your home is on the city's Historic Resources Inventory, the ADU needs a historic clearance, within the limits of state law. [1][5][6]

Size, height and setbacks

Size. A detached ADU can be up to 1,200 sq ft. An attached ADU can be up to 50% of the main home's living area, or 800 sq ft if 50% would be smaller. A conversion must stay within the existing building, plus up to 150 sq ft for an entrance. The ADU must meet your zone's lot coverage limits unless they would block an 800 sq ft ADU; state-exempt ADUs skip those limits.

Setbacks. A new ADU needs 4-foot side and rear setbacks. It must meet your zone's front setback unless that would prevent an 800 sq ft ADU. A conversion, or an ADU built in the same spot and size as an existing accessory building, needs no setback unless one is needed for fire safety.

Height. ADUs can be as tall as state law allows in Gov. Code 66321(b)(4), and no more than two stories. HCD's handbook lists those state heights: 16 feet for a detached ADU; 18 feet within a half mile of a major transit stop or high-quality transit corridor (HCD's description includes an allowance for a roof pitch that matches the home) or on a lot with a multistory multifamily building; and 25 feet or the main home's height limit, whichever is lower, for an attached ADU.

Design. For a second-story ADU, the entrance, porches, balconies and stairs must face away from the nearest residential lot line. Windows facing a neighbor need clerestory design, privacy glass, fencing or landscaping to block a direct view. The Director may also adopt written objective design standards.

Fire sprinklers. An ADU or JADU needs sprinklers only if the main home has or needs them. Building an ADU can't by itself trigger sprinklers in an existing home. [1][5]

Parking

Napa's code asks for one off-street space for each ADU, except where state law (Gov. Code 66322 and 66323) says no parking can be required. The space can be in a setback or tandem in a driveway. HCD's handbook lists those state exemptions: ADUs within a half-mile walk of public transit, in a historic district, part of the home or an existing accessory building, within one block of a car share vehicle, or where on-street permits are required but not offered. State-exempt ADUs under Gov. Code 66323 need no parking either.

If you demolish or convert a garage, carport or covered parking to build an ADU, you don't have to replace it. A junior ADU needs no parking. [1][5]

Fees, sewer and utilities

Napa charges no impact fees on a junior ADU or on an ADU under 750 sq ft. For an ADU of 750 sq ft or more, impact fees are charged in proportion to the main home's size. State law, as HCD explains it, exempts ADUs of "750 square feet of interior livable space or less," so if your ADU is exactly 750 sq ft, ask the city how it will be charged. Building permit fees are based on the project's valuation; call the city for an estimate.

Every ADU needs a sewer permit from Napa Sanitation before the city will issue the building permit. Napa Sanitation has its own application; call (707) 258-6000 ext. 401. An attached ADU or a JADU can't be required to get a new utility connection or pay connection fees, unless an attached ADU is built with a new home. For other ADUs, the city says new connections may be required, with fees in proportion to the ADU's impact. For water, call the City of Napa Utilities Department at (707) 257-9521. See our ADU cost guide. [1][3][5]

How long it takes

The city says first plan reviews are complete within 21 business days, and resubmittals are reviewed within 14 business days. Expect at least one round of comments. A soils report may be needed if your lot is on an earthquake fault or in a flood zone.

State law requires the city to tell you within 15 business days whether your application is complete, and to approve or deny a complete application within 60 days when a home already exists on the lot. Once your permit is approved, your Napa Sanitation permit is verified and fees are paid, you can pick up the building permit. The city says construction commonly takes 12 to 18 months. See our ADU permit process guide. [2][3][5]

Junior ADUs and the Junior Unit Initiative

A junior ADU (JADU) in Napa can be up to 500 sq ft, not counting a bathroom shared with the home. It needs an efficiency kitchen with cooking appliances, a food preparation counter and storage cabinets. If it doesn't have its own bathroom, it must have an inside door to the main home. Before occupancy, you record a deed restriction that bars selling the JADU separately and limits its size. The city provides a deed restriction template.

Napa's code says that if a lot has a JADU, one of the units must be owner-occupied. State law has changed: HCD's handbook says owner occupancy is required only when the JADU shares a bathroom with the main home. If your JADU has its own bathroom, confirm with the Planning Division.

