ADU Builders in Marin County: How to Choose One and Check Them
No list of "top 10 builders" here. This page tells you what kind of builder your project needs, how to check one in five minutes, what a comparable bid looks like, and how long Marin projects have actually taken on the record.
The short answer
We do not have a Marin builder to recommend today — and we will not pretend to.
As of November 2026, no builder is signed to the ADU Build Works roster in Marin County. The roster is capped at three licensed builders per territory, and those seats are empty. So nobody is waiting for your details, no quote is available through us today, and nothing on this page is a paid placement.
What you can use right now:
- Three clocks, from Marin permit records. Permit wait, application to issuance: a median of 186 days in Mill Valley (12 permits) and 205 days in unincorporated Marin (189 permits). Build time, issuance to completion: a median of 502 days in Mill Valley (50 permits) and 440 days in the unincorporated county (123 permits). These are records of what happened — not a promise about your project.
- One price frame. Marin planning ranges, as of Nov 2026: $120k–$220k for a garage conversion, $80k–$180k for a JADU inside the house, $300k–$500k for a detached one-bedroom ADU. Planning ranges — estimates, not bids.
- Two dates. The unincorporated-Marin fee waiver runs through Dec 31, 2026. From Jan 1, 2027, state law will allow two detached ADUs on a single-family lot.
- One free step. Run the lot check at the bottom of this page first. A builder's first three questions are your address, your lot's flags and the unit type you want — the check answers all three on one screen.
Sources, as of November 2026: Mill Valley and Marin County permit records (application→issuance and issuance→completion medians, evidence pack read Nov 2026); ADU Build Works Marin planning ranges, as of Nov 2026 (estimates, not bids); Marin County fee-waiver programme, marincounty.gov, checked Nov 2026; AB 956 (Chapter 791, Statutes of 2026). Roster status is stated as of the date on this page.
Who builds ADUs here
Three kinds of ADU builder — and which project each one fits
Every company that turns up when you search "ADU builders Marin County" is one of three things. Knowing which saves you weeks.
1. Design-build firms
One company draws the plans, pulls the permit and builds. One contract, one party responsible when the drawings and the site disagree.
Fits: sloped lots, design-review towns, above-garage units — projects where design and construction decisions keep colliding. That describes a lot of Marin.
Ask: who actually stamps the structural drawings, and is the design fee credited if you build with them.
2. A general contractor, with your own architect or designer
You hire the designer first, then put finished plans out to bid. More work for you; more control over the design; and the only route where two or three builders price exactly the same drawings.
Fits: owners who want real competitive bids, and conversions where the design is simple and the price is the question.
Ask: whether the contractor has built an ADU in your jurisdiction before, and for the permit number so you can look it up.
3. Prefab and modular companies
The unit is built in a factory and set on your lot. The factory price is real — and it is only part of the job. Foundation, delivery, craning, utility trenching and the permit still happen on your property, and on a hillside or a tight street they can decide whether prefab works at all.
Fits: flat lots with clear truck and crane access.
Ask: for the installed price on your lot in writing, with site work, foundation, delivery, crane and utility connections as separate lines — and who holds the building permit.
A note on kits. An ADU kit is materials, not a permitted home. It still needs plans your jurisdiction accepts, a foundation, utilities, inspections and someone licensed to do the work that requires a licence. Price the whole project, not the box.
The five-minute check
How to check any builder before you spend an hour with them
Do this before the site visit, not after you like them.
- Look up the licence yourself. Use the Contractors State License Board lookup at cslb.ca.gov. You want an active licence, in the name of the company you are actually signing with, in a classification that covers the work. A general building contractor's licence is the usual one for a ground-up ADU.
- Read the whole licence record, not just "active". The CSLB record shows the bond, workers' compensation status, and any disclosed complaints or disciplinary actions. A builder with employees and a workers' compensation exemption on file is a question to ask out loud.
- Ask for ADU permit numbers in your jurisdiction. Not photos — permit numbers. Mill Valley, San Rafael and the County each keep public permit records. A builder who has finished ADUs in your town can give you two or three numbers without effort.
