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SB 9 Lot Splits and Duplexes in Sonoma, Napa and Marin

By Matthew Martinez, Founder, CEO & Managing Broker, Diamond Real Estate Group. California DRE #01869679.

An SB 9 lot split lets the owner of a qualifying single-family lot divide it into two lots, and SB 9’s companion rule lets each lot hold up to two primary homes, all approved ministerially without a public hearing. The parcel must be in a single-family zone, in a city that includes part of a U.S. Census urbanized area or urban cluster (or, in unincorporated areas, wholly inside one), outside excluded sites, with no tenant-occupied or rent-controlled housing affected, and the applicant for a split must sign a three-year owner-occupancy affidavit. Each new lot must be at least 1,200 square feet, unless a local ordinance allows smaller, and at least 40 percent of the original.

SB 9 has been in effect since January 1, 2022, and SB 450 (Chapter 286, Statutes of 2024) tightened it in owners’ favor, most notably with a 60-day decision deadline. Below is what the statute requires, what changed, and how Sonoma County, Santa Rosa, Petaluma and unincorporated Marin apply it.

What SB 9 allows

SB 9 lives in two sections of the Government Code. Section 65852.21 covers a “two-unit development,” meaning up to two primary homes on one lot. Section 66411.7 covers the “urban lot split,” dividing one lot into two.

Two-unit development only: add a second primary home to a lot with an existing house, or build two on a vacant lot. Units can be attached as a duplex or detached.
Lot split only: divide the lot and sell or build on the new lot later.
Both: split the lot and put up to two homes on each, for as many as four primary homes where there was one.

Units created under SB 9 must be rented, if rented at all, for terms longer than 30 days. Lots created by a split are limited to residential use.

Is the parcel eligible?

Eligibility is the step where most SB 9 plans fail. The statute screens the parcel, its location and its occupants.

Test	Requirement
Zoning	A single-family residential zone
Location	In a city whose boundaries include part of a U.S. Census urbanized area or urban cluster; in unincorporated areas, wholly inside one
Excluded sites	Prime farmland or farmland of statewide importance, wetlands, protected-species habitat, land under a conservation easement or identified for conservation, hazardous waste sites
Conditional sites	Very high fire hazard severity zones and, under the current text of Gov. Code 65913.4(a)(6)(D), the State Responsibility Area; earthquake fault zones, FEMA special flood hazard areas and regulatory floodways, allowed only if the site meets the mitigation or building standards the law specifies
Historic status	Not in a historic district or on a historic resource list
Occupancy	No demolition or alteration of rent-restricted or rent-controlled housing, or housing occupied by a tenant in the last three years
Ellis Act	No withdrawal of the property from the rental market in the last 15 years
Prior splits (lot splits only)	The parcel wasn’t created by an earlier SB 9 split, and the owner hasn’t split an adjacent parcel under SB 9

The tenant rule is a frequent surprise for buyers of rented houses. If you’re buying a home that has been leased, read our guide to buying a tenant-occupied property and confirm the three-year history before you count on a split.

Fire hazard zones in Sonoma, Napa and Marin

Fire mapping matters more here than almost anywhere in the state. The State Fire Marshal’s map for State Responsibility Areas took effect April 1, 2024, and its maps for Local Responsibility Areas were released in four phases between February 10 and March 24, 2025. Under the county counsel association’s summary, local agencies then had 120 days to adopt them by ordinance.

Jurisdictions describe the SB 9 fire exclusion differently, so read the local guidance.

Santa Rosa’s January 2022 interim guidance says two-unit developments and lot splits aren’t allowed in a very high fire hazard severity zone unless the site is subject to adopted fire hazard mitigation measures under existing building standards. Marin County’s SB 9 page lists very high, and high or very high, fire hazard severity zones among its core exclusions. Our fire hazard severity zone guide explains how to look up a parcel.

Rules for the lot split

Section 66411.7 sets the geometry and the ownership conditions for a split.

Size: two parcels of approximately equal area, neither smaller than 40 percent of the original lot, and each at least 1,200 square feet unless a local ordinance allows smaller.
Owner occupancy: the applicant signs an affidavit stating they intend to live in one of the units as their principal residence for at least three years after approval. Community land trusts and qualified nonprofits are exempt, and the city or county can’t add further owner-occupancy conditions.
Improvements: the agency can require easements for utilities and access, but can’t require right-of-way dedications or offsite improvements, and can’t require you to correct nonconforming zoning conditions as a condition of approval.
Unit cap: the agency doesn’t have to allow more than two units on a lot created by an SB 9 split, and “unit” includes primary homes, ADUs and JADUs.

Without a split, a two-unit development can still be paired with ADUs and JADUs under local ADU rules. For ADU economics, see our ADU investment guide.

Standards the city or county can and can’t impose

Local agencies may apply objective zoning, subdivision and design standards, meaning measurable rules with no personal judgment. Since SB 450, those standards must apply uniformly in the underlying zone, though they can be more permissive for SB 9.

Size protection: standards can’t physically preclude two units or keep either from being at least 800 square feet.
Setbacks: up to four feet from side and rear lot lines, and none for an existing structure or one rebuilt in the same location and dimensions.
Parking: no more than one space per unit, and none within a half mile of a high-quality transit corridor or major transit stop, or within one block of a car-share vehicle.
Septic: for units on an onsite wastewater system, the agency may require a percolation test done within the last five years, or ten years if recertified.
Connected structures: an application can’t be rejected only because structures are adjacent or connected, if they meet building code safety standards and can be sold separately.

The agency may deny a project only if its building official finds, by a preponderance of the evidence, a specific adverse impact on public health and safety that can’t feasibly be mitigated.

