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

What multifamily in San Carlos looks like

San Carlos income property ranges from duplexes and ADUs on single-family lots to older garden apartments and condominium buildings in the RM districts near El Camino Real and the downtown, and newer mid-rise apartments in the mixed-use districts along El Camino, San Carlos Avenue and around the Caltrain station. The city’s zoning names each multifamily and mixed-use district by its maximum density, and its RM districts also set minimum densities, so a small project on a large RM lot may not be approvable. For two to four units, read financing 2-4 unit properties; for larger buildings, see apartment building loans.

Where the zoning allows it

Chapter 18.04, with its use table last amended by Ordinance 1619 in 2024, sets these residential districts.

Minimum densities. RM-59 requires at least 45 units an acre and RM-100 at least 75, and duplexes are not permitted in either. An existing duplex or small building in those districts may be nonconforming as to density; check what can be rebuilt after a loss.

Mixed-use options. The downtown core and edges allow 100 or 120 units an acre, the station area around the historic train depot 50, El Camino Real’s North and South Boulevard districts up to 120, and the neighborhood transition districts 40 to 120. Single room occupancy housing generally needs a conditional use permit.

ADUs, JADUs and urban infill units. ADUs are permitted in all five residential districts under Section 18.23.210, while JADUs are permitted only in RS-3 and RS-6. The RS districts also allow "urban infill units" under the state SB 9 statutes, Government Code Sections 65852.21 and 66411.7. See our ADU investment guide and SB 9 lot split guide.

SB 79 near Caltrain. Under Chapter 18.25A, adopted by Ordinance 1634 in 2026, parcels within a quarter mile of the station zoned RM-100, MU-DC-100, MU-D-120 or MU-SC-120 are temporarily excluded from the state’s SB 79 standards, because the city finds they already allow at least half the required intensity. Those sites follow local zoning until one year after the next housing element is adopted.

Rent rules: state law only

The Attorney General’s rent cap page, updated July 16, 2026, places San Mateo County in the San Francisco Area, with covered units allowed up to 8.8% for increases from August 1, 2026 through July 31, 2027. Its chart lists East Palo Alto as the only San Mateo County city with local rent stabilization, and the San Carlos code’s table of contents shows no tenant protection chapter. San Carlos rentals therefore follow AB 1482’s cap and just-cause rules for covered units, with the usual exemptions for buildings under 15 years old and qualifying individually owned homes. See our AB 1482 guide and just cause eviction guide.

San Carlos rental property rules

• Transfer tax. The County’s documentary transfer tax is $0.55 per $500. San Carlos’s Chapter 3.20, dating to 1967, sets 27.5 cents per $500 under the state enabling law, which typically shares the County rate; confirm the total in escrow.
• Short-term rentals. We did not find a stand-alone short-term rental chapter. The city’s transient occupancy tax is in Chapter 3.24 and Chapter 18.23 sets standards for specific uses, but we did not confirm how either treats home rentals; confirm with the city before listing any unit nightly.
• Affordable housing. Chapters 18.16 and 18.17 set the city’s affordable housing programs and incentives; new projects may carry affordable unit or fee obligations, so ask Planning early.
• Industrial edge. Housing is not listed as a use in the industrial districts east of the tracks, so do not underwrite a residential conversion there unless the Northeast Area Specific Plan allows it.

Hillsides, the airport and older buildings

The RS-3 lot coverage figure drops to 25% in the Hillside overlay, a sign of the steeper western neighborhoods where grading and access limit added units. Near San Carlos Airport, the city’s Airport district caps heights at 50 feet and defers to the County’s airport plan, which can affect nearby residential sites. The city’s older wood-frame apartments merit a seismic retrofit review, and buildings of three or more units with elevated balconies need SB 721 balcony inspections.

Underwriting a San Carlos multifamily purchase

1. Confirm the district and whether minimum density makes the existing building nonconforming.
2. Classify units under AB 1482 and model increases at or below 8.8% for covered units.
3. For a redevelopment near Caltrain, confirm whether Chapter 18.25A or SB 79 applies to the site.
4. In RS districts, check JADU, ADU and urban infill unit potential, and any Hillside overlay limits.
5. Collect leases, the rent roll and estoppel certificates.
6. Rebuild expenses with reassessed taxes, SB 721 and seismic costs in our rental property analysis.

Questions people ask

Sources & notes

1. San Carlos Chapter 18.04, Residential Districts (GoCodebook)
2. Chapter 18.05, Mixed-Use Districts
3. Chapter 18.25A, SB 79 applicability
4. Chapter 18.07, Industrial Districts
5. Chapter 18.09, Airport District
6. Chapter 3.20, Real Property Transfer Tax
7. San Mateo County Clerk-Recorder, documentary transfer tax
8. California Attorney General, rent caps

Note: We did not review Section 18.23.210 (ADUs), the RM development standards beyond the table, the affordable housing chapters, Chapter 18.23’s other use standards or any short-term rental rules. Mixed-use heights were not reviewed.

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

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