Every single-family lot in Sonoma County can hold at least an 800 sq ft accessory dwelling unit (ADU), 16 ft tall, with 4 ft side and rear setbacks. California law guarantees it. What changes from town to town is how much more you can add: Santa Rosa allows up to three units on one lot, while the City of Sonoma allows one.
I work with buyers and sellers in five ADU jurisdictions: Healdsburg, the City of Sonoma, Windsor, Santa Rosa, and unincorporated Sonoma County. Their rules look alike on paper because the state has flattened most of the differences. The gaps that remain can swing a project by tens of thousands of dollars and change what a property is worth at resale.
How state and local ADU rules fit together
California Government Code sections 66313-66342 set a floor. Cities and the county can be more generous, but never stricter. The latest changes took effect January 1, 2026.
The 800 sq ft guarantee at the bottom of that chart is the single most useful tool in California ADU law. If lot coverage, setbacks, or a local size cap would block every ADU on a parcel, the state floor overrides them.
ADU vs JADU: the state baseline
An ADU is a full second home on the lot, attached or detached. A junior ADU (JADU) is a smaller unit carved out of the main house. These state rules apply in all five jurisdictions unless a local ordinance is more generous.
| Rule | ADU | JADU |
|---|---|---|
| Maximum size | Up to 1,200 sq ft (cities must allow at least 800 sq ft) | 500 sq ft |
| Where it goes | Attached, detached, or a converted structure | Inside the walls of an existing or proposed single-family home |
| Kitchen | Full kitchen | Efficiency kitchen with a sink |
| Bathroom | Own bathroom | May share with the main house |
| Side and rear setbacks | 4 ft (none for a conversion in the same footprint) | Not applicable |
| Owner-occupancy | Set locally | Required (owner lives in the house or the JADU) |
| Parking | Up to 1 space, with many exemptions | None can be required |
| Impact fees | None under 750 sq ft; proportional above | No water or sewer connection fees |
| Approval | Ministerial, decided within 60 days of a complete application | Same |
Ministerial approval matters more than most owners realize. There is no public hearing, no neighbor notice, and no discretionary design review.
How tall a detached ADU can be under state law
| Situation | Height allowed |
|---|---|
| Most single-family lots | 16 ft |
| Within 1/2 mile of a major transit stop, or on a multifamily lot | 18 ft |
| Matching the height of the main house, or some two-story ADUs | Up to 20 ft |
| Above a garage (in many cases) | 25 ft |
Parking rules are loose too. No space is required if the ADU sits within 1/2 mile of transit, in a historic district, inside an existing structure, within one block of a car-share, or is a studio. Convert a garage and no city can make you replace the lost parking.
How the five jurisdictions compare
All five local ordinances now comply with state law, so the differences are narrower than they once were. Where a rule says "per zoning," the answer can change parcel to parcel.
| Rule | Healdsburg | City of Sonoma | Windsor | Santa Rosa | Unincorporated county |
|---|---|---|---|---|---|
| Max ADU size | 1,200 sq ft | 850 sq ft detached; 30% of main home if attached | 850 sq ft (studio or 1-BR); 1,200 sq ft (2+ BR) | 1,200 sq ft | 1,200 sq ft |
| Max JADU size | 500 sq ft | 500 sq ft | 500 sq ft | 500 sq ft | 500 sq ft |
| Minimum size | 150 sq ft | 150 sq ft | Per CA code | Per CA code | Per CA code |
| Side / rear setback | 4 ft / 4 ft | 4 ft state minimum | 4 ft / 4 ft | 4 ft (8 ft corner side) / 4 ft | 4 ft / 4 ft |
| Detached height | Up to 25 ft, 2 stories | Single story | 16 ft; 25 ft above a garage | 16 ft (1-story); 25-27 ft (2-story) | 18 ft or base zoning limit |
| Lot coverage | ADUs up to 850 sq ft exempt | Per SMC 19.50.090 | Per zoning | 40%, cannot block an 800 sq ft ADU | Per base zoning |
| Code | HMC 20.20.010 | SMC 19.50.090 | Title 17 | Chapter 20-42 | Section 26-88-060 |
For investors and multigenerational buyers, unit count is the headline. Santa Rosa and the unincorporated county allow three units on one single-family lot. The City of Sonoma allows one.
Size tells a different story. A Healdsburg owner can add a 1,200 sq ft ADU plus a 500 sq ft JADU with no parking required: up to 1,700 sq ft of new legal living space, often without losing the garage. If you are buying a home in Healdsburg with family or rental income in mind, that is a real edge.
Fees: where the math swings
Impact fees are where two similar ADUs in neighboring towns part ways. State law bans impact fees under 750 sq ft, but each jurisdiction sets its own curve above that line.
| Jurisdiction | Impact fee rule |
|---|---|
| Windsor | No town development impact fees on any ADU |
| Healdsburg | Exempt up to 850 sq ft; 850-1,200 sq ft pays proportional fees, capped at 50% of single-family fees |
| Santa Rosa | 0% up to 750 sq ft; 25% for 751-950 sq ft; 50% for 951-1,200 sq ft |
| Unincorporated county | Under 750 sq ft exempt from park and traffic fees; school fees may apply above 500 sq ft |
| City of Sonoma | State law: exempt up to 750 sq ft |
Take a 900 sq ft ADU. In Santa Rosa it pays 25% of the impact fees a new single-family home would pay. In Healdsburg it pays a proportional share. In Windsor it pays no town impact fees at all.
