Junior ADU (JADU): The In-Law Unit Inside Your House

The difference

JADU vs. ADU: what you give up and what you get

A JADU has its own entrance from outside and at least an efficiency kitchen. Ask your planning counter what it will accept for each before you draw anything.

The money

What a JADU costs in Marin — and why the range is wide

Our Marin planning range for a JADU is $80k–$180k, as of November 2026. That is an estimate for planning, not a bid, and it is a Marin County figure only.

It is the lowest range we publish because the expensive parts of a new building already exist: foundation, walls, roof, and a utility connection. What moves a JADU from the bottom of the range to the top:

• The bathroom. Sharing the house's bathroom is the low-cost route — and it is also what triggers the owner-occupancy rule. Adding a bathroom costs more and removes that condition.
• The kitchen. New plumbing and electrical runs to a room that never had them.
• The separate entrance. A new exterior door is simple on a ground-floor room and less simple on a hillside lower level.
• What the space was. A finished bedroom wing converts more cheaply than an attached garage that needs insulation, a raised floor and fire separation.
• Hillside and age of house. Older Marin houses can need electrical or structural upgrades once walls are opened.

For every other type and the four cost buckets a bid should show, see ADU cost in Marin County. For the lowest-cost routes ranked, see the cheapest way to build an ADU in Marin.

The fine print

The three conditions that come with a JADU

1. A deed restriction, recorded against your property. State law requires it for every JADU. It runs with the land and tells a future buyer what the unit is. This is the one place the JADU rule is stricter than the ADU rule — for a full ADU, the state housing department's position is that a city may not require a deed restriction at all.

2. Owner-occupancy, in one case. Since January 1, 2026, the owner must live on the property only where the JADU shares a bathroom with the main house. Give the JADU its own bathroom and that condition falls away. If you plan to move and rent out both the house and the unit one day, this decides your bathroom layout now.

3. No rentals shorter than 30 days. A JADU is long-term housing. It is not a short-stay rental.

Where you are

The local layer: Marin and San Francisco

Unincorporated Marin County. The county's fee waiver covers JADUs: up to $1,500 per JADU in county building and planning fees, with no income test, through December 31, 2026. It is applied by staff at permit stage and requires your affirmation that the unit will not be a short-term rental. What must happen by December 31 — an application, an issued permit, or something else — is not confirmed in the county's published material; ask Marin County CDA in writing. The waiver applies only to unincorporated addresses. A postal address that says "Mill Valley" or "San Rafael" can still be unincorporated — check before you assume either way. Details: Marin ADU fee waiver.

Mill Valley. A JADU of up to 500 sq ft inside the home, with the state-law deed restriction. The city's own ADU ordinance is under a state non-compliance finding (HCD letter, April 9, 2025) and the city has said it will process ADU applications under state law. Guide: Mill Valley ADU rules.

San Rafael. JADUs are covered by Municipal Code §14.16.286: up to 500 sq ft. No parking space is required for a JADU. Guide: San Rafael ADU rules.

San Francisco. State JADU law applies. Two things to know: a JADU cannot be sold separately under San Francisco's AB 1033 condo programme, and we do not publish a San Francisco JADU cost range. If your San Francisco plan is a garage or lower-level conversion, the fuller unit types are covered in the basement conversion and Sunset District guides.

Other towns. We have not published a verified guide yet and will not guess. State JADU law applies; ask the planning counter for its JADU handout.

From 2027

The 2027 trade-off: two backyard ADUs, or a JADU

From January 1, 2027, AB 956 will let a single-family lot have two detached new-construction ADUs by right. The trade: if two detached ADUs are built, the town is not required to also approve a JADU on that lot.

That makes the JADU decision part of a bigger one:

• One detached ADU + a JADU — available today. One household across the yard, one under your roof.
• Two detached ADUs — from 2027. Two fully separate households, a much larger budget (Marin planning range $550k–$900k, as of Nov 2026 — an estimate, not a bid), and no assurance of a JADU on top.

If you already have, or plan, a JADU, do not assume a second detached unit can be added later. More: two ADUs on one Marin lot from 2027 · all 2027 law changes.

Is it the right one

When a JADU fits — and when it does not

A JADU usually fits when:

• the person moving in is family, and sharing a wall is a feature, not a problem;
• you have a bedroom wing, attached garage or lower level you are not using;
• the yard is steep, small, or taken up by a septic field;
• the budget is closer to the low hundreds of thousands than the mid.

Look at a full ADU instead when:

• you need more than 500 sq ft, or two bedrooms;
• you want a rental with no owner-occupancy condition and no recorded deed restriction;
• you may want to sell the unit separately one day, where that is allowed;
• sound and privacy between the two households matter more than cost.

Questions, answered

Junior ADU FAQ

Keep going

Related guides

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