Multifamily and Duplex Properties for Sale in Mill Valley: Zoning and Rent Rules

What multifamily in Mill Valley looks like

Mill Valley’s apartments are concentrated close to downtown and along Miller Avenue, where small 1950s to 1970s buildings, older duplexes and converted houses sit on flatter lots near the bus lines. Upper-floor units over downtown shops add a few more. Much of the hillside housing stock is single-family, and many "Mill Valley" multifamily listings are actually in unincorporated Strawberry, Tam Valley or Almonte, where county rules apply. Two local factors shape almost every deal inside the city: a new mandatory soft-story retrofit law, and a seller-paid sewer lateral inspection. For financing, see financing 2-4 unit properties and apartment building loans.

Where the zoning allows it

Mill Valley’s Title 20 allows two or more units in these districts:

District	Multifamily rule	Density / FAR	Height
RM-3.5 Multiple-Family	Multifamily permitted	3,500 sq ft of lot area per unit, 8–15 units/acre. 50% coverage. 450 sq ft open space per unit	25 ft at setbacks, up to 35 ft farther in
RM-B / RM-M (Bayfront, Marsh) on Miller Avenue	Rental and for-sale multifamily permitted	FAR 0.5, 50% coverage. 100 sq ft outdoor space per unit	35 ft, three stories. 25 ft within 20 ft of single-family zones
RM-PA / RM-P (Passage, Parkway)	Rental multifamily permitted. For-sale needs a CUP unless inclusionary	FAR 0.4 on sites over 30,000 sq ft, 0.6 below	35 ft with 1:1 stepback above the second story
DR Downtown Residential	Rental multifamily permitted. For-sale by CUP	FAR 0.60, up to 0.75 with design review. Efficiency units count as half a unit	25 ft, up to 35 ft with stepback
C-D, C-N, C-G, C-L commercial	Mixed-use housing	17–29 units/acre	35 ft
Housing overlays (SLHO, OSHO, OCHO)	Housing Element sites	17–40 or 20–40 units/acre	40 ft

The DR chapter says its aim is to preserve existing rental housing, and it does not allow existing apartments to be merged into a single-family home. That limits an investor’s exit as well as a developer’s plans.

ADUs. Second units are permitted in RM and DR under Chapter 20.90. See our ADU investment guide.

SB 9. Chapter 20.91, adopted by Ordinance 1331 in 2021 and amended in 2025, makes two-unit projects a permitted use only in the RS single-family district. Sites in historic districts, flood hazard areas, fault zones or wetlands are excluded, as are projects that would remove rent-restricted or recently tenanted units. Units may not be rented for less than 30 days, the height limit is 25 feet (35 if both units share one structure), and side and rear setbacks are 4 feet. See our SB 9 lot split guide.

Rent rules: state law only

No local rent cap. We found no rent stabilization or just cause chapter in Mill Valley’s code index. The Attorney General’s rent cap page places Marin in the San Francisco area region, with a maximum AB 1482 increase of 8.8% from August 1, 2026 through July 31, 2027. See our AB 1482 guide. State just cause rules apply to covered tenancies; see our just cause eviction guide.

No county mediation. Marin’s mandatory mediation program covers only unincorporated Marin, San Rafael and Fairfax, so Mill Valley increases are not subject to its over-5% rule. Remember that an address in Strawberry or Tam Valley is in unincorporated Marin and is covered.

Source of income. Mill Valley’s Title 5 includes Chapter 5.33, which bars discrimination against tenants based on source of income. Screening criteria should treat voucher holders the same as other applicants.

Mill Valley rental property rules

Condominium conversions. Under Chapter 20.59, a conversion needs a conditional use permit and findings that it will not substantially affect rental availability in the city. Tenants get 60 days’ notice before filing, a purchase right of at least 90 days, 180 days’ notice to vacate and three months’ rent in relocation aid. Lower-income, disabled and 68-and-older tenants must be offered lifetime leases with increases capped at half the regional CPI.
Business license. Chapter 5.08 charges apartment and rental house operators a flat $15 per dwelling unit per year.
Short-term rentals. Stays of 30 days or less owe the 10% transient occupancy tax, and operators must register with the Tax Collector.
County inspections. Marin County Environmental Health’s multi-unit rental inspection program covers buildings of three or more units everywhere except San Rafael and Novato. Ask for the operating permit.