- Ask who pulls the permit and who answers plan-check comments. If the answer is "you do", the price you were quoted is missing a job.
- Know the deposit rule before you are asked for one. California law caps the down payment on a home-improvement contract. Check the current cap on cslb.ca.gov, and treat a request for more as your answer about that builder.
None of this needs us, and none of it costs anything.
The record
How long Marin ADU projects have actually taken
Builders quote timelines. Permit records keep them. These are medians from Marin permit records in our evidence pack, read November 2026.
| Stage | Mill Valley | Unincorporated Marin | What it measures |
|---|---|---|---|
| Permit wait — application to issuance | 186 days (12 permits) | 205 days (189 permits) | From the day an application went in to the day the permit was issued. Includes the owner's and designer's time answering corrections, not only the agency's. |
| Build — issuance to completion | 502 days (50 permits) | 440 days (123 permits) | From permit issued to the project recorded complete. Includes slow starts, weather, change orders and final inspections. |
Read them carefully:
- They are medians of past permits, not a schedule for yours. Half were faster, half slower.
- Mill Valley's permit-wait figure rests on 12 permits. That is a small sample; treat it as an indication.
- The two stages come from different sets of permits, so do not add them into one "total time".
- San Rafael: we hold a figure and are not publishing it — it has not been reconciled to our standard. When it is, it goes here with its sample size.
How to use this with a builder: if someone promises a finished ADU in a few months, ask which of their Marin permits did that, and look it up.
The bid
What a bid you can compare looks like
Two bids only help if they describe the same project in the same buckets. Ask every builder for this, in writing:
- One scope. Same unit type, size and finish level, described identically to every bidder.
- Four buckets, itemised separately: build cost; design and engineering; permits and fees; utility and site work. Never one lump sum.
- Your lot's drivers as their own lines. Hillside and foundation work. Septic or sewer work. Fire-zone (Chapter 7A) construction. If your lot has one of these and the bid does not show it, the bid has not priced your lot.
- Fees by name. Which permit, impact and utility fees are inside the number and which you pay directly. In Mill Valley the planning line alone is $1,204 for the ADU Permit plus a $342 Southern Marin Fire District fee (2026 Planning Fee Schedule, effective July 1, 2026) — and recorded total fees on 12 recent standalone Mill Valley ADU permits had a median of $17,812.56. A bid that carries "permits: $1,500" for Mill Valley is not finished.
- Allowances and exclusions, listed. Every allowance is a number you can overspend.
- A site visit behind the number. A bid written from a floor plan is a planning range with a signature line.
The planning ranges those bids should land near, for reference — Marin County, as of Nov 2026, estimates, not bids:
| Type | Planning range |
|---|---|
| JADU — inside the house | $80k–$180k |
| Garage conversion | $120k–$220k |
| Above-garage ADU | $250k–$400k |
| Prefab ADU (installed) | $250k–$450k |
| Detached ADU — 1 bed | $300k–$500k |
| Two detached ADUs (legal per lot from Jan 1, 2027) | $550k–$900k |
Where a bid sits inside its band is mostly your lot, not your floor plan. The full breakdown is in the Marin ADU cost guide.
Town by town
What a builder has to know about your jurisdiction
A builder who works in Marin should be able to answer these without looking them up. If they cannot, you have learned something.
Mill Valley. Does the design stay inside the ministerial class — 800 sq ft or less, 16 ft or less, 4-ft side and rear setbacks — or does it need design review? What does the builder do about the city's deed-restriction practice? (The city requires a recorded deed restriction in practice; the state housing department's position is that state law does not allow a city to require one for an ADU. Both are true today — see the Mill Valley ADU guide.)
San Rafael. The ordinance of record caps a detached ADU at 1,000 sq ft (SRMC §14.16.285, Ordinance 2002). A builder who designs to 1,200 sq ft without raising it has not read it. Under 750 sq ft, the city waives a published list of fees. Details in the San Rafael ADU guide.
Unincorporated Marin. Your permit is a county permit. Septic capacity is the first question on many lots. The county waives up to $2,500 per ADU and $1,500 per JADU through Dec 31, 2026; what exactly has to happen by that date is not confirmed in the county's published programme, so ask Marin County CDA in writing. See the fee-waiver guide.