What SB 450 changed

SB 450 was approved on September 19, 2024. The changes owners notice most:

A 60-day clock. The agency must approve or deny a complete application within 60 days, or it’s deemed approved. A denial must come with a full set of comments explaining the defects and how to fix them.
No 25 percent wall limit. The old rule capping demolition of existing exterior walls is gone. HCD’s December 2025 letter to Santa Clarita told that city to delete the limit from its ordinance.
Uniform standards. A city can’t impose SB 9-only standards that are stricter than those in the underlying zone.
Narrower denials. The ground of a specific adverse impact on the physical environment was removed; the health and safety ground remains.

How North Bay jurisdictions process SB 9

Unincorporated Sonoma County (Permit Sonoma)

Permit Sonoma’s Policy 8-0-6 names R1, RR and AR as the county’s single-family zones for SB 9. A split alone is processed as a minor subdivision, a two-unit project alone as a zoning permit, and both together as a planning project.

The county lists forms PJR-141 to PJR-144, including an owner-occupancy affidavit, and says it will tell you within 30 days of fee payment whether the application is complete, then act within 60 days. Policy 8-0-5 says no more than two dwellings, ADUs and JADUs included, may be permitted on a lot created by a split, and a note limiting future development is recorded on the parcel map.

Santa Rosa

Santa Rosa applies SB 9 in its RR and R-1 single-family districts and single-family planned developments; its January 2022 interim guidance lists RR-40, RR-20, R-1-15, R-1-9, R-1-7.5 and R-1-6. The guidance treats a SMART station or a bus route with 15-minute service as a high-quality transit corridor for the parking exemption. Start with the city’s eligibility checklist; our Santa Rosa guide covers the city’s neighborhoods.

Petaluma

Petaluma’s FAQ says SB 9 applies only in single-family zones, and a planned unit development qualifies only if it allows single-family homes exclusively and has no CC&Rs. A split uses a tentative parcel map with a $4,265.17 deposit plus staff time, and SB 9 homes are primary dwellings that need their own utility connections and full impact fees. See our Petaluma guide for the city’s housing stock.

Unincorporated Marin County

Marin County cites Development Code sections 22.32.184 and 22.80.064, publishes an SB 9 eligibility map, and records a deed restriction requiring rentals longer than 30 days. Its Planning Division handles applications at 3501 Civic Center Drive in San Rafael.

Napa and other cities

We couldn’t confirm a current SB 9 ordinance for the City of Napa or Napa County in this review. Ask the planning department for its SB 9 checklist and any objective standards before you design, and confirm cities such as San Rafael and Novato the same way.

Step by step: from idea to recorded map

Screen the parcel. Zoning, the city or Census urban boundary test, fire, flood and fault maps, historic status and three years of occupancy history.
Talk to planning staff. Permit Sonoma offers walk-in planner time, but notes a planner’s preliminary determination isn’t binding.
Check utilities and access. Separate sewer and water service, or septic and well feasibility, and a route for an access easement to the rear lot.
Have a surveyor and designer lay out the split. Confirm the 40 percent and 1,200-square-foot tests and where two units of at least 800 square feet fit.
File the complete application with the affidavit and the local forms. The 60-day clock runs from a complete application.
Record the parcel map and pull building permits. Building and encroachment permits follow planning approval.

A split is often the first step toward a sale, so it can make sense to compare it with our guide to subdividing land in Sonoma and Napa under a regular parcel map, and with the SB 684 and SB 1123 small-lot subdivision rules if the lot is larger or vacant.

Questions people ask

Do I have to live on the property after an SB 9 lot split?

For a lot split, the applicant must sign an affidavit of intent to occupy one of the units as a principal residence for at least three years after approval. The law bars the city or county from adding more owner-occupancy conditions, and a two-unit development without a split has no such affidavit under the statute.

Can I sell one of the lots after the split?

SB 9 doesn’t prohibit selling a lot, but the affidavit states your intent to live on the property for three years. Talk to a land use attorney before planning an immediate sale.

How many homes can I end up with?

Up to two primary homes on each lot after a split, so four in total, and the agency doesn’t have to allow ADUs or JADUs on top of that. Without a split, two primary homes plus ADUs and JADUs under local rules.

How long does approval take?

Since SB 450, the agency has 60 days from a complete application to approve or deny, or the application is deemed approved. The parcel map, building permits and utility work come after that.

Can SB 9 units be short-term rentals?

No. Rentals of units created under SB 9 must be for terms longer than 30 days.

Does SB 9 work on a hillside lot with a septic system?

It can, if the lot is in a qualifying zone and outside excluded hazard areas. The agency may require a recent percolation test, and slope, access and septic area often decide whether a second unit fits.

Does a lot with SB 9 potential sell for more?

Sometimes, but buyers pay for what they can confirm. A parcel with a clean eligibility screen, a survey and utility answers usually tells a stronger story than one marketed on “SB 9 potential” alone; our guide to buying a teardown covers similar pricing questions.

If you own a single-family lot and want to know whether a split or second home changes what it’s worth, we can pull the zoning, hazard and occupancy records and give you a realistic number. Request a property valuation, or see how SB 9 fits with the other tools on our land development hub and our investor services. For rural parcels, our Sonoma County land and ADU guide covers wells, septic and access.

This is general information, not legal or tax advice; talk to a California land use attorney or tax adviser about your situation.

Sources & notes

The official statute pages on leginfo.legislature.ca.gov could not be opened during this review, so the statute was checked against the chaptered text of SB 450. Local SB 9 rules for the City of Napa, Napa County, San Rafael and Novato weren’t confirmed.

Last reviewed October 2026 by Matthew Martinez.