Soft-story retrofits and sewer laterals

Mandatory seismic retrofit. Mill Valley adopted Chapter 14.36 by Ordinance 1363 on December 8, 2025. ABAG describes the city as the first North Bay jurisdiction with a mandatory soft-story program. It covers wood-frame buildings with three or more long-term rental units, built or permitted before January 1, 1978, with a weak or open story such as tuck-under parking. Every owner must file an engineer’s screening within one year. Buildings with a unit or business in the weak story are Tier 3 and have six years to finish work. Buildings in high landslide risk areas are Tier 2 with five years. All others are Tier 1, with a retrofit permit due in two years and construction in three. Price the retrofit before you remove contingencies. See our seismic retrofit guide.

Sewer lateral at sale. Under Section 17.04.270, adopted in 2015, the seller must give the City Engineer a licensed plumber’s lateral inspection report before title transfers, and disclose the rule to the buyer in writing. If the seller does not, the buyer must provide it within 30 days and pay for repairs. Laterals replaced within 20 years, or inspected or repaired within 3, are exempt, as are condo sales and shared multiple-connection laterals. Repairs are due within 180 days of notice. New units pay a sewer connection charge, listed at $10,900 per dwelling unit.

Balconies. Buildings with three or more units and elevated decks or stairs need SB 721 inspections.

Underwriting a Mill Valley multifamily purchase

Confirm the parcel is in the city, then identify the district and test the unit count against RM-3.5 lot area or DR and RM FAR rules.
Determine whether the building falls under Chapter 14.36, which tier it is in, and whether the screening has been filed.
Get the sewer lateral report status and any repair notice from the City Engineer.
Classify each unit under AB 1482 and model increases at or below the state cap.
Collect leases, the business license receipt, the county inspection permit and estoppel certificates.
Build retrofit costs, reassessed taxes and real expenses into our rental property analysis.

Questions people ask

Does Mill Valley have rent control?

No local ordinance appeared in our review of the code. Covered units follow AB 1482, with an 8.8% cap for the San Francisco area from August 2026.

Which buildings must do a soft-story retrofit?

Wood-frame buildings with three or more rental units, built or permitted before 1978, with a weak or open story. Screening is due one year after the ordinance takes effect.

How many units can I build in RM-3.5?

One per 3,500 square feet of lot area, or 8 to 15 units per acre.

Can I convert my apartments to condominiums?

Only with a conditional use permit and findings about rental supply, plus tenant notices, relocation payments and lifetime lease offers for protected tenants.

Who pays for the sewer lateral inspection?

The seller must deliver the report before closing. If not, the buyer must provide it within 30 days and handle repairs.

Can I use SB 9 on a multifamily lot?

No. Mill Valley allows SB 9 two-unit projects only in the RS district.

Does the county rent mediation program apply?

Not inside Mill Valley. It does apply in unincorporated areas with Mill Valley addresses, such as Strawberry.

Sources & notes

Mill Valley Title 20 (GoCodebook, Oct. 2026): index, Ch. 20.24, Ch. 20.26, Ch. 20.30, Ch. 20.40, Ch. 20.59, Ch. 20.91
Ch. 14.36 mandatory seismic retrofit (Ord. 1363)
Ch. 17.04 sewers, including § 17.04.270
Mill Valley Title 5: Ch. 5.08, Ch. 5.28
ABAG, Bay Area soft story ordinances
California Attorney General, Rent Caps
Marin County, Mandatory Mediation Program and Multi-Unit Rental Inspection

Note: The soft-story chapter’s effective date was not shown; confirm deadlines with the Building Division. We did not read Chapter 5.33’s text, the ADU chapter or the zoning map, and the RM chapter we read did not show RM sub-districts other than RM-3.5.

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

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