How Much Does an ADU Cost in Mill Valley?
Mill Valley uses the same Marin planning ranges as the rest of the county — what changes here is where inside those ranges real projects tend to land, what the city actually charges, and what the city's own permit record shows.
The short answer
Plan on the Marin ranges — and expect Mill Valley projects to sit toward their upper ends
These are planning ranges for Marin, as of Nov 2026 — estimates, not bids:
- Detached ADU, 1 bed: $300k–$500k
- Garage conversion: $120k–$220k
- JADU (inside the existing home): $80k–$180k
- Above-garage ADU: $250k–$400k
Mill Valley does not get its own, separate set of ranges. Hillside work, design review and the town's review path push real Mill Valley projects toward the upper ends of the ranges above — that is a direction, not a different number. A builder's site visit produces the exact figure for your lot.
Sources, as of Nov 2026: City of Mill Valley Planning Fee Schedule (current schedule effective July 1, 2026; prior schedule effective July 1, 2025) and Building Fee Schedule (effective July 1, 2026); City of Mill Valley monthly “Permits Issued with Fees and Values” reports, pulled Nov 2026 — individual reports are linked in the permit-evidence section below. Planning ranges are the locked Marin set used across this site, including on the Marin County cost guide.
Start with the rules pair
Cost is only half the Mill Valley question
What Mill Valley allows — and whether your project takes the ministerial path or a design-review path — shapes the budget before any builder prices it.
This page owns the money. Its pair, the Mill Valley ADU guide, owns the rules: the size caps, the ministerial-versus-discretionary fork, hillside triggers and the permit steps from Municipal Code Ch. 20.90. Read that guide alongside this one — a cheaper type that needs a hearing can cost more in time and design work than a larger type that passes ministerially, and the rules decide which side of that line your lot sits on.
Flat lot vs hillside
The driver matrix: what moves you inside the range
Two Mill Valley projects of the same type and size can land at opposite ends of the same planning range. The difference is usually the lot, not the floor plan.
| Cost driver | Flatter, simpler lot | Hillside or constrained lot |
|---|---|---|
| Access | Materials and equipment reach the build area directly; staging is straightforward. | Narrow or steep access means more hand-carrying, smaller equipment and slower staging — labour time rises across every trade. |
| Foundation and retaining | A conventional foundation on workable ground, with little or no retaining. | Sloped ground can require engineered foundations, retaining work and drainage designed for the slope — structural scope a flat lot simply does not have. |
| Excavation and drainage | Limited grading; storm water handled with standard drainage. | Cut, haul-off and hillside drainage add scope before the building itself starts, and slope conditions can disqualify simpler conversion approaches. |
| Design and review | A design that fits the ministerial path moves through review with fewer iterations. | Projects drawn into design review carry more design development, more drawings and more rounds of response — professional time, not just construction time. |
| Utilities | Short, direct runs for water, sewer and power from the existing house or street connections. | Longer or uphill runs, and any capacity work the connections require, add site-work cost that never shows in a per-square-foot figure. |
| Where it tends to land | Toward the lower or middle part of the planning range for its type. | Toward the upper end of the planning range for its type — sometimes the deciding factor between one type and another. |
Planning ranges are estimates, not bids. The matrix above is qualitative on purpose: no honest per-driver dollar figure exists without a survey, a design and a site visit, and any page that prices your hillside from a table is guessing.
Mill Valley's real fees
The city's fee schedules, in plain numbers
Mill Valley publishes its fees. The planning fee below is real and specific — and it is not the whole fee picture.
The planning fee
Under the city's current 2026 Planning Fee Schedule, effective July 1, 2026, an Accessory Dwelling Unit Permit is $1,204, plus a $342 Southern Marin Fire District fee. The prior 2025 schedule listed the same permit at $1,175 + $342 — that figure is now only the prior schedule, and it is the one older pages still quote.
The rest of the fee picture
Building permit fees are additional, and they are valuation-based: they are calculated from your project's declared valuation under the city's building fee schedule, effective July 1, 2026, with plan check charged at 65% of the basic permit fee. That is why two ADUs of the same type can pay very different total city fees — the fee follows the valuation, not just the permit type. The permit-evidence section below shows what that looks like in practice: recorded total fees on real standalone Mill Valley ADU permits ran well into five figures, far above the $1,204 planning fee alone.
