Multifamily and Duplex Properties for Sale in San Martin: Zoning and Rent Rules

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What multifamily in San Martin looks like

San Martin has almost no apartment buildings, and County policy is built to keep it that way. Income property here usually means a rural home with an accessory dwelling unit, a property with a second legal residence, or a small number of older multi-unit or mixed-use buildings that predate today’s rules. The County’s General Plan says the community will remain rural, nonurban and residential, so an investor’s realistic options are ADUs, JADUs, SB 9 where it qualifies and legal nonconforming buildings, not new apartment projects. For two to four units, read financing 2-4 unit properties; for larger buildings, see apartment building loans.

Where the zoning allows it

San Martin is unincorporated, so the County Zoning Ordinance and the General Plan’s San Martin policies control.

Designation	Housing allowed	Density and limits
Rural Residential (RR)	Single-family homes with agriculture and open space; ADUs	5 to 20 acre variable slope-density formula; land under 10% slope may be zoned RR-5 acres
Clustered rural lots	Homes grouped with protected open space	1-acre minimum parcel when open space is permanently protected
Other General Plan designations	Per the base County designation	Density follows that designation
-sm Commercial Use Permit Area	Nonresidential uses; housing per base district	Use permit and architecture and site approval
Master plan requirement	No major density increase	A land use and infrastructure master plan must come first

The County’s urban R2 and R3 districts, which allow duplexes and apartments elsewhere, are aimed at pockets inside city urban service areas, not at rural San Martin. A new building of three or more units is therefore unlikely without a General Plan change and the master plan the County requires before any significant increase in intensity.

ADUs and SB 9. County ADU and SB 9 rules come from a 2023 ordinance. In a June 24, 2025 letter, the state housing department found parts of it inconsistent with state law, including a 180-day minimum rental term for SB 9 units where state law allows 30, ADU height limits, a ban on separate ADU sales and the handling of agricultural and hillside zones. Check whether the County has amended the ordinance before relying on a rule, and confirm whether a rural San Martin parcel qualifies for SB 9 at all. See our ADU investment guide and SB 9 lot split guide.

Rent rules: state law plus County tenant rules

The Attorney General’s rent cap page, updated July 16, 2026, places Santa Clara County in All Other Counties, with covered units allowed up to 8.6% for increases from August 1, 2026 through July 31, 2027. Only San Jose and Mountain View appear in its chart of local ordinances in this county. Covered San Martin rentals follow AB 1482’s cap and just-cause rules; many single-family homes with ADUs may qualify for the individually owned home exemption if the required notice is given.

County rules. County Code Division C5 already requires relocation help when tenants must leave unsafe or red-tagged units, according to a County tenant protection presentation. That 2025 proposal would extend protections to single-family homes and newer buildings with no-fault relocation of three months of Fair Market Rent. Local press reported Board approval on June 2, 2026; we did not find the adopted text on a County site, so confirm final terms. Read our AB 1482 guide and just cause eviction guide.

San Martin rental property rules

No city layer. There is no city rental registry, business tax or transfer tax; County rules apply directly.
Transfer tax. Santa Clara County’s documentary transfer tax is $0.55 per $500 of value.
Community review. The Board-appointed San Martin Planning Advisory Committee advises on local land use applications, including second units that need discretionary review.
Airport. Changes to South County Airport policies or facilities need prior review with residents and owners; check airport-area parcels for added limits.

Septic, groundwater and flood limits

Septic capacity, not zoning, is usually what caps added units in San Martin.

Sewer. Private projects cannot connect to a sewer until an area-wide infrastructure plan is approved.
Llagas Creek. No new septic systems are allowed in high-permeability soils near Llagas Creek, and flood hazard zones restrict activities that add septic or nitrate loads.
Groundwater studies. Land division density is tied to the 1981 San Martin Area Water Quality Study and later groundwater work.
Wells. Many properties rely on private or small water systems; confirm water supply for any added unit.

Older rural buildings and converted outbuildings should also be checked for permits and structural condition; see our seismic retrofit guide.

Underwriting a San Martin multifamily purchase

Confirm the County zoning and General Plan designation, and that every existing unit is permitted.
Get septic permits, capacity and any repair history; size additions to what the system and soils allow.
Decide whether AB 1482 applies, and give the exemption notice if the property qualifies.
Check the current County ADU and SB 9 rules, including the rental term for SB 9 units.
Collect leases and estoppel certificates for any tenants.
Rebuild expenses with reassessed taxes, well and septic costs in our rental property analysis.

Questions people ask

Can I build apartments in San Martin?

Generally not. County policy keeps San Martin rural, and a master plan is required before significant density increases.

Can I add an ADU to a San Martin home?

Usually, under County ADU rules, if septic and water allow. Check for amendments after the state’s 2025 findings.

Does SB 9 apply in San Martin?

Possibly on some parcels; confirm eligibility, since state law excludes certain rural and agricultural land.

Is there rent control in San Martin?

No. AB 1482 applies to covered units at up to 8.6% from August 1, 2026, plus County relocation rules.

Can I connect to sewer?

Not until the County approves an area-wide infrastructure plan.

What limits septic near Llagas Creek?

No new systems are allowed in high-permeability soils there.

Sources & notes

Note: The HCD letter and County tenant presentation were reviewed by our team for the County page; the June 2, 2026 Board vote is from local news, not a County source. The well-water point is general practice. We did not review the zoning map or mobile home park inventory.

General information, not legal or tax advice. Last reviewed October 2026 by Matthew Martinez.

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