The Promise
A few years ago, California passed a pair of laws that were supposed to fundamentally change what's possible on a typical single-family lot. SB 9 would let homeowners split their lot in two and build a duplex on each half, theoretically turning one house into four homes on the same land. SB 10 would let cities opt in to allow up to ten units on parcels near transit and job centers, without the years-long environmental review that has historically made denser infill projects slow and expensive to pursue. Housing advocates called it the biggest change to single-family zoning in California history. It's worth asking, now that these laws have been in effect for a few years, whether that promise has actually materialized.
What SB 9 and SB 10 Actually Do
The two laws work quite differently, which matters for understanding why their results have looked so different. SB 9 applies statewide and is mandatory: a qualifying homeowner has a legal right to split their lot and build up to two units on each resulting parcel, four units total where one single-family home used to stand, and cities must approve a qualifying application through a ministerial process rather than a discretionary hearing where neighbors can show up and object. It's a homeowner-initiated tool, available to essentially anyone who owns a qualifying single-family lot.
SB 10 works the opposite way. It doesn't give homeowners any new rights directly. Instead, it gives cities the option to rezone parcels near transit and job-rich areas for up to ten units, exempt from full CEQA environmental review if they choose to adopt it locally. That's a meaningful difference: SB 9's impact depends on how many individual homeowners decide to use it, while SB 10's impact depends entirely on whether individual city councils decide to adopt it in the first place, which many haven't.
The Uptake Problem
Here's the honest assessment, and it's less dramatic than either supporters or critics predicted. SB 9 in particular has produced far fewer lot splits and new units than early projections suggested when the law passed. The zoning barrier that used to make this kind of small-scale infill illegal or nearly impossible to permit has genuinely been removed in most of the state. But removing a legal barrier and making a project financially and practically viable turn out to be two very different things.
The reasons are largely the same ones that make replacing an old apartment building difficult, covered elsewhere on this site: construction costs, financing complexity, and the sheer difficulty of a homeowner without development experience navigating a lot split, new utility connections, and construction financing, all for a project that, unlike a straightforward renovation, requires real technical and financial sophistication to execute well. A right to build isn't the same as an easy or obviously profitable path to building, and a meaningful share of eligible homeowners who looked into it concluded the juice wasn't worth the squeeze.
Why Zoning Reform Alone Wasn't Enough
This is the core lesson from SB 9 and SB 10's first several years in effect: zoning was a real barrier, but it wasn't the only one, and in a lot of cases it wasn't even the primary one. Construction costs have to actually support the economics of a smaller project. Financing has to be available and reasonably priced for what is, from a lender's perspective, a fairly unusual and higher-risk type of project compared to a conventional home purchase or a large, professionally developed apartment building. And homeowners have to actually want to take on the complexity, cost, and risk of becoming a small-scale developer on their own property, which is a bigger ask than simply being legally allowed to do it.
State lawmakers seem to have absorbed this lesson. The 2025 legislative session included measures aimed more directly at the process and cost barriers that zoning reform alone didn't fix, including an infill exemption from full environmental review for qualifying housing projects and continued work on transit-oriented housing legislation aimed at unlocking density near major transit investments. The throughline across this newer wave of bills is a recognition that housing production requires clearing barriers at every stage, zoning, environmental review, permitting timelines, and construction cost, not just the first one.
How an SB 9 Lot Split Actually Works
For homeowners genuinely considering it, the process generally follows a consistent sequence. First, confirm eligibility: the lot needs to meet minimum size requirements after the split, generally around 1,200 square feet per resulting parcel depending on local rules, and can't be in certain protected zones like high fire-hazard areas, wetlands, or historic districts without additional review. Next comes a survey and the legal lot-split application itself, which, because it's a ministerial process, doesn't require a public hearing, though it still requires accurate paperwork and, often, a civil engineer or land use professional to prepare it correctly.
Once the split is approved, each resulting parcel can be developed with up to two units, subject to standard building permits and objective design standards the city applies uniformly rather than case-by-case discretion. That means running new utility connections to parcels that previously shared one connection, which is often one of the more expensive and time-consuming parts of the entire project. Financing typically requires either significant cash, a construction loan, or a partnership with a developer or investor, since most homeowners don't have the liquidity to fund a multi-unit construction project from savings alone. All told, from initial application to finished, occupiable units, the timeline typically runs well over a year, and often closer to two, once financing and construction are factored in realistically.
What This Means for San Diego Specifically
San Diego has seen some SB 9 activity, lot splits and small-scale duplex and fourplex projects scattered across the city, but nothing close to the scale that would meaningfully move the needle on the region's 171,685-unit state housing mandate on its own. Where these tools have had more visible impact is in combination with local policy, San Diego's own complete communities and density bonus programs, rather than as a freestanding solution. SB 10 adoption has been more limited locally, since it requires an affirmative local decision to opt in, and San Diego's use of it has been more targeted than sweeping.
The practical reality is that missing-middle housing, duplexes, fourplexes, small apartment buildings, is being built in San Diego today, but mostly through a combination of these state tools, local zoning changes, and the ADU boom covered elsewhere, rather than through any single law functioning as the silver bullet it was sometimes described as when it passed.
Is a Typical Homeowner Actually Going to Do This?
For most single-family homeowners, splitting a lot and building a duplex on each half remains a significant undertaking, closer in complexity to a small development project than a home renovation. It generally makes the most sense for homeowners on larger-than-typical lots in areas with strong rental or resale demand, who either have development experience themselves or are willing to partner with someone who does. For a homeowner simply looking to add space or rental income with less complexity, an ADU remains the more accessible tool, covered in more detail elsewhere on this site.
The Investor Angle
Where SB 9 and SB 10 have arguably had their most concrete impact is among investors and small developers who specialize in exactly this kind of project, identifying underbuilt single-family lots in strong locations, executing the lot split or entitlement work, and either building and selling the resulting units or holding them as rentals. For a buyer with the capital and expertise to execute well, the legal pathway these laws created is real and usable, even if it hasn't turned into the mass homeowner movement some early advocates envisioned.
The Takeaway
SB 9 and SB 10 didn't fail, but they also didn't deliver the transformation some of their loudest supporters predicted. What they did was remove a real legal barrier and shift the conversation toward the next set of barriers, cost, financing, and execution complexity, that state lawmakers are now working through in subsequent legislation. For San Diego's housing shortage, that's a meaningful but partial answer: another tool in a toolkit that still needs several more pieces before it adds up to enough new housing to meet the region's actual demand.