Budget honestly: $1,204 + $342 is the planning line item you can know today. Your building permit, plan check and other valuation-based charges are set once your project's valuation is set — your builder and the city's fee schedule, not a guide page, produce that total.
The county fee waiver does not apply here
Marin County's ADU fee waiver does not apply inside incorporated Mill Valley — it is a county programme for unincorporated Marin only, and Mill Valley is an incorporated town with its own fees. If your property is in unincorporated Marin instead, read the Marin County ADU fee waiver guide for the amounts, tiers and the December 31, 2026 deadline. Do not subtract a waiver from a Mill Valley budget.
Permit evidence
What Mill Valley ADU permits actually show
The City of Mill Valley publishes a monthly “Permits Issued with Fees and Values” report — its own issuance ledger, listing each permit's type, declared valuation, total fees and issue date. We read those reports so this page can show recorded figures, not folklore.
Mandatory caveat, wherever these figures appear: Declared valuation is what was declared for fee calculation — it is not a final construction cost, a bid, or a sale price.
| Project | Size | Declared valuation | Total fees recorded | Issued | Permit |
|---|---|---|---|---|---|
| New detached ADU | 800 sq ft | $895,000 | $29,400.17 | Mar 12, 2026 | B-046827 — Mar 2026 report |
| New detached ADU | 562 sq ft | $375,000 | $14,455.98 | Jun 24, 2026 | B-050416 — Jun 2026 report |
| Conversion of storage space to an ADU | 727 sq ft | $350,000 | $29,414.88 | Dec 18, 2025 | B-049366 — Dec 2025 report |
Read the three rows together and the lesson is the spread, not any single number. Two detached ADUs of similar size paid very different total fees, and a conversion paid about the same in fees as the largest detached example — because fees follow declared valuation and project scope, not a flat per-type charge. Declared valuation is what was declared for fee calculation — it is not a final construction cost, a bid, or a sale price.
The wider picture, and the method
Across the reports we pulled, 1,501 permits were screened and 27 were ADU-construction permits: 12 standalone ADU projects and 15 mixed with larger home projects. The mixed permits are never blended into the standalone figures — their valuations are dominated by the main-house work, so treating them as ADU costs would badly overstate what an ADU alone declares. On the 12 standalone permits, the median declared valuation was $397,500 and the median total fees were $17,812.56. That is a small sample (n=12), so treat both medians as indicative, not definitive. Declared valuation is what was declared for fee calculation — it is not a final construction cost, a bid, or a sale price.
Method note: our dataset covers 11 monthly reports, September 2025 through September 2026, pulled Nov 2026. The November 2025 and July 2026 reports could not be located in the city's DocumentCenter, so the run is 11 of the 13 months in that span — stated here rather than papered over. Examples are anonymised: no addresses and no owner names appear anywhere on this page; permit numbers are shown so you can verify each row against the city's own report.
What this evidence can and cannot do: it proves what Mill Valley homeowners declared and what the city charged on real, recent ADU permits. It does not prove what those projects finally cost to build, and it does not replace a bid. Use it to sanity-check a quote and to budget fees realistically — not as a price for your project.
Design review
Design review: cost and time effects, honestly stated
Design review rarely appears as a line item. It shows up as time, design work and carrying costs while a project waits and revises.
In cost terms, a project that goes through a fuller review path generally needs more from its design team: a more developed design earlier, more complete drawings, and responses to comments across more than one round. That is professional time you pay for before construction starts, and redesign can also change the project itself — materials, massing or detailing chosen to satisfy review become part of the build cost.
In time terms, review stretches the front end of a project: preparation, review, revision and re-review all sit before a building permit, and construction cannot start until they finish. Time is not free — design and holding costs run while it passes, and construction pricing can move between quote and start. We do not put a number on either effect here, because no verified Mill Valley figure for “the cost of design review” exists; anyone quoting you a precise review surcharge is inventing it. What decides your exposure is which review path your specific project takes, which is why the Mill Valley rules guide leads with that fork — and why the checker asks about your lot before anyone talks about design.
One related question we will not dress up with figures: what a finished Mill Valley ADU might rent for, or how fast it might pay back, depends on the unit, the tenancy and the market at completion, and no verified Mill Valley rent dataset stands behind a number we could publish — so this page carries no rent or payback figures.
