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

What multifamily in San Mateo County looks like

Most apartments in the county are in its cities, from mid-century buildings in San Mateo, Burlingame and Daly City to newer complexes in Redwood City and South San Francisco. The unincorporated County holds a meaningful share of older small buildings too: duplexes, fourplexes and small apartment buildings in North Fair Oaks, Broadmoor and unincorporated Colma, cottages and second units on the Midcoast, and seven mobile home parks. For two to four units, read financing 2-4 unit properties; for five or more, see apartment building loans.

Where the zoning allows it

Inside the twenty cities and towns, local codes apply; see our pages for San Mateo, Redwood City, Daly City, East Palo Alto and South San Francisco. In unincorporated areas, Title 8 of the County Code applies, as described on the County’s zoning regulations page.

The R-3 chapter allows multiple-family dwellings, dwelling groups and dormitories, with hotels and clinics by use permit. Unit density comes from S-numbered residential density combining districts listed in Chapter 8.08, including neighborhood-specific ones for North Fair Oaks, Devonshire, Ladera and the Midcoast. The new districts page maps the 2026 rezonings.

ADUs. The County’s ADU page puts inland ADU rules in Chapter 8.392 and Coastal Zone rules in Chapter 8.396, allows ADUs in every Coastal Zone residential district, sets four-foot side and rear setbacks, and imposes no owner occupancy for ADUs, though JADUs without their own sanitation need a recorded owner-occupancy restriction. See our ADU investment guide and SB 9 lot split guide.

Rent rules: state law, County mobile home rent control and city ordinances

The Attorney General’s rent cap page, updated July 16, 2026, places San Mateo County in the San Francisco Area: 6.3% for increases from August 1, 2025 and 8.8% for increases from August 1, 2026 through July 31, 2027. Its chart of local rent ordinances lists East Palo Alto as the only San Mateo County city with local rent stabilization; see our East Palo Alto page for that city’s rules.

Mobile home rent control. The County has regulated space rents in unincorporated parks since 2003. A September 2017 amendment limits increases to once a year at the lesser of 75% of the Bay Area CPI change or 5%, extended coverage to motor homes and trailers on park spaces, and added change-of-use permits and relocation assistance before a park closes. It covers seven unincorporated parks with more than 750 spaces.

For apartments and duplexes in unincorporated areas, AB 1482 applies to covered units. Read our AB 1482 guide and just cause eviction guide, and check the city page for any city building.

San Mateo County rental property rules

Short-term rentals. In the Coastal Zone, the R-3 chapter allows short-term rentals only with a permit under the County’s coastal zoning rules. Operators in unincorporated areas register for transient occupancy tax through the County’s contractor, HdL.

The urban Midcoast unit cap. LCP Policy 1.23 limits the urban Midcoast to 40 new housing unit building permits a year, ADUs included. Because 102 units were permitted in 2025, the County says only 18 residential permits can be issued across 2026 and 2027. It will not accept new residential building permit applications until January 1, 2028 or until an LCP amendment providing relief takes effect, and approved projects go on a waitlist. The Coastal Commission has scheduled a November 4, 2026 hearing on amendment LCP-2-SMC-26-0048-1; the County does not state the outcome. A Montara or El Granada buyer counting on adding an ADU should treat that income as uncertain.

Coastal, geologic and older-building issues

Coastside parcels carry the Coastal Development combining district, and some areas carry the Geologic Hazards district, both listed in Chapter 8.08. The County’s Planning page also tracks flood hazard information and a Seal Cove page for the Moss Beach area. Older bayside apartment buildings with elevated decks need SB 721 balcony inspections at three or more units, and wood-frame buildings over garages merit a seismic retrofit review.

Underwriting a San Mateo County multifamily purchase

1. Confirm city or County, and for County parcels the base district plus any S density district.
2. In East Palo Alto, apply local rent stabilization; elsewhere model AB 1482 increases at or below 8.8%.
3. For a mobile home park, model space rents under the County’s CPI-based limit.
4. On the urban Midcoast, do not count new ADU income until a permit can issue under the cap.
5. Collect leases and obtain estoppel certificates.
6. Rebuild expenses with reassessed taxes in our rental property analysis.

Questions people ask

Sources & notes

1. San Mateo County Planning, zoning regulations
2. Title 8, Chapter 8.08 districts
3. Title 8, Chapter 8.28 R-3
4. San Mateo County Planning, new zoning districts
5. San Mateo County Planning, ADUs
6. San Mateo County, mobile home tenant protections (2017)
7. San Mateo County, urban Midcoast building permit cap
8. San Mateo County, transient occupancy tax
9. San Mateo County Planning and Building
10. California Attorney General, rent caps (updated July 16, 2026)

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

Related: San Mateo County real estate guide · Commercial real estate in San Mateo County · Multifamily investing guide · Investor services