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

What multifamily in San Geronimo looks like

Multifamily in San Geronimo is mostly small: homes with second units, a few duplexes and cottage groups, and units in village buildings. The 1997 community plan counted, from the 1990 Census, 1,394 occupied homes across the valley. That included 1,247 single-family homes, 103 multi-unit homes and 44 mobile homes or other types. The plan also noted that many second units had been built without permits. Planned developments shaped the village. The 1983 Hendricks/Horne master plan allowed 117 homes, with 17 more possible if septic conditions could be met, and set aside a site for senior housing. The French Ranch approval allowed 33 homes, including 7 density bonus units and 3 affordable units, wrapped around the former golf course. For financing, see financing 2-4 unit properties and apartment building loans.

Where the zoning allows it

San Geronimo is unincorporated and inland, so Marin County’s Title 22 applies. A 2022 Planning Commission resolution lists the valley’s districts, including ARP, RSP, R1-B, RA-B4, CP, H1, OA, RCR and VCR. It also adds the San Geronimo Valley combining district on top of all of them.

In the county code, two-family and multifamily housing are principally permitted only in R2 and RMP among the residential districts, and neither appears on the valley’s district list. In San Geronimo, added rental units therefore usually come from ADUs, JADUs or village mixed use.

ADUs and SB 9. ADUs and JADUs are permitted in every residential district. See our ADU investment guide. SB 9 can apply in single-family zones such as R1-B and RSP, but stream conservation areas, fire hazard and septic capacity often rule out a second unit or a lot split. See our SB 9 lot split guide.

Rent rules: state law plus Marin County’s just cause and mediation

Rent caps. No local cap applies. AB 1482 covers qualifying units, and the Attorney General’s chart sets the San Francisco Area maximum at 8.8% from August 1, 2026 through July 31, 2027. See our AB 1482 guide.

County just cause. The county’s just cause ordinance, Chapter 5.100 (Ordinance 3705, December 2018), applies in San Geronimo to lots with three or more dwelling units. Permitted ADUs and JADUs are not counted, but unpermitted units are. The plan’s note about unpermitted second units matters here, because an older cottage group may be covered even if only one unit is permitted. Before ending a tenancy, a landlord needs a county rental business license, a tenant-rights notice, Rental Registry registration and an Environmental Health permit if one applies. See our just cause eviction guide.

Mediation. Increases above 5% in 12 months can go to free mediation under the county’s mandatory mediation program (Chapter 5.95).

San Geronimo rental property rules

• Stream conservation rules. Under the valley’s updated rules, described in a county release, development along streams faces a minimum 35-foot no-touch zone and site plan review. Residential additions in stream areas are capped at a 300-square-foot footprint. A creekside duplex or cottage group may not be able to add a unit or expand. The rules also offer a voluntary point-of-sale inspection program.
• Short-term rentals. The San Geronimo township cap under Chapter 5.41 is 10 licenses initially, falling to 7. Multifamily units and newer ADUs are ineligible.
• Multi-unit inspections. County Environmental Health covers rentals of three or more units outside San Rafael and Novato.
• Transfer tax. Only the county’s $0.55 per $500 applies, per the Recorder.

Septic and water: the real unit limits

No sewer district serves the valley. The community plan warns that the valley floor’s high water table causes septic problems, that a conventional leach field generally needs about an acre, and that many small lots cannot support a system. It suggests merging adjoining substandard lots under one owner and notes that mound systems may make some lots usable. Water comes from the Marin Municipal Water District, which has served the valley from its lakes since it absorbed the local water company in 1951. Older cottages may need a seismic retrofit review. Buildings with three or more units and elevated decks need SB 721 inspections.

Underwriting a San Geronimo multifamily purchase

1. Confirm zoning, the planned-district suffix and the legal status of every unit.
2. Map stream conservation areas and the 35-foot no-touch zone against existing and planned buildings.
3. Get the Environmental Health septic file and an inspection, and confirm bedroom capacity.
4. Count dwellings the county’s way to see whether just cause applies, and model increases at or below 8.8%.
5. Collect leases, the rent roll and estoppel certificates.
6. Rebuild expenses with reassessed taxes, septic upkeep, wildfire insurance and the county license in our rental property analysis.

Questions people ask

Sources & notes

1. San Geronimo Valley Community Plan (1997)
2. Marin County, Board approves San Geronimo Valley policies and Resolution PC22-009
3. Marin County Code Chapters 22.10 and 22.12 (GoCodebook, October 2026)
4. Marin County, Just Cause Eviction and Mandatory Mediation
5. Marin County Code Chapter 5.41 and Environmental Health rental program
6. California Attorney General, Rent Caps and Marin County Recorder

Note: Census figures are from the 1997 plan and cover the whole valley. We did not read the zoning map for San Geronimo village parcels or the stream ordinance text.

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

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