Multifamily and Duplex Properties for Sale in Forest Knolls: Zoning and Rent Rules

What multifamily in Forest Knolls looks like

The San Geronimo Valley Community Plan (p. IV-4) notes that Forest Knolls, Woodacre and Lagunitas contain some apartment buildings and other multifamily units. It also says many valley homes have second units in converted basements, attics, garages or cottages. Some were built without permits or do not fit current zoning. That is the Forest Knolls rental market: small buildings over village shops, a few older multi-unit parcels, and houses with second units on hillside lots above San Geronimo Creek. For financing, see financing 2-4 unit properties and apartment building loans.

Where the zoning allows it

Forest Knolls is unincorporated and inland, so Marin County’s Title 22 applies, guided by the 1997 community plan. No Coastal Development Permit is needed.

Check the county zoning map for multifamily or two-family districts on specific parcels. The plan estimated about 1,566 homes in the valley in 1995 and a theoretical buildout of 2,369. It stressed that the figures are maximums that septic and slope would reduce, and they exclude second units.

ADUs and SB 9. ADUs are the main way to add units, but the plan warns that second units strain water, parking and septic. A legal ADU here usually needs septic approval from Environmental Health. See our ADU investment guide. SB 9 projects need adequate services and are barred on some hazard parcels, and a lot on septic often cannot support two primary units. Confirm with the county and see our SB 9 lot split guide.

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

Rent caps. The Attorney General’s rent cap page puts Marin in the San Francisco Area region. The maximum increase for covered units is 6.3% from August 1, 2025, and 8.8% from August 1, 2026 through July 31, 2027. No local cap applies. See our AB 1482 guide.

County just cause. Marin County’s just cause ordinance (Ordinance 3705, Chapter 5.100, December 2018) covers unincorporated lots with at least three dwelling units. Unpermitted units count toward the three, so a Forest Knolls parcel with a main house and two unpermitted cottages is likely covered. Permitted ADUs and JADUs are exempt. See our just cause eviction guide.

Mediation. Under the county’s mandatory mediation program, tenants can require free mediation of increases over 5% in 12 months. Bad-faith refusal can invalidate the increase.

Forest Knolls rental property rules

• Short-term rentals. The county’s Chapter 5.41 caps Forest Knolls at 8 licenses, one per owner. Multifamily units, deed-restricted units and ADUs created or legalized after 2020 or 2021 are ineligible. Underwrite second units as long-term rentals.
• Business license. Rentals need a county business license, renewed each July.
• Transfer tax. Only the county’s $0.55 per $500 applies, per the Recorder’s sheet.

Septic, creek and fire

The county’s wastewater Q&A says no sewer district serves the valley. Septic failures there come from poor soils, high groundwater, small lots and age, with most systems over 50 years old, and county permit records exist for fewer than half of developed parcels. The community plan says a leach field generally needs about an acre and urges merging small contiguous lots. It also identifies San Geronimo Creek as key coho and steelhead habitat. On a multi-unit Forest Knolls property, expect to pay for a septic inspection, possibly a replacement or mound system, and creek-setback review for any addition. Hillside units face wildfire exposure and defensible-space requirements. Older wood-frame buildings on slopes may warrant a seismic retrofit review, and buildings with three or more units and elevated decks need SB 721 inspections.

Underwriting a Forest Knolls multifamily purchase

1. Confirm the zoning and community plan designation, and test unit count against VCR, CP or H1 density.
2. Pull the Environmental Health septic file. If none exists, treat every unit’s legality as an open question.
3. Count all dwelling units, permitted or not, to see whether the county’s just cause rule applies, and classify units under AB 1482.
4. Model increases at or below 8.8%, and plan for mediation above 5%.
5. Collect leases, the rent roll and estoppel certificates, and check for unpermitted second units.
6. Rebuild expenses with reassessed taxes, septic upkeep and fire insurance in our rental property analysis.

Questions people ask

Sources & notes

1. San Geronimo Valley Community Plan (Dec. 2, 1997) (historical society copy)
2. Marin County Code Ch. 22.12 and Ch. 5.41 (GoCodebook)
3. Marin County Environmental Health, valley wastewater Q&A
4. Marin County, Just Cause Eviction and Mandatory Mediation
5. California Attorney General, Rent Caps
6. Marin County Recorder, transfer tax and Business License

Note: We did not verify parcel-level zoning, Marin Municipal Water District rules or the county’s current creek setback standard.

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

Related: Forest Knolls real estate guide · Commercial real estate in Forest Knolls · Multifamily investing guide · Investor services