California's Duplex Law Promised Homes on Every Lot and Delivered Few

In mid-September 2021, a few days after beating back a recall election, Governor Gavin Newsom signed a pair of housing bills that had been fought over in Sacramento for years. The more famous of them, Senate Bill 9, carried by Senate President pro Tem Toni Atkins, did something that would have been unthinkable in California a decade earlier: on most lots zoned for single-family homes across the state, a homeowner could now build a second unit, or split the lot in two and put up to two homes on each half. Supporters described it as the end of single-family-only zoning in the nation's largest state. Opponents warned of neighborhoods being bulldozed for fourplexes. Then the law took effect on January 1, 2022, and something much quieter happened. Almost nothing.

In short: SB 9 legalized duplexes and lot splits on most single-family lots in California, but in its first years it produced only a small number of new homes. Construction costs, owner-occupancy rules, local resistance and the difficulty of financing small projects held it back, while the state's earlier backyard-cottage laws quietly produced far more. California has since tightened the law and passed others, and the lesson for every state is that legalizing a building is only the first step.

What the law actually allowed

SB 9 came in two parts, and the details explain much of what came after.

The first part let an owner build up to two homes on a single-family lot, by right, meaning a city could not demand a discretionary public hearing if the project met objective standards. The second part let an owner split a lot into two, as long as each new lot met certain minimum sizes, and then build up to two homes on each. Combine the two with California's separate rules on accessory dwelling units, and supporters argued a single lot could, in principle, hold several homes.

There were limits. The law did not apply in historic districts, on certain environmentally sensitive land, or where the project would require demolishing housing occupied by tenants in recent years or rent-controlled housing. And for the lot-split option, the owner had to sign an affidavit promising to live in one of the homes for at least three years, a provision added to answer fears that investors would buy up houses and carve them up.

Why so few people used it

In the law's first years, researchers at the Terner Center for Housing Innovation at UC Berkeley surveyed cities and found that applications under SB 9 were scarce in most places and that actual completed homes were rarer still. A small number of cities accounted for much of the activity. Statewide, the numbers were modest compared with California's housing shortage, which state officials measure in the millions of homes.

The reasons fell into a few buckets.

The math. In most of California, land is extraordinarily expensive and construction is costly. Building a second home on an existing lot means paying for design, permits, utility connections, fees and construction at small-project prices. For many homeowners, the cost of a new detached home simply exceeded what it could rent or sell for, especially once interest rates rose sharply in 2022 and 2023.

The owner-occupancy rule. Lot splits required the owner to live on the property, which shut out the small builders who might otherwise have made SB 9 projects a line of business. Individual homeowners, meanwhile, rarely have the experience or appetite to act as developers.

The financing. Banks are set up to lend on a single-family house or on an apartment building. A homeowner who wants to split a lot and build a second house in the backyard falls between categories, and lenders were slow to develop products for it.

The local rules. SB 9 allowed cities to apply objective standards, such as setbacks, parking near limited transit and design rules, as long as they did not physically preclude two units of at least 800 square feet. Some cities wrote standards that pushed right up against that line, making projects harder and costlier.

The cities that fought back

Some local resistance was not subtle. In early 2022, the town of Woodside, a wealthy community on the Peninsula, declared that the entire town was potential habitat for mountain lions and therefore exempt from SB 9. The state attorney general, Rob Bonta, warned the town that this violated the law, and Woodside backed down within days. The episode became a national symbol of what housing advocates call NIMBY creativity.

Pasadena adopted an urgency ordinance expanding its landmark districts, which drew a similar warning from the state. A number of charter cities, including several in Los Angeles County, sued to argue that SB 9 could not override their local authority, and in 2024 a Los Angeles County judge ruled in their favor on the law as written, a decision that set off further legal and legislative maneuvering. Huntington Beach, which has fought the state over housing on several fronts, also challenged state housing mandates in court.

Not all local hesitation was bad faith. Some cities worried about wildfire risk, water supply or sewer capacity, and some residents feared that tall new buildings next door would cast shadows or crowd out trees. But the most theatrical refusals made it harder to take the sincere concerns seriously, and they helped persuade lawmakers that the state needed stronger tools.

The law that did work: backyard cottages

The most useful comparison is with a different set of California laws. Starting in 2016 and continuing through a series of bills, the Legislature made it far easier to build accessory dwelling units, the backyard cottages and garage conversions often called ADUs or granny flats. It cut parking requirements, capped fees, required cities to approve compliant projects quickly and limited the ability of homeowners associations to block them.

The effect was dramatic. ADU permits in California grew from a few thousand a year to tens of thousands, making them a meaningful share of all new housing in the state. Small companies sprang up to design and build them. Lenders created products for them. Homeowners built them to house aging parents, adult children or tenants.

Why did ADUs take off when SB 9 did not? ADUs were cheaper, since they can be small and can reuse a garage. They did not require splitting the lot or recording a new parcel. The state kept fixing the ADU rules year after year as cities found ways around them, building up a body of law that pre-empted local obstacles one by one. And over several years, an industry formed around a standardized product. SB 9, by comparison, was asked to do something more complicated with fewer follow-up fixes in its early years.

