Lot size is one of the first SB9 questions, but it is not only a square-footage question. A practical SB9 lot split has to satisfy the 40 percent rule, the minimum parcel-size rule, and the physical realities of the property.
What homeowners need to know first.
For an SB9 urban lot split, California Government Code Section 66411.7 generally requires the split to create no more than two new parcels of approximately equal lot area, with one parcel not smaller than 40 percent of the original lot area. Both newly created parcels generally must also be at least 1,200 square feet unless the local agency has adopted a smaller minimum. That is the starting point, not the whole feasibility answer.
The practical details matter more than the headline.
40/60 split limit
The law does not require a perfect 50/50 split, but it generally prevents carving off a tiny leftover parcel. A common practical range is 40/60 to 50/50.
1,200 square feet
Both newly created parcels generally need to be at least 1,200 square feet unless a local ordinance allows a smaller minimum lot size.
Usable lot shape
A mathematically compliant split can still fail if the new parcels are awkward, narrow, landlocked, or hard to build on.
Access and frontage
Each parcel needs practical access, and agencies may require access to or adjacency with a public right-of-way.
Utilities and drainage
Water, sewer, power, stormwater, easements, grading, and service connections can change whether the split is practical.
Local objective standards
Cities can apply objective standards, but those standards cannot physically preclude qualifying SB9 outcomes where the statute protects them.
Riechers reviews the real property, not just the idea.
The SB9 40 percent rule is a parcel-map rule, not a design slogan. It asks whether the proposed urban lot split creates two new parcels that are close enough in size to qualify under the statute. In practical terms, the smaller resulting parcel generally cannot be less than 40 percent of the original parcel area.
That means SB9 usually does not allow a homeowner to split off a very small side yard and call it a separate buildable lot. If the original parcel is 10,000 square feet, a split that creates one 2,000-square-foot parcel and one 8,000-square-foot parcel would usually be a problem under the 40 percent rule. A 4,000-square-foot and 6,000-square-foot split is the kind of ratio the rule is designed around, assuming the rest of the requirements also work.
The 1,200-square-foot minimum is the second lot-size checkpoint. Government Code Section 66411.7 generally says both newly created parcels must be at least 1,200 square feet, unless the local agency has adopted a smaller minimum lot size by ordinance. A parcel can satisfy the 40 percent relationship and still fail if one new lot is below the applicable minimum.
Square footage is only the first screen. The stronger question is whether each resulting parcel can function as a real property. Shape, frontage, driveway access, fire access, utility routing, drainage, slope, trees, easements, existing structures, setbacks, and title conditions can all matter once the split moves from an idea to a map.
Two properties with similar lot areas can have very different SB9 outcomes. A rectangular corner lot with clean access may be easier to split than a larger flag-shaped lot with awkward frontage, steep grade, utility conflicts, or an existing home sitting exactly where the new parcel line would need to go.
Local objective standards also matter. HCD guidance recognizes that jurisdictions may apply objective zoning, subdivision, and design standards, but those standards cannot be used in ways that conflict with SB9 or physically preclude protected SB9 outcomes. The local review still has to be checked for the specific city or county.
This is why Riechers treats the 40 percent rule as an eligibility checkpoint, not a complete answer. A lot can pass the math and still need careful engineering review before the owner spends serious money on architectural plans, lender conversations, or a sale strategy.
The right next step is to test the property address against the legal rule, the local standards, and the physical site constraints together. That gives you a practical SB9 answer instead of a guess based on acreage alone.
Keep building the full picture.
State rules and local review both matter.
SB9 Lot Size Requirements and the 40 Percent Rule FAQs
What is the SB9 40 percent rule?
For an SB9 urban lot split, the parcel map generally must create no more than two new parcels of approximately equal lot area, and one parcel generally cannot be smaller than 40 percent of the original parcel proposed for subdivision.
Does SB9 require a 50/50 lot split?
No. SB9 generally allows an approximately equal split. The practical limit is that one resulting parcel generally cannot be smaller than 40 percent of the original parcel area, so a 40/60 relationship may work where the rest of the requirements also work.
What is the minimum lot size for an SB9 split?
Both newly created parcels generally must be at least 1,200 square feet unless the local agency has adopted a smaller minimum lot size by ordinance.
Is total lot size enough to know if my property qualifies?
No. Lot size is only one checkpoint. Access, frontage, utilities, drainage, easements, existing structures, setbacks, environmental constraints, title, lender issues, and local objective standards can all affect feasibility.
Can my city use local standards to stop an SB9 split?
A local agency may apply objective standards, but those standards cannot conflict with SB9 or physically preclude protected SB9 outcomes. The exact answer depends on the local standards and the parcel.
Why can two similar-size lots have different SB9 answers?
Parcel shape, street frontage, existing building placement, slope, easements, utility routing, drainage, fire access, and local standards can make two similar-size lots perform very differently.
Should I check the 40 percent rule before starting design?
Yes. The 40 percent rule, minimum lot size, and physical site constraints should be checked before spending heavily on design, mapping, lender strategy, or sale planning.
Find out what your lot can actually do.
Bring the property address and your goal. Riechers Engineering can help you understand whether SB9 is worth a closer look before you spend serious money.
Check SB9 Eligibility