In unincorporated San Mateo County, qualifying multifamily-zoned sites of up to five acres and vacant single-family sites of up to 1.5 acres can use the small-lot subdivision pathway. SB 684 eligibility also depends on urban location, housing history, utilities and the proposed homes. Statutory parcel criteria. Subdivision approval does not itself authorize construction, and coastal requirements remain.

Part of our California housing coverage, this guide addresses requirements operative on September 21, 2026, including SB 1123 and the relevant 2025 amendments in AB 130.

Confirm county jurisdiction first

Start with the assessor’s parcel number, or APN, and the legal description. Ask Planning and Building to confirm that the property is in unincorporated county territory. The county processes development applications in those areas; a property inside an incorporated city follows that city’s implementation of state law. A postal address does not establish jurisdiction. County planning responsibilities, updated August 20, 2026.

That confirmation determines where you obtain zoning information, submit applications and resolve interpretation questions. Keep it in the acquisition file as you compare zoning and land-use pathways.

Screen zoning, acreage and parcel history

SB 1123 expanded the pathway to qualifying vacant single-family land beginning July 1, 2025, as explained in HCD’s April 2026 fact sheet. Apply these thresholds first:

State law permits a local ordinance to establish smaller minimum parcels. Qualifying remainder parcels receive separate treatment, discussed below. Government Code section 66499.41(a)(1)–(3).

The site must be a legal parcel. A parcel previously created through this small-lot statute or an SB 9 urban lot split is excluded. AB 130 expressly includes designated remainder parcels in the prohibition on repeating this pathway. AB 130, section 28.

Document both urban-location tests

An unincorporated site must lie within a qualifying Census urbanized area or urban cluster in a county exceeding 600,000 residents. The statute’s geographic definitions specifically reference Census designations published on March 27, 2012. Ask the county to identify the applicable boundary rather than substituting a current map labeled “urban.” Statutory location requirements.

San Mateo County clears the population threshold: its July 1, 2025 Census estimate was 743,568, from the county estimates released in March 2026. That countywide figure does not establish an individual parcel’s location.

A separate test requires the site to be “substantially surrounded” by qualified urban uses. At least 75% of its perimeter must adjoin parcels developed with qualified urban uses, allowing separation by an improved public right-of-way. The remaining perimeter must adjoin parcels designated for qualified urban uses under a zoning, community or general plan with a certified environmental impact report, with the same right-of-way allowance. Have the project planner document the calculation. Public Resources Code section 21159.25(a)(2), reproduced in the 2025 CEQA handbook.

Verify vacancy and housing history together

For the single-family pathway, an unoccupied house is not necessarily a vacant lot. The statute requires no permanent structure unless it is both abandoned and uninhabitable. Its vacancy definition excludes specified affordability-restricted and rent- or sales-price-controlled housing, and housing occupied by tenants during the five years before application—even if subsequently demolished or vacated. SB 1123’s vacancy definition.

Both pathways must also satisfy section 66499.41(a)(8). A project cannot require demolition or alteration of housing with recorded or legal rent restrictions for low-, very-low- or extremely-low-income households; housing subject to local rent or price control; or housing occupied by tenants within five years before application, including housing subsequently demolished or vacated. The restriction also covers housing on a parcel where an owner exercised Ellis Act rights to withdraw accommodations from rent or lease within 15 years before application. These restrictions are separate from the single-family vacancy definition and can disqualify a multifamily project. AB 130, section 28.

Request permit and demolition records, leases, occupancy documentation and recorded restrictions before relying on a seller’s description. The county’s April 2026 occupancy affidavit, required when existing units would be demolished or altered, addresses five years of rental occupancy and 15 years of Ellis Act withdrawal history, including actions by predecessor owners. Support the signed affidavit with underlying records.

Test the unit count and site plan

The pathway generally caps the subdivision at ten parcels and the project at ten residential units. AB 130 permits a qualifying remainder parcel outside the ten-parcel count: it must retain existing uses or structures, contain no new residential units and not exist exclusively to serve the project. Parcel-count and remainder provisions.