Windsor still has two outside fees, set per square foot by the school and fire districts:
Keeping a Windsor ADU at 500 sq ft or less avoids both. Fees are only one line in the budget; our guide to the cost to build in Sonoma County covers the rest. Healdsburg also keeps a library of pre-reviewed ADU plans (Abodu, Adore, Mayberry Workshop and others) that shortcut plan check, and a prefab home can trim both time and cost.
Parking, owner-occupancy and rentals
| Jurisdiction | Parking | Owner-occupancy | Short-term rental (under 30 days) |
|---|---|---|---|
| Healdsburg | None for ADU or JADU | JADU only | Prohibited; deed restriction recorded |
| City of Sonoma | 1 off-street space (driveway or tandem OK), with exemptions | Required: owner lives in the main home or the ADU | Prohibited |
| Windsor | Per state law, with exemptions | Not specified in the town handout | Prohibited |
| Santa Rosa | 1 space for some new detached ADUs; none at 750 sq ft or less, for JADUs, or conversions | JADU only | ADU rentals must be 30+ days |
| Unincorporated county | 1 space, with state exemptions | JADU only | Prohibited |
Two rows deserve a closer look.
Owner-occupancy is a moving target. The state suspended it for ADUs through 2024. Since January 1, 2025, cities may bring it back. Most have not, but the City of Sonoma has. JADUs require an owner on site everywhere, because that rule comes from the state.
No ADU in any of these five jurisdictions can be an Airbnb. The 30-day minimum is recorded against title, so it follows the property to the next owner. If short-term rental income is the goal, start with our guide to Sonoma County's vacation rental rules and look at homes that already hold a permit.
Healdsburg: the most room to add
Healdsburg allows the largest combined footprint and the tallest detached units, up to 25 ft and two stories. That makes a cottage over a garage practical. An "exception ADU" path (850 sq ft, 16 ft, 4 ft setbacks) skips most local standards. Front setbacks follow zoning: 20 ft in R-1-6, 25 ft in R-1-12.5, 30 ft in R-1-20/40, and none downtown. In the Historic District Overlay, exteriors must use natural materials, not vinyl or plastic.
City of Sonoma: the tightest rules
Detached ADUs top out at 850 sq ft and one story, and any detached unit over 650 sq ft cannot exceed 50% of the main home's living area. Attached ADUs are capped at 30% of the main home. A detached ADU must sit at least 10 ft from the house. Sonoma is also the one city here that still requires owner-occupancy.
Windsor: simple rules, lowest fees
Windsor's handout is the shortest of the five, and its lack of town impact fees is a real advantage. Two catches: attached and new detached ADUs must follow Chapter 7A wildfire-resistant construction, and protected oaks must appear on the site plan.
Santa Rosa: the most units
Santa Rosa permits one new ADU, one conversion ADU and one JADU on a single-family lot. On multifamily lots, owners can convert up to 25% of existing units (at least one) and add up to eight detached ADUs on an existing building. Attached conversions are capped at 45% of the existing living area. Owners of unpermitted ADUs built before March 3, 2020 can use an enforcement deferral program.
Unincorporated Sonoma County: start with septic and water
Outside city limits (Russian River, Glen Ellen, Kenwood, Forestville, Geyserville and the rural edges of Healdsburg, Windsor and Santa Rosa), the rules on paper are generous. The real limits are underground.
A buyer can have the right lot, zoning and budget and still be stopped by a marginal septic system or a Class 4 groundwater area. Pull a parcel report at parcelsearch.permitsonoma.org before you write an offer. Coastal Zone parcels follow a separate ordinance, Section 26C-325.1.
Which jurisdiction fits your goal
| If your goal is | Look first at | Why |
|---|---|---|
| The most units on one lot | Santa Rosa, unincorporated county | Up to 3 units (2 ADUs + 1 JADU) |
| The most added square footage | Healdsburg | 1,200 sq ft ADU + 500 sq ft JADU, no parking |
| A two-story cottage | Healdsburg, Santa Rosa | 25 ft (Healdsburg) and 25-27 ft (Santa Rosa) detached |
| The lowest fees on a larger ADU | Windsor | No town impact fees at any size |
| Renting both units with no owner on site | Anywhere but the City of Sonoma, and skip the JADU | Sonoma requires owner-occupancy; JADUs require it everywhere |
| Short-term rental income | None of them | Every ADU here carries a 30-day minimum |
Local ordinances change often. Before you buy, design or price a property around an ADU, confirm the current rules with that jurisdiction's planning department.
Thinking about adding an ADU, or buying a home with one? Email me directly at david@bruingtonhargreaves.com.
About the Author
David Hargreaves is the co-founder of BruingtonHargreaves, one of Sonoma County's top-ranked real estate teams and part of W Real Estate. Originally from the UK and an Oxford University graduate, David built and ran a digital marketing agency serving Google, Facebook, and other major brands before becoming one of Sonoma County's top agents within three years of entering real estate.
Today he and business partner Jonathan Bruington have sold more than $250 million in Sonoma County homes over the past three years, earning recognition as a RealTrends No. 2 team in the county and the No. 1 team in Healdsburg. David specialises in helping Bay Area buyers and sellers with luxury properties and vacation rentals across Healdsburg, Windsor, Santa Rosa, and the Russian River communities.
He lives in Sonoma County with his wife Nancy and is happiest cycling the back roads, exploring local wineries, or behind a camera. Have a question about buying, selling, or building in Wine Country? Book a free call.