Questions, answered
Mill Valley cost questions
Are Mill Valley's ADU ranges different from the rest of Marin?
No separate Mill Valley range set exists on this site, on purpose. The planning ranges are the locked Marin set — detached 1-bed $300k–$500k, garage conversion $120k–$220k, JADU $80k–$180k, above-garage $250k–$400k, as of Nov 2026, labelled planning ranges and estimates, not bids. Mill Valley's hillside work, design review and review path push real projects toward the upper ends of those ranges; that is a direction, not a different number.
What are Mill Valley's actual city fees for an ADU?
The planning fee is $1,204 for an ADU Permit plus a $342 Southern Marin Fire District fee, under the current 2026 Planning Fee Schedule effective July 1, 2026 (the prior 2025 schedule was $1,175 + $342). That is not the whole picture: building permit fees are additional and valuation-based under the building fee schedule effective July 1, 2026, with plan check at 65% of the basic permit fee. Recorded total fees on recent standalone Mill Valley ADU permits had a median of $17,812.56 in the city's own monthly reports.
What have real Mill Valley ADU permits declared and been charged?
Three verified examples from the city's monthly reports: a new detached ADU of 800 sq ft declared $895,000 with $29,400.17 in total fees (permit B-046827, issued Mar 12, 2026); a new detached ADU of 562 sq ft declared $375,000 with $14,455.98 in fees (B-050416, issued Jun 24, 2026); and a conversion of storage space to an ADU of 727 sq ft declared $350,000 with $29,414.88 in fees (B-049366, issued Dec 18, 2025). Declared valuation is what was declared for fee calculation — it is not a final construction cost, a bid, or a sale price.
Does the Marin County fee waiver apply in Mill Valley?
No. The county waiver is for unincorporated Marin County only, and Mill Valley is incorporated — its own fee schedules apply. Unincorporated readers should use the fee waiver guide instead; Mill Valley readers should not subtract any waiver from their budget.
Does design review add cost, or just time?
Both, indirectly. Review itself is a process, but projects on a fuller review path generally need more design development, more drawings and more rounds of response — professional time before construction — and a longer front end while carrying costs run. There is no verified standard figure for that effect, so this page does not invent one; which review path your project takes is the thing to establish first.
Why is the spread inside each range so wide?
Because the lot and the review path do as much to the price as the building does. A flat lot with direct access and a ministerial path sits low in its range; a hillside lot with engineered foundations, retaining, drainage and design review sits high. The driver matrix on this page shows the mechanism; the checker and a site visit place your lot on it.
Related guides
Keep reading
The rules pair — read this next
Building an ADU in Mill Valley: The Ch. 20.90 Rules, Plainly
Size caps, the ministerial-versus-design-review fork, hillside triggers and the permit steps — the rules that decide which side of the cost range your project lands on.
County cost flagship
How Much Does an ADU Cost in Marin County?
The same locked planning ranges with the full county anatomy: what is included, what is not, and what drives cost up or down across Marin.
Scope check — unincorporated only
Marin County's ADU Fee Waiver Ends December 31, 2026
The county waiver does not apply inside incorporated Mill Valley. If your property is unincorporated Marin, this guide has the tiers, conditions and deadline.
Your next step
Check your Mill Valley lot
The checker is pre-set to Mill Valley. Tell it your address and what you want to build, and your plan applies these ranges, the town's rules and your lot's flags — hillside included — to your specific property, in plain language.
Check my lot
FREE · FAST VERDICTStep 1 of 2 — your address and one tap on what you want to build. That is enough for a first verdict; sharpen it afterwards only if you want to.
Every type, answered — not just the one you picked
True cost — the planning band, the fees on record, and what the band leaves out
Not in the band — budget these separately; no honest source lets us put a number on them for your lot, so none is invented:
How long until someone can move in?
Dates that change this decision
What the permit record shows
What would change this answer
Lay your quotes side by side
Print this sheet and write each contractor's figures in a column. Every row is something a comparable bid has to state; a blank in a quote is a question for that contractor, not a saving. The first column is your brief, filled from your answers.
| Line every quote must state | Your brief | Quote A | Quote B | Quote C |
|---|
Check each contractor's licence number on the Contractors State License Board's public lookup (cslb.ca.gov) before you compare prices. Figures in your brief are the rule caps and planning estimates shown above, not bids.
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Stored and confirmed
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