California SB 10 Housing Law: Opt-In, Eligible Parcels, and Limits

California Senate Bill 10 is a 2021 state law, codified at Government Code Section 65913.5, that lets a city council or county board of supervisors pass an ordinance allowing up to 10 residential units on a single qualifying parcel, even in neighborhoods zoned for single-family homes.¹ It is entirely voluntary: nothing changes in a given city until that city’s elected officials affirmatively adopt an SB 10 ordinance.

What SB 10 Lets Cities Do

The core mechanic is simple. A local government can rezone a qualifying parcel for up to 10 units, and it sets the height limit for those buildings in its own ordinance, so a city keeps control over how tall these projects can get.¹

The target is what planners call “missing middle” housing: duplexes, triplexes, fourplexes, and small apartment buildings that fill the gap between single-family houses and large apartment complexes.² Decades of single-family zoning across California had effectively banned these forms in many neighborhoods.

It Only Works If Your City Opts In

This is the detail that catches most people off guard. SB 10 rezones nothing on its own. It is an enabling statute, and no city or county is required to use it. The local government must pass its own ordinance, declaring it adopted under SB 10, before any of the law’s provisions apply in that jurisdiction.²

If your city council has not acted, SB 10 has no practical effect where you live. Adoption has been slow statewide, partly because of political resistance to density in established neighborhoods and partly because some cities find other housing laws more useful for their circumstances.

Which Parcels Qualify

A parcel has to fall into one of two geographic categories.

Transit-Rich Areas

A parcel qualifies if it sits within half a mile of a major transit stop or on a high-quality bus corridor. The corridor standard has three frequency thresholds: buses at least every 15 minutes during weekday morning and evening peaks, every 20 minutes between 6 a.m. and 10 p.m. on weekdays, and every 30 minutes between 8 a.m. and 10 p.m. on weekends.¹

Urban Infill Sites

An urban infill site has to meet all three of these: the city contains at least part of a U.S. Census Bureau urbanized area or urban cluster; at least 75 percent of the parcel’s perimeter borders land already in urban use; and the parcel is zoned residential or mixed-use, with at least two-thirds of any project’s square footage designated for housing.¹

What SB 10 Does Not Cover

SB 10 cannot be used on parcels in very high fire severity zones or on land voters have designated as open space, parks, or recreational areas.²

The law also contains no affordability mandate. Nothing in Section 65913.5 requires any of the new units to be rent-restricted or set aside for lower-income households. Local governments remain free to apply their own inclusionary housing rules, and many California cities already do.

CEQA and Voter-Approved Rules

SB 10 gives cities two legal shortcuts worth understanding.

First, the rezoning ordinance itself is exempt from the California Environmental Quality Act. Zoning changes normally trigger environmental review that can add months or years to the process, and SB 10 lets a local government skip that step for the rezoning.² Individual projects built on the rezoned parcels may still face their own CEQA review depending on scope and potential impacts.

Second, SB 10 lets a local legislative body override voter-approved density caps and land use restrictions, but only with a two-thirds supermajority vote.² A simple majority is not enough to undo what voters put in place.

ADUs on Top of the 10

Each parcel rezoned under SB 10 can also include up to two accessory dwelling units or junior ADUs, and these do not count toward the 10-unit cap.² A single parcel could in theory hold 12 units: 10 primary plus two ADUs.

SB 10 vs. SB 9

SB 9, the California HOME Act, was signed the same day as SB 10 and the two are often confused. They work differently. SB 9 is mandatory statewide and lets a homeowner split a single-family lot in two and build a duplex on each, for up to four units, without any local opt-in. SB 10 is voluntary and permits cities to allow up to 10 units on qualifying parcels. A homeowner in any California city can invoke SB 9 as a matter of right; SB 10 only helps if the local government has chosen to use it.

When the Opt-In Window Closes

Governor Gavin Newsom signed SB 10 on September 16, 2021, as part of a broader housing package.³ It took effect on January 1, 2022. Local governments have until January 1, 2029, to adopt an SB 10 ordinance. Ordinances adopted before that date can remain operative past 2029, so early-adopting cities are not racing a clock on their projects.¹

1
California Legislative Information. California Government Code GOV 65913.5
2
Southern California Association of Governments. SB 10: Local Rezoning for Missing Middle Housing Production
3
Governor of California. Governor Newsom Signs Historic Legislation to Boost California’s Housing Supply and Fight the Housing Crisis