Any other Marin town. We have not published a guide for it yet, and we do not guess. Ask the builder for their last ADU permit number in that town.
Our roster, honestly
Where ADU Build Works fits — and where it does not, yet
We are a research and matching service. We are not a builder or a licensed contractor.
The model. At most three licensed builders per territory hold a roster seat. A seat requires a current California contractor's licence in good standing and ADU permit history. Builders pay a seat fee; that fee buys a seat, not a recommendation.
The status, as of November 2026. Zero builders are signed in Marin. If you save a brief through the lot checker, it is stored, and you are told plainly that no quotes are available today. When a roster builder can take a brief like yours, a person contacts you first, by the channel you chose. Nothing is passed to anyone without your separate OK.
What that means for you. Use the check, the ranges, the permit clocks and the bid list on this page with any builder you find yourself. If our roster can help later, it will be because a builder met the bar — not because this page needed a name on it.
The full model is published at How it works.
Questions, answered
Marin ADU builder FAQ
Who are the best ADU builders in Marin County?
We do not rank builders, and we have none signed to our roster as of November 2026, so we have no one to name. The checks on this page — the CSLB licence record, local permit numbers, and a four-bucket bid — will tell you more about a builder than a list would.
How do I verify an ADU contractor's licence?
Search the business name or licence number at cslb.ca.gov. Confirm the licence is active, is in the name on your contract, and covers the work. Read the bond, workers' compensation and complaint-disclosure sections too.
How many bids should I get?
Enough to compare like with like — and only if each bidder prices the same written scope in the same four buckets. Two bids on identical drawings beat five on five different ideas.
How long does an ADU take in Marin?
On the record: a median of 186 days from application to permit in Mill Valley (12 permits) and 205 days in unincorporated Marin (189 permits); then a median of 502 days (Mill Valley, 50 permits) and 440 days (unincorporated county, 123 permits) from permit to completion. Those are medians of past permits, not a promise for yours.
Is a prefab ADU faster or cheaper in Marin?
Sometimes, on a flat lot with crane access. The Marin planning range for an installed prefab ADU is $250k–$450k, against $300k–$500k for a site-built detached one-bedroom — the bands overlap. Site work, foundation and delivery decide it.
Can ADU Build Works get me quotes today?
No. No builder is signed to our Marin roster as of November 2026. You can run the lot check and save your brief; we will tell you it is stored and that no quotes are available yet.
Should I wait for 2027 to build two ADUs?
Only if two detached units is really the project. The right begins January 1, 2027, and it comes with a JADU trade-off. Read two ADUs in Marin from 2027 before you decide — and note the county fee waiver's window closes the day before.
Keep going
Related guides
- How much does an ADU cost in Marin County? — planning ranges by type, and what moves them.
- Mill Valley ADU rules and Mill Valley ADU cost — the ministerial class, design review, and the city's own fee records.
- San Rafael ADU rules — the 1,000 sq ft cap and the under-750 fee waiver list.
- Marin County ADU fee waiver — ends Dec 31, 2026; unincorporated Marin only.
Check your lot
Before you call a builder: know what your lot can take
Type your address and tap twice. You get a plain-language verdict on screen — what your lot can likely take, the planning range for it, and the flags a builder will ask about — before we ask for any contact details. If you choose to save your brief, we store it and tell you so. No builders are signed to our roster yet, so no quotes are available today and no builder will contact you.
Check my lot
FREE · FAST VERDICTStep 1 of 2 — your address, what kind of home it is, and what you want to build. Two taps. That is enough for a first verdict; sharpen it afterwards only if you want to.
Stored and confirmed
Anything wrong in that summary? Email us at [email protected] and we'll correct the record.
We already have this brief from you
If something changed — a different project, a new budget — email us at [email protected] and we'll update the record. Your verdict itself is above, and it stays yours.
That didn't go through — nothing was sent
Your verdict is right here on this page, and it stays yours. Prefer to do this by email? Write to us at [email protected] with your address and what you'd like to build, and we'll set it up with you — no dead end.
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.
Make your verdict more exact (optional)
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