Building an ADU in Mill Valley: The Ch. 20.90 Rules, Plainly

Mill Valley at a glance

The Ch. 20.90 rules table

Sizes are gross floor area, measured to the outside walls — Mill Valley's own definition, and one reason its numbers do not line up neatly with towns that measure differently.

The Mill Valley details that decide real projects: the ministerial-class test itself, the 850 / 1,000 / 1,200 cap structure, the 25-ft above-garage rule, the hearing path, the 50-cubic-yard conversion disqualifier, and the deed-restriction and utility requirements below — each one traceable to Chapter 20.90 or the city's handout.

The fork that decides your project

Ministerial vs discretionary — the decision table

Run your project down the left column. One “hearing” answer does not kill a project — Mill Valley approves ADUs through that path too — but it changes the timeline, the cost of the process, and the certainty you can plan around.

The ministerial class, in the code's own terms

New detached or attached ADUs that are 16 ft or less, 800 sq ft or less, and set back at least 4 ft from the side and rear lines are exempt from lot-coverage, floor-area, and rear and side setback requirements. Conversions are treated as-sited — the existing footprint stands, subject to fire and building code. That class is the fast lane, and designing into it on purpose is the single most useful decision a Mill Valley homeowner can make.

What the hearing path actually looks like

This is not a theoretical fork. In a documented staff report (case PL25-0096, June 23, 2026), a proposed 1,096 sq ft ADU at 19 ft 11 in needed exceptions on size, floor area and height, and went through discretionary review and a public hearing before the Zoning Administrator. Read that case the right way: projects outside the class are not banned — they are argued, in public, on a longer clock. If your design needs the extra size or height, go in knowing that, with the exceptions named up front.

The state-law caveat: Chapter 20.90 dates from 2021. State law has moved since — on heights in particular, current state law can allow more than the printed 16 ft in defined situations. Where state law is more permissive than the 2021 text, state law controls. A project that looks “over” on this table may still have a state-law route; that is a question to confirm with the city against current law, not a reason to abandon the design at the kitchen table.

Size, height and setbacks

850, 1,000 or 1,200 — which cap is yours?

Template pages claim Mill Valley allows “1,200 sq ft detached.” That conflates three different caps. The general ADU cap is 1,000 sq ft. A studio or one-bedroom caps lower, at 850 sq ft. The 1,200 sq ft figure belongs to one class only: an internal conversion inside an existing single-family home. An attached ADU is measured differently again — up to 50% of the primary home's maximum adjusted floor area, with a 150 sq ft minimum and the 1,000 sq ft maximum. Knowing which cap is yours, before design, is the difference between the ministerial lane and an avoidable hearing.

Above-garage: the combined 25-ft rule

A detached ADU on its own is limited to 16 ft. Built above a garage, the test changes: the combined structure — garage plus the unit above it — must come in at 25 ft or less. That combined measurement is the design constraint to work from on an above-garage project, not the 16 ft figure alone. Attached ADUs are 16 ft by right, with the zone standard available through the ordinance path.

Front setbacks and slope

The standard front setback is 15 ft, slope-reduced under MVMC §20.60.080 — Mill Valley's terrain is written into its setback code, and a sloped lot can change the front measurement. Side and rear setbacks for the ministerial class are 4 ft. One trap for the unwary: decks, breezeways and trellises attached to an ADU must meet the full district setbacks — the 4-ft ADU allowance does not extend to them. Conversions sit as they are, subject to fire and building code rather than new setback measurements.

When a “conversion” stops being a conversion

Two things can take a project out of the conversion class. Excavation beyond 50 cubic yards is the bright line: dig more than that — for a lowered floor, a foundation fix or a hillside footing — and you are no longer converting, you are building. Roof-raising beyond what the code's conversion definition allows does the same. Both matter because the conversion class is where the 1,200 sq ft cap and the as-sited setback treatment live. If your garage conversion needs serious digging or a raised roof, get its classification confirmed before you rely on conversion treatment.

