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Napa County, CA Accessory Structures: ADU Rules (2026)

Light Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified June 2026

ADU rules in Napa County, CA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Code Section: § 18.104.180
Max ADU Size: 1,200 sq ft
Max JADU Size: 500 sq ft
Detached Height (waived to allow): 16 feet
Side/Rear Setback (new detached): 4 feet
Allowed Zones (ADU): RS, RC, AW, PD
Allowed Zones (JADU): RS, RC, AW, PD, AP
Parking Required: 1 space per unit/bedroom (many exemptions)
Impact Fees: None if under 750 sq ft
Review Type: Ministerial

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in California under recent ADU streamlining laws. The Napa County, CA rules below cover local size, height, and setback requirements.

Summary

Napa County Code Section 18.104.180 ministerially permits Accessory Dwelling Units (ADUs, called 'second units') and Junior ADUs (JADUs) on legal lots zoned RS, RC, AW, or PD, with a 1,200 sq ft maximum for ADUs (500 sq ft for JADUs) and four-foot side/rear setbacks for new detached units up to 16 feet tall.

These county ordinances apply to unincorporated areas of Napa County. Incorporated cities within the county may have their own rules that take precedence over county-level regulations.

A. Pursuant to the provisions of Government Code Sections 65852.2 and 65852.22, the following requirements apply to all second units and junior accessory dwelling units, as applicable: 1. The lot is a legal lot as defined by Section 18.08.340 of this title. 2. The lot is zoned RS, RC, AW, or PD when the lot contains an existing or proposed single family dwelling or existing multifamily unit that is precluded from transient occupancy. 3. The lot contains at least one legal single-family dwelling or multifamily dwelling, except when an applicant is applying for a permit to build a single-family dwelling and a second unit at the same time. 4. The total floor space of a junior accessory dwelling unit shall not exceed five hundred square feet as measured from the inside of the exterior walls. The total floor space of a second unit shall not exceed 1,200 square feet as measured from the inside of the exterior walls. 5. Except as modified in this Section 18.104.180, a junior accessory dwelling unit or second unit shall conform to all height, setback, lot coverage and other zoning requirements applicable to a primary (main) dwelling in the zone in which the property is located, unless they are inconsistent with the provisions of this chapter, in which case the standards of this chapter shall apply. ... 14. Limits on lot coverage, floor area ratio, open space, and size must permit or shall be waived to allow an 800 square foot detached or attached second unit 16 feet high with four-foot side and rear yards, if the proposed second unit is in compliance with all other development standards, including but not limited to front yard setbacks. ... 16. Except as specified below, a second unit shall be required to comply with the setback requirements of the zone in which the unit is to be located. a. No setback is required for an existing living area or an existing accessory structure converted to a second unit, or for a new second unit constructed in the same location and built to the same dimensions as an existing structure. b. For all other second units, a setback of four feet is required from the rear and side property lines.

View official code

Full Breakdown

180(D)(1)). The total floor space cap is 1,200 sq ft for an ADU and 500 sq ft for a JADU (measured from inside the exterior walls). 180(A)(14)). 180(A)(16)(a)). 180(A)(16)(b)). 180(A)(11)). 180(A)(6)(c)). 180(A)(10)). 180(A)(13)).

Violations & Fines

Building or occupying an ADU without ministerial permit approval is a code-enforcement violation under Title 18. Napa County operates an ADU Delayed Enforcement Program (per AB 2533) under which unpermitted ADUs may qualify for delayed enforcement if a Building Inspector confirms no inherent health and safety risk and the unit meets minimum setback, size, and zoning standards listed on the Code Compliance ADU Delayed Enforcement Program Inspection Checklist.

Frequently Asked Questions

What size ADU can I build in unincorporated Napa County?

Detached or attached second units may be up to 1,200 sq ft total floor space. Napa County Code § 18.104.180(A)(14) specifically requires the County to permit or waive lot-coverage limits to allow an 800 sq ft second unit at 16 feet high with four-foot side/rear setbacks.

Do I need a parking space for my ADU?

Generally one space per ADU or per bedroom (whichever is less), provided as tandem parking on the existing driveway. But § 18.104.180(A)(11) waives the parking requirement entirely if the ADU is within 1/2 mile of transit, in a historic district, part of the existing primary residence or an accessory structure, on-street permits aren't available to the ADU occupant, or there's a car-share pickup within one block.

Can I rent my ADU as a vacation rental?

No. § 18.104.180(A)(13) requires the owner to acknowledge that neither the second unit nor the single-family dwelling may be used for short-term residential rentals, and the owner must record a covenant before the building permit issues.

Are there owner-occupancy requirements?

JADUs require the owner to occupy either the primary dwelling or the JADU as principal residence (§ 18.104.180(D)(6)). Standard ADUs do not have owner-occupancy requirements under current state law (Gov. Code § 65852.2), so Napa cannot impose one.

Sources & Official References

Other rules in Napa County

Parking Rules: Overnight ParkingSome Restrictions Parking Rules: Street Parking LimitsSome Restrictions Shed & Carport RulesSome Restrictions Parking Rules: RV & Boat ParkingSome Restrictions Garage ConversionsSome Restrictions Noise Ordinances: Aircraft NoiseSome Restrictions Parking Rules: Commercial & Oversized VehiclesFew Restrictions Parking Rules: Abandoned VehiclesLight Restrictions

All Napa County rules

How Napa County compares: Easiest Cities to Build an ADU·California rules heatmap·Compare Napa County to another location·View the California accessory structures overview

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ADU Rules in Cities Across Napa County

Napa, CA