That exception matters when a site contains a house. In its October 7, 2025 guidance, HCD explains that an existing single-family residence can remain on a qualifying remainder without affecting the proposed subdivision’s vacancy, maximum parcel size or density tests. The development area must still meet the other eligibility requirements. Obtain written confirmation of the proposed boundaries and treatment before relying on this approach.

Acreage alone does not establish how many homes qualify. The development must also meet these requirements:

• Housing-element capacity: A site identified in the applicable compliant housing element must deliver at least its projected units, including projected lower-income units where required. For other sites, the minimum-density calculation generally uses 66% of the greater of local maximum zoning density or the applicable statutory density.
• Home size and affordability: Proposed units may average no more than 1,750 net habitable square feet, and applicable local inclusionary housing requirements remain.
• Utilities: Newly created parcels must be served by a public water system and municipal sewer system. Individual wells or septic systems do not satisfy that requirement.

These requirements appear in Government Code section 66499.41(a), as amended by AB 130. Request a written density calculation and service-provider confirmation before pricing the land around ten homes.

Screen environmental constraints at the same stage. The statute excludes specified farmland, wetlands, protected habitat and conservation land. Its fire exclusions distinguish very-high-hazard local responsibility areas from high- or very-high-hazard state responsibility areas. Contaminated sites, mapped fault zones, flood-hazard areas and regulatory floodways have specific statutory exceptions requiring appropriate clearance, compliant design or documentation. Document how each mapped constraint is resolved. Statutory site exclusions.

Separate the map, housing approval and building permit

Each approval addresses a different question:

Qualifying map and housing applications receive ministerial consideration: compliance review without discretionary approval or a hearing, subject to the coastal qualification below. Each statutory 60-day decision period begins when the agency receives the completed application. Those deadlines do not establish a construction start date. Housing approval provisions, map provisions.

The sequence can overlap. Section 65913.4.5 provides for building-permit issuance after tentative-map or parcel-map approval and a complete building-permit application, subject to its conditions. Required improvements must be guaranteed; the county may require security and a recorded agreement withholding occupancy approval until final-map recordation. Map approval alone does not permit construction, but final-map recordation need not always precede building-permit issuance. Building-permit statute.

Check the intended exit strategy as well. Separate sales, leases and financing of resulting parcels face statutory restrictions, exceptions and possible local authorization. An approved map should not be treated as permission for an immediate vacant-lot selloff. Transfer provisions.

Assess coastal approvals before setting a schedule

Small-lot eligibility does not remove coastal permitting requirements. The county’s September 2021 Local Coastal Program requires a Coastal Development Permit for development in the Coastal Zone, subject to exemptions. Its definition of development expressly includes subdivisions and other land divisions, so coastal review can apply before buildings are proposed. Local Coastal Program policies 1.1–1.2.

Before acquisition, request a written coastal determination identifying required permits, any public hearings, applicable resource policies and appeal jurisdiction. Account for that process separately in the schedule and budget. Passing the acreage and vacancy tests does not establish an entirely ministerial approval route or resolve coastal development potential.

Assemble the evidence before committing to a yield

Organize the acquisition file around the eligibility tests:

1. Parcel identity and creation: Recorded deed, subdivision maps, legal description, title report and any certificate of compliance. Establish acreage and whether an earlier statutory split bars another.
2. Planning controls: Written zoning and jurisdiction confirmation, housing-element site-inventory entry, applicable objective standards and evidence for both urban-location tests.
3. Existing conditions: Permit history, occupancy and demolition records, recorded restrictions, hazard documentation and any coastal approvals.
4. A feasible layout: A tentative map and site plan showing proposed homes, any remainder, access, easements and utility connections. Obtain provider responses addressing the proposed demand.

Ask Planning and Building which office holds missing records and request written answers to unresolved eligibility questions. Its August 2026 program description identifies public zoning information and project research among staff functions. Keep those findings in the broader land acquisition due-diligence file.

Request a project-specific fee estimate covering planning, mapping, technical review, utilities and building permits. Before basing a purchase on the proposed yield, establish a documented approval route, a supportable unit count and an infrastructure budget. Have a California land-use attorney and the project’s civil engineer review unresolved eligibility and transfer questions.