Design review, hillside and JADUs

The realities behind the rules

Mill Valley's design friction is not a separate design-review overlay pasted on top of the ADU rules — it is the fork itself. Meet §20.90.040's standards and §20.90.050's objective design guidelines and review stays ministerial. Miss them and the same project becomes a discretionary hearing. Hillside conditions feed that test directly: slope changes the front setback, excavation limits the conversion class, and height is measured against terrain that rarely sits flat. On a hillside lot, the honest sequence is classification first, design second.

JADUs in Mill Valley

A junior ADU is up to 500 sq ft inside the existing single-family home, under MVMC 20.90's JADU standards in the city handout. A deed restriction is recorded, and owner-occupancy applies as current state law provides — since January 1, 2026, state law requires owner-occupancy for a JADU only where it shares a bathroom with the primary home; a JADU with its own bathroom does not carry that requirement. As everywhere in California, an ADU or JADU may not be rented for under 30 days.

Deed restrictions and utility disconnects

Bottom line for your budget: plan as if Mill Valley will require the deed restriction, because today it does. Whether it lawfully can is a dispute between the city and the state — not one you need to win to build. If recording one would change your financing or a future sale, ask Planning in writing which authority it relies on before you record anything.

Two paperwork items need care, and the first has two layers that are both true at once. Layer one — what Mill Valley requires today: the city requires a deed restriction for an ADU in practice. The requirement is codified at §20.90.030(B)(2); the Planning Department's April 2026 handout states “Deed restrictions are required for all ADUs” (recorded with the County of Marin and then submitted to the City); the Building Division's Completeness Checklist (current revision dated October 30, 2026) requires the deed restriction to be submitted before final inspection; and 2026 staff reports condition approvals on the city's deed-restriction template. Layer two — how the state housing department reads state law: §66315 bars any standard beyond those listed in §66314, and HCD's 2025 ADU Handbook and its February 27, 2026 letter to the city treat a deed restriction as one of those barred "additional standards" — HCD's position is that state ADU law prohibits deed restrictions on ADUs. Plan on the city requiring the restriction — and ask the city which authority it relies on, because HCD's stated position is that state law does not allow it. For a JADU, state law itself requires a deed restriction. And the city's handout calls for separate utility disconnects for the ADU, so the unit can be shut off independently of the main house. Neither is a reason not to build; both belong in the budget and the timeline from the start.

Permit steps

Follow the city's own checklist

Mill Valley publishes the two documents that govern your application: the Planning Department's ADU handout (2026 update) and the Building Division's ADU Completeness Checklist, current revision dated October 30, 2026. Work from those, in this order.

The handout also points Mill Valley homeowners to ADU Marin, the county's shared ADU resource, for early feasibility help — a free first conversation before design money is spent.

Fees — summary only

The one fee line that belongs on this page

In brief: the ADU Permit fee is $1,204 + $342 fire district under the current 2026 schedule — full fee treatment and the permit record live on the cost page. Building permit fees beyond that planning fee are valuation-based, and the complete picture — the schedule in full, what real Mill Valley ADU permits declared and were charged, and the planning ranges by type — is on Mill Valley's ADU cost page. This rules page deliberately carries no cost ranges beyond that pointer: one page owns Mill Valley prices, and it is that one.

And the waiver question, answered plainly because Mill Valley homeowners ask it: the Marin County fee waiver does not apply inside incorporated Mill Valley. That programme covers unincorporated Marin County only. The county fee-waiver guide sets out the scope in full. The countywide cost picture, for comparison, is on the Marin ADU cost guide.

Questions, answered

Mill Valley homeowners ask us these

Related guides

Where to go next

Check your lot

Check your Mill Valley address

Tell us the address and what you want to build. The checker is pre-set to Mill Valley, so your verdict starts from the question this page is built around — which side of the ministerial fork your lot and your design are on — in plain language, in about 60 seconds.