What California did next

The Legislature did not leave SB 9 alone. In 2023, it passed SB 684, which created a streamlined path for building up to ten homes on small lots in multifamily and some other zones. In 2024 it passed SB 450, which tightened SB 9 itself by limiting the local standards cities could apply to these projects and requiring faster review. The state has also increasingly used its housing element law, which requires each city to plan for its share of the region's housing needs, to pressure local governments, with consequences for cities that fall out of compliance.

Each of these steps responded to a specific way the earlier law fell short. The pattern is a familiar one in California housing policy: pass a reform, watch cities find the gaps, pass another reform. It is slow, and it is also how ADU policy eventually succeeded.

The blue-state problem, stated honestly

The opposing case is often put this way: California, governed by Democrats for years, has some of the highest housing costs and the largest homeless population in the country, so progressive governance must be the cause. Critics from the right make this argument often, and in a period when blue states position themselves as the counterweight to the Trump administration, it carries political weight.

The fair answer has two parts. First, it is true, and progressives should say so plainly, that many of the most restrictive local land-use rules in America are in blue states and blue cities. Strong local control, layered environmental review and powerful homeowner groups made building hard, and much of that was defended in the language of progressive values. SB 9 and the ADU laws exist because a new generation of Democrats decided that pattern had to change.

Second, California is now among the states doing the most to reform its rules, often over the objections of its own cities. Many Republican-led states have followed with zoning reforms of their own, and the housing shortage is national. The question is not which party has a perfect record; neither does. It is who is willing to change course. California is changing, unevenly, slowly and with plenty of internal fights.

What other states can learn

For states considering their own versions, the SB 9 story offers practical guidance:

1. Legalize, then make it buildable. Allowing a duplex on paper does little if setbacks, height limits, lot-size rules and fees make it impossible on most real lots.
2. Let small builders in. Owner-occupancy requirements soothe neighbors but shrink the pool of people who can do the work. States that want production should think hard before adding them.
3. Watch financing. Work with lenders, state housing finance agencies and credit unions on loan products for small infill projects.
4. Plan for the next bill. Expect cities to test the edges, and keep a follow-up law ready, as California did for ADUs.
5. Pair supply with protections. Exclusions for tenant-occupied and rent-controlled housing help ensure new homes add to the stock rather than displace the people who live there now.

A homeowner's view of the paperwork

Picture an ordinary SB 9 candidate: a retired couple in a Sacramento suburb with a deep backyard who would like to build a small house for their daughter's family. Under the law, they can. Here is what they face.

They need a survey and probably a soils report. If they want to split the lot so their daughter can own the new house, they must apply for a parcel map, and the city will check that each new lot meets size requirements and has access to the street. They need an architect or designer, plans that meet the city's objective standards, and separate utility connections, which can mean new water and sewer lines and fees from agencies other than the city. They will pay impact fees, though the state has limited some fees for smaller homes. They need a construction loan, which their bank may not offer for this kind of project, or they need savings. And they must sign the affidavit promising to live on the property for three years if they split the lot.

None of those steps is unreasonable on its own. Together they amount to a part-time job lasting a year or more, with tens of thousands of dollars spent before construction begins and no guarantee of the final cost. Many couples who start down this road stop, and choose an ADU instead, which can be permitted with fewer steps.

That is the core reason the law underdelivered. It legalized a project that was still too complicated for most homeowners to take on and too small for most builders to bother with.

What the shortage actually requires

It is worth keeping the scale in view. California's state housing agency has set goals in the millions of new homes for the coming years, far above the pace the state has built in recent decades. No single law, least of all one aimed at backyards, was ever going to meet that goal by itself. The bulk of new homes will have to come from apartments on commercial corridors, near transit and on underused land such as old shopping centers and parking lots, alongside public investment in affordable housing.

Seen that way, SB 9 was less a production tool than a statement of principle: single-family zoning would no longer be untouchable. As a principle, it mattered. As a production tool, it was one modest piece of a much larger puzzle.

That is not a reason for cynicism. It is a reason to judge housing laws by the homes they produce, measured honestly, and to keep adjusting until the numbers move.

What you can do

• See what your state allows. If you live in California, your city's planning department should explain how it handles SB 9 and ADU applications. Elsewhere, look up whether your state has passed similar laws. Keep up with how blue states are handling housing on the blue states vs. Trump topic page.
• Press your legislators on the follow-up fixes. Housing laws rarely work perfectly the first time. Find your state legislators, with their phone numbers and recent votes, and ask whether they support the next round of fixes.
• Back the housing groups in your state. Coalitions of tenant groups, homebuilders and environmental organizations have pushed most of these reforms. Look them up in the organizations directory and see which fit your values.
• Speak up for your neighbor's cottage. When someone on your street applies to build a backyard home or a duplex, a friendly word at the hearing, or simply not filing an objection, matters more than most people think.