The city's Junior Unit Initiative helps owners create a JADU or convert an existing unit or garage into an ADU. Its 2026 flyer lists forgivable loans of up to $75,000, forgiven 5% a year and fully after 20 years, plus up to $25,000 more in deferred, no-interest loans for a converted ADU. It also offers grants of up to $5,000 for accessibility work, design and engineering grants, and free construction management. One unit must stay owner-occupied and the other must be rented to a low-income tenant at an affordable rent. Call the Housing Division at (707) 257-9430. (An older city handout lists loans up to $50,000; the 2026 flyer is newer.) [1][3][4][5][7]

Renting, selling and legalizing an ADU

You can rent the ADU or the main home, or let someone live there rent-free. Rentals of an ADU or JADU must be longer than 30 days, and the city's vacation rental rules bar using an ADU for transient (short-term) stays. An ADU can't be sold separately from the main home, except under Gov. Code 66341. The city won't issue a certificate of occupancy for the ADU before the main home has one.

If you built an ADU without a permit, the city says there is no fine or penalty for legalizing it, and no enforcement will result from asking about it. Call (707) 257-9530 or email planningdepartment@cityofnapa.org. [1][3][8]

Lot splits (SB 9) and where Napa and state law differ

Napa's code bars an ADU or JADU on any property that has used both SB 9 laws, the two-unit development rule (Gov. Code 65852.21) and the urban lot split (Gov. Code 66411.7). See our SB 9 lot split guide.

In January 2024, the state housing agency HCD found Napa's 2020 ADU ordinance out of compliance with state law. Among other items, it flagged a two-bedroom limit, the 1,200 sq ft cap applied to conversions, an administrative permit for ADUs over 16 feet, and a limit on combining ADU types. The city rewrote Section 17.52.015 in September 2025; the current code drops the bedroom limit and the administrative permit and ties unit counts and heights to state law. We found no later HCD letter on the new code.

The city's ADU Guide still describes the older rules, such as one ADU and one JADU per lot and a 16-foot height limit. The code was amended more recently than the guide; confirm with the Planning Division. The JADU owner-occupancy rule and the 750 sq ft fee line also differ from state law (see above). See our California ADU law overview. [1][2][6]

Permit steps in Napa

1. Learn the rules and get advice

   Read Napa Municipal Code 17.52.015 and the city's ADU process handout. Bring a sketch of your lot to a planner, or call the Planning Division at (707) 257-9530. [3][7]

2. Apply for your sewer permit

   Contact Napa Sanitation at (707) 258-6000 ext. 401. Every ADU needs a sewer permit before the building permit can be issued. [3]

3. Prepare the application package

   Prepare a building permit application, 6 sets of plans (site, floor and foundation plans, construction and elevation details) and the Residential High-Performance Building Checklist. The Building Department also accepts "pre-reviewed" ADU plans, which still get a full review. [3][7]

4. Submit and pay fees

   Submittal fees are based on project valuation. Reviews are completed within 21 business days, and resubmittals within 14 business days. [3]

5. Record the JADU deed restriction

   For a JADU, record the deed restriction before occupancy, using the city's template. [1][7]

6. Get the permit, build and pass inspection

   Once your application is approved, the Napa Sanitation permit is verified and fees are paid, collect your building permit. Your builder schedules the final inspection; call (707) 257-9590. Public Works assigns the ADU's address. [3]

Pre-approved and sample plans

• Pre-reviewed ADU plans

  The Building Department accepts "pre-reviewed" ADU plans with a permit application; they have been checked for completeness but still get a full code review. [7]

Who to call

• Planning Division

  ADU zoning rules, setbacks and legalizing an existing ADU (planningdepartment@cityofnapa.org)

  707-257-9530

  Community Services Building, 1600 First Street, Napa, CA 94559

  [2][3]

• Building Division (Permit Techs)

  Building permits and plan review (PermitTechs@cityofnapa.org)

  (707) 257-9540

  [2][7]

• Housing Division, Junior Unit Initiative

  Forgivable loans and help to create a JADU or converted ADU

  707-257-9430

  1115 Seminary Street, Napa, CA 94559

  [4]

Questions people ask