Larkspur rewrote its ADU rules in 2025. Here’s an ADU contractor’s honest read.

In August 2024, the state’s Department of Housing and Community Development sent Larkspur a findings letter: six ways the city’s 2023 ADU ordinance fell short of state law, including lot-coverage math that effectively capped every ADU at 800 square feet. The city’s answer was Ordinance 1090, adopted September 3, 2025 — a complete replacement of the ADU chapter, written to current state law and applying to applications filed after it took effect that fall. It even codifies the tiebreaker: where the chapter and state law disagree, state law controls.

The centerpiece is the by-right fast lane. One detached new-construction ADU up to 800 square feet, with four-foot side and rear setbacks, must be approved without applying the city’s development or design standards — no floor-area, lot-coverage, parking, or design-material requirements. And by-right categories stack: a single-family lot can hold a conversion ADU carved out of existing space, a junior ADU inside the house, and a detached ADU, all together. Conversions of existing accessory structures can even expand up to 150 square feet to handle entry and exit.

Process-wise, everything runs through the Building Division at 400 Magnolia Avenue, and submittal is digital-only through the city’s eTRAKiT portal. The city has 30 days to tell you whether the application is complete, then 60 days to decide — and a complete application the city sits on past 60 days is deemed approved. The city quotes roughly three to four weeks for initial plan review on new construction, with two-to-three-week resubmittal rounds. Clean first submittals are what make those clocks work in your favor.

Outside the by-right lane, the size tiers are generous by Marin standards: 850 square feet for a studio or one-bedroom, 1,000 for two bedrooms, and for three or more bedrooms the lesser of 1,200 square feet or half the primary home — never less than 800. A junior ADU runs 150 to 500 square feet inside the walls of the house. The overall cap is two ADUs plus one JADU on a single-family lot: four dwellings, total.

Height is where Larkspur is quietly unusual. The baseline for a detached ADU is 16 feet — but state law grants 18 feet within a half-mile walk of a major transit stop, plus two more feet to match the primary home’s roof pitch, and Larkspur Landing has two qualifying stops side by side: the ferry terminal and the SMART station. Lots within that half-mile walk can plausibly build a 20-foot detached ADU with a real second story of headroom. Whether your parcel qualifies is a Planning determination, so we confirm it per address rather than assume.

Two more rules worth knowing. Lot coverage can’t be applied in a way that limits a conforming ADU — and an ADU built after July 1, 2023 banks up to 800 square feet of additional coverage and floor-area allowance the lot can spend later on a main-house expansion — an unusual bonus almost nobody knows about. For non-by-right ADUs, the design standards that do lawfully apply are objective: exterior materials and colors matching the main home, shielded lighting, and privacy rules for second-story windows within 30 feet of a property line.

Larkspur’s fire authority is the Central Marin Fire Department, and the wildland-urban interface is a fact of life on the King Mountain side of town. The city has an adopted WUI boundary, your plans must declare WUI status on the title page, and CMFD runs annual defensible-space and home-hardening inspections — 30 to 150 feet around structures depending on slope and terrain, funded through the Marin Wildfire Prevention Authority. New state fire-hazard maps released in 2025 are working through adoption, so check CMFD’s current map for your parcel before you start designing.

One thing we deliberately won’t state as settled: sprinklers in a new detached ADU. The ADU ordinance says sprinklers aren’t required in an ADU if they’re not required in the primary home, but the city’s own submittal checklist says sprinklers are required on all new construction. For any new-construction ADU we get CMFD’s determination in writing early — at (415) 927-5077 — rather than designing around a guess.

Down in the flats, Corte Madera Creek is the constraint. Parcels adjacent to a creek or inside the 100-year flood boundary need a FEMA flood elevation certificate with the application, and FEMA is actively restudying the Ross Valley watershed, so mapped zones can move. On the geotechnical side, Larkspur requires a soils report for new construction and most additions — not just on obvious hillsides — and any earthwork beyond small hand-dug excavations triggers a grading permit.

City permit fees are valuation-based, with plan check billed as a percentage on top and a pair of small percentage-of-valuation citywide fees; the schedule resets every July 1, and the current one took effect July 1, 2026. Rather than quote numbers that age out annually, we price permits per project and confirm the current schedule with the city at application. The number that doesn’t reset every July: under 750 square feet, state law currently bars impact fees entirely, and larger ADUs pay proportionally to the main home.

Utilities are a two-district story. Sewer collection is Ross Valley Sanitary District — a flat $513 ADU permit, no capacity charge at all for ADUs converted from existing space, and $335 per drainage fixture unit for expansions and new detached units, so every sink and shower is a line item you control in design. The regional treatment charge scales in proportion, which means conversions skip it too. Water is Marin Water: interior-conversion ADUs pay no capacity charge, while new detached units pay $507 per fixture unit. One catch — a small slice of Larkspur is actually served by Corte Madera’s sanitary district, so we verify the district per address.

Two boundary traps to check before you fall in love with a design. School impact fees apply when a project adds 500 or more square feet of habitable space, paid directly to the district — and depending on where in Larkspur you are, that’s Larkspur-Corte Madera, Kentfield, or San Rafael City Schools, each with its own rates. And Greenbrae is split down the middle: part sits inside Larkspur city limits, part is unincorporated Marin County with entirely different ADU rules and a different permitting office. Confirming which side of the line you’re on is step zero.

Historic downtown carries its own adder: a new detached ADU on a site in the Downtown Historic District, or on (or eligible for) the city’s Historic Resources Inventory, needs a report from a qualified architectural historian finding the design meets federal preservation standards. It’s a real cost and a real lead time that almost nobody budgets for — worth knowing before you buy plans off the internet for a Magnolia Avenue-area property.

Larkspur’s new ordinance builds in an amnesty path: an unpermitted ADU constructed before January 1, 2020 can be legalized, the city can’t deny the permit based on the prior violation, and you can start with a confidential third-party inspection by a licensed contractor before anything goes on file with the city. Owners who’ve received a violation notice can request up to five years of enforcement delay while they legalize — but that window narrows after January 1, 2030, so the clock favors acting sooner.