The rules for building an accessory dwelling unit (ADU for short) in Napa County, California have changed — in a good way. State law now guarantees a faster, more straightforward approval process, and local policies are catching up. This post covers what you need to know if you’re planning to build in unincorporated Napa County.
Napa County ADU rules apply to all the following California towns:
The following cities in Napa County have their own rules:
Before we dive into the local rules, it helps to understand the three main types of ADUs in California:
Detached ADU
A completely separate structure, like a small house in your backyard. It has its own kitchen, bathroom, and entrance.
Attached ADU
Built as an addition to your main home. It shares a wall but has its own living space and entrance.
Junior ADU (JADU)
A smaller unit created within your existing home, like converting a bedroom or part of the garage.
Each type comes with slightly different rules, especially around size, parking, and whether the owner has to live on the property. The table below gives a quick overview — then we’ll break down Napa County’s specific requirements section by section.
| Feature | Detached ADU | Attached ADU | Junior ADU (JADU) |
|---|---|---|---|
| Description | Backyard tiny home | Duplex add on | Converted bedroom / bathroom or Garage conversion |
| Maximum Size | 1,200 square feet | 1,200 square feet | 500 square feet |
| Construction Impact | Built out back, peace and quiet inside your home | You’ll hear hammering on shared walls and roof | Construction happens inside your home — expect sawdust |
| Home Design Options | Full creative control | Full creative control | Limited to existing space |
| Owner Can Live Elsewhere | |||
| Maximum Privacy | |||
| Eligible for Separate Utilities? | |||
| Resell to Anyone* | |||
| Split & Sell Potential (SB 9) |
JADUs require a permanent title restriction that forces all future owners to live on the property.
How California’s ADU Size Tiers Work
California uses four ADU size tiers that affect what you can build and how much it costs. As units get larger, the rules can change — especially when it comes to impact fees, zoning limits, and design restrictions. To make things easier (and more affordable), we’ve designed our homes to align with these tiers — staying within the most favorable size ranges whenever possible.
| Tier 1 | Tier 2 | Tier 3 | Tier 4 | |
|---|---|---|---|---|
| Size Range | Under 500 sq ft | 501–750 sq ft | 751–800 sq ft | 801–1,200 sq ft |
| Rules | Favorable state rules. Allowed by right. | Favorable state rules. Allowed by right. | Favorable state rules. Allowed by right. | Subject to local zoning requirements. |
| Permit Fees | Lowest – school and impact fees waived | Low – impact fees waived | High – no fees waived | High – no fees waived |
| Our Models | 1 bedroom | 2 bedroom | (We don’t build in this tier) | 3 bedroom |
What can I use an ADU for?
You can use an ADU in Napa County as a rental unit, a guest house, a place for family members, a home office, or even for yourself. Many people build one for aging parents, adult children, or a caregiver. Others use it for extra income or simply as more flexible space. As long as someone can legally live there full-time, it’s allowed.
Where can I put an ADU on my property?
Our one and two bedroom homes can be placed as close as 4 feet from the side and back property lines. However, a three bedroom home must be 5 feet from the side and back property lines. The distance from the front of your property follows the same rules as your main house — usually 20 feet.
Do I Need to Add Parking for an ADU?
You will need to provide one off-street parking space for your ADU. This space can be located in tandem with existing parking, on an uncovered driveway, or in another legal off-street spot — no garage or carport is required. Parking requirements are waived if your property is within ½ mile of public transit, or you’re converting an existing structure (JADU). In those cases, no new parking is required.
Design Requirements
Napa County doesn’t impose any design restrictions on ADUs. That means there are no rules about matching the main house’s style, roofline, materials, or colors. As long as your unit meets basic building and zoning requirements—like size, setbacks, and safety codes—you’re free to design it however you like.
Each ADU must include a bedroom, bathroom, living room, kitchen, and dining area—though only the bathroom needs to be fully enclosed with four walls. JADUs can share a bathroom with the main home.
How Many Can I Build?
If you own a single-family home in unincorporated Napa County, you can build up to two additional units:
One full-size ADU (up to 1,200 sq ft, detached or attached)
One Junior ADU (JADU) (up to 500 sq ft, converted from space inside the home)
That means you can have a total of three units on the property — your main home, one ADU, and one JADU. However, if you include a Junior ADU, the property owner must live on site, either in the main home or the JADU.
Permit Costs for Napa County
Once you submit a complete ADU application, Napa County is required by state law to approve or deny it within 60 days. The table below give an estimated range of fees:
| Tier 1 Up to 500 sq ft | Tier 2 Up to 750 sq ft | Tier 3 Up to 800 sq ft | Tier 4 Up to 1200 sq ft | |
|---|---|---|---|---|
| Estimated Valuation | $200,000 | $225,000 | $240,000 (est.) | $250,000 |
| Estimated Permit Fees | ~$9,200 | ~$9,600 | ~$10,200 | ~$10,800–$12,000 |
| Impact Fees | $0 | $0 | ~$4,000–$6,000 | ~$6,000–$10,000 |
| School Fees | $0 | ~$2,250–$3,000 | ~$2,400–$3,200 | ~$2,700–$3,600 |
| Total Estimated Fees | ~$9,200 | ~$11,850–$12,600 | ~$16,600–$19,400 | ~$19,500–$25,600 |
Free Site Assessment
We’ll evaluate your property’s layout, zoning, and access — so you’ll know exactly what’s possible for an ADU. No cost. No pressure.
What we check:
- ✓ Lot size & layout
- ✓ Zoning and setback rules
- ✓ Access for delivery
- ✓ Your best ADU options
