The short version
ADU law lets you add one or more secondary units to your existing lot — you keep one property, with extra housing on it. SB 9 (the California HOME Act, effective 2022) is a different tool entirely: it lets qualifying single-family lots be split into two separate parcels, with each resulting parcel potentially supporting up to a duplex. They're not competing options for the same project — they solve different problems, and in some cases can even be combined.
Side-by-side comparison
| Factor | ADU Law | SB 9 |
|---|---|---|
| What it does | Adds a secondary unit to your existing lot | Splits one lot into two, each potentially holding a duplex |
| Does your lot get subdivided? | No — you keep one property | Yes — that's the core mechanism |
| Maximum units created | Up to 3 additional units (JADU + conversion + detached), see our unit-stacking guide | Up to 4 total units across the two new parcels (2 per parcel) |
| Owner-occupancy requirement | Generally not required for a standard ADU | Generally requires the owner to occupy one of the units for a period after approval |
| Review type | Ministerial, no discretionary hearing | Ministerial, no discretionary hearing |
| Where it doesn't apply | Broadly available statewide with few exceptions | Excluded in historic districts, high fire-risk areas, wetlands, and several other protected/sensitive categories |
Why homeowners mix these up
Both laws share a lot of DNA: both were designed to add housing capacity in single-family zones without requiring a full rezone or discretionary approval process, and both are processed ministerially by the city. Media coverage sometimes lumps them together as "California's new housing laws," which blurs the very different mechanics underneath.
Can you use both on the same property?
In some cases, yes — a homeowner could pursue an SB 9 lot split and still add ADUs to the resulting parcels, subject to each parcel's own zoning and lot-size math. This is a more complex scenario than either path alone, and worth discussing directly with a land use attorney or your city's planning department if you're considering it.
Which one is actually relevant to you
If your goal is rental income, housing for family, or added value without splitting your property, ADU law is almost certainly the right framework — it's also what United Build Partners specializes in helping homeowners navigate. If your goal is closer to subdividing your lot and selling or building on a separate parcel, SB 9 is a different, more involved process that a real estate attorney and land use professional are better positioned to guide you through.
Frequently asked questions
No — they're separate statutes that can apply independently or, in some cases, together on the same property. SB 9 addresses lot splits and duplexes; ADU law addresses secondary units on an existing lot.
Our focus is specifically ADU advisory and contractor matching — SB 9 lot splits involve real estate and land use considerations beyond that scope. A real estate attorney or land use consultant is the better resource for an SB 9 project specifically.
No — it excludes historic districts, high fire-hazard severity zones, wetlands, and several other protected or sensitive categories, and carries its own eligibility requirements. Confirm your specific property's eligibility with your city's planning department.