The Cheapest Way to Build an ADU in Marin County

Ranked

Six routes, from lowest planning range to highest

The ranges overlap. An awkward garage on a hillside can cost more than a simple detached unit on a flat lot. Rank is a starting point, not a quote.

Route 1

The junior ADU: lowest cost, three strings attached

A JADU is a separate home of up to 500 sq ft made from space inside a single-family house. It is the lowest range we publish because nothing new is built outside.

The strings:

• A recorded deed restriction — required by state law for every JADU.
• Owner-occupancy if the bathroom is shared. Since January 1, 2026 the owner must live on site only where the JADU shares a bathroom with the house. The shared bathroom is also the cheapest layout — so the cheapest JADU is the one that ties you to living there.
• No rentals under 30 days.

If those are acceptable — and for a parent or adult child they usually are — this is the answer. Full explanation: junior ADUs.

Route 2

The garage conversion: cheap until it isn't

A garage conversion keeps the slab, walls and roof. It is a full ADU, so there is no owner-occupancy condition, and San Rafael requires no parking space for a conversion.

What turns a low-range conversion into a high-range one:

• The slab. Garage floors slope and sit low. Raising and insulating one is routine; replacing a cracked one is not.
• Earth-moving. In Mill Valley, a conversion that involves more than 50 cubic yards of excavation falls out of the simple conversion path.
• Utilities. Distance from the garage to the sewer line and the electrical panel.
• Fire zone. In a high fire-severity zone, exterior work is built to Chapter 7A wildfire standards.

Ask each bidder to price the slab, the utility run and any fire-zone upgrades as separate lines. That is where conversion bids quietly differ.

Fees

Five ways to pay less in fees — without cutting the building

Fees are the smaller bucket, but they are the one you can lower with a decision instead of a compromise.

1. Stay under 750 sq ft. An ADU under 750 sq ft is exempt from impact fees under state law. From January 1, 2027, SB 1117 softens the cliff: a larger unit will pay impact fees only on the area above 750 sq ft — an 800 sq ft unit pays on 50 sq ft — for up to two ADUs per lot.

2. In unincorporated Marin, use the county waiver before it ends. Up to $2,500 per ADU and $1,500 per JADU in county planning and building fees, no income test, through December 31, 2026. It does not cover sewer, water or school fees. What has to happen by December 31 is not confirmed in the county's published material — ask Marin County CDA in writing. An extra $5,000 or $10,000 is available for a unit deed-restricted as affordable, reimbursed after the Certificate of Occupancy. Details: Marin ADU fee waiver.

3. In San Rafael, know the under-750 list. The city publishes the fees it waives for an ADU under 750 sq ft: impact fees, general plan maintenance, street maintenance and bedroom taxes. Guide: San Rafael ADU rules.

4. In Mill Valley, stay in the ministerial class. A new ADU of 800 sq ft or less, 16 ft or less, with 4-ft side and rear setbacks is approved without design review. Go larger or taller and you buy a design-review process: in a 2026 case, a 1,096 sq ft, 19 ft 11 in ADU needed exceptions and a public hearing (PL25-0096). The city's flat ADU permit fee is $1,204, plus $342 for the Southern Marin Fire District (2026 schedule). The median of all recorded fees on 12 standalone Mill Valley ADU permits was $17,812.56 — so the flat fee is the start of the fee bill, not the whole of it. More: Mill Valley ADU cost.

5. Start from a pre-approved plan. San Rafael runs a pre-approved ADU plans programme, and the countywide ADU Marin site (adumarin.org) lists pre-approved plans. A plan a building department has already reviewed can shorten design and plan check. It does not remove site-specific engineering on a slope. We have no measured saving to give you, so we do not give one.

Read the label

Are kits and prefab ADUs cheaper?

Sometimes — but compare the right numbers.

A kit or prefab price is the price of the unit. In Marin, the unit is only one of four buckets: the build, design and engineering, permits and fees, and utility and site work. A kit price typically leaves out the foundation, the utility trenching and connections, the permits, delivery and crane access, and the labour to put it together. On a narrow hillside street, getting a factory-built unit onto the lot can be the hard part.

That is why our Marin planning range for a prefab detached ADU ($250k–$450k) overlaps almost entirely with a site-built one ($300k–$500k). Prefab can save time and reduce surprises. On a flat, accessible lot it can save money. It is not automatically the cheap route, and on a difficult site it can be the dear one.

If you are comparing a kit quote with a builder's bid, ask the kit seller for an all-in number for your address in the same four buckets.

Marin's cost drivers

What makes a cheap ADU expensive here

1. Hillside. Engineered foundations, retaining walls, drainage. The single biggest reason a Marin project lands at the top of its range.
2. Septic, sewer and water. A septic system at capacity, or a long run to the sewer lateral, is priced before the building is.
3. Fire zone. Chapter 7A construction in high fire-severity zones.
4. Design-review towns. Leave the by-right envelope and you add months and consultant fees.

None of these shows up in a per-square-foot figure or a kit price. All of them show up on your lot. Screen for them before you fall in love with a floor plan.

Don't be misled

A warning about "permit value" numbers

You will see Marin ADU "costs" quoted from permit records. Be careful. A permit records a declared valuation — a figure entered for fee purposes — not what the owner paid.

In Mill Valley, the median declared valuation on 12 standalone ADU permits was $397,500. That is a declared valuation only. It is not a construction cost, it does not include design, fees or site work in any consistent way, and it should not be used as a budget. We publish it so you can recognise the figure when someone else misuses it.

False economies

What not to cut

• The site visit. A bid written from a floor plan alone is a range, not a price.
• The permit. San Rafael publishes a whole process for legalising unpermitted units built before 2020 — a sign that skipping the permit is a problem deferred, not a saving.
• The licence check. Five minutes at cslb.ca.gov. How to check a builder.
• The soils and septic answers. They are cheaper to learn before design than during construction.
• A grant that no longer exists. The CalHFA ADU Grant Program is closed. Do not build a budget around it.

Questions, answered

Cheapest ADU FAQ

Keep going

Related guides

Check your lot

Find the lowest-cost route your lot allows

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 to check — 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.