Two Backyard Homes on One Marin Lot: What Changes on January 1, 2027

Before and after

2026 vs. 2027 on a Marin single-family lot

This is a summary of a law change. It is not a finding about your parcel.

The trade-off

Two detached ADUs replace the JADU path — they do not stack on it

The headline version is "three extra homes". The law does not say that. If two detached ADUs are built on the lot, the town is not required to also approve a JADU there.

So the real choice in Marin looks like this:

Option B is not a consolation prize. On a sloped lot where a second foundation is the expensive part, a JADU inside the existing house can be the second unit that actually gets built. Do not budget, promise family, or plan rent on a combination the town is not required to approve.

More on the smaller unit: junior ADUs, explained.

Will it fit

The Marin question: can your lot physically carry two?

Two 800 sq ft buildings, 4-ft setbacks on the sides and rear, and room left to live in. In much of Marin the lot decides before the law does.

Slope. A second detached unit means a second foundation. On a hillside that can mean a second set of engineered footings, retaining and drainage — the main reason a Marin project lands at the top of its range. In Mill Valley, a 2026 staff report shows what happens when a design leaves the standard envelope: a proposed 1,096 sq ft ADU at 19 ft 11 in needed exceptions on size, floor area and height and went to a public hearing (case PL25-0096, June 23, 2026). Two by-right units at 800 sq ft each avoid that path only if they stay inside the envelope.

Septic. Much of unincorporated Marin is on septic. Two added homes are two more households on the system. Capacity, and where the leach field sits, can decide the question before any drawing is made. Get the septic answer for your address first.

Fire zone. In a high fire-severity zone, both units are built to Chapter 7A wildfire standards. That shapes materials and budget for each building.

Access and utilities. Two units need two utility runs and construction access to the back of the lot. A long, narrow or flag lot can be legal for two and impractical for two.

Where two is realistic: flat, deep, sewer-connected lots with clear access to the rear. Where it usually is not: steep hillside lots, small in-town lots, and septic lots already near capacity. We cannot promise either way from a page. The check at the bottom sorts your address into one group or the other as a first screen.

Town by town

The local layer — what we have verified, and what we have not

State law sets the two-unit right. Your jurisdiction still runs the permit. As of November 2026, we have not located a Marin town or county ordinance updated for AB 956 — local codes and handouts still describe one detached ADU. The state right takes effect on January 1, 2027 regardless; expect local paperwork to lag behind it.

Mill Valley

• The city's ministerial class already matches the AB 956 envelope: 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. Above that, design review applies — the general cap is 1,000 sq ft, or 850 sq ft for a studio or one-bedroom.
• Ordinance status: the state housing department (HCD) told the city on April 9, 2025 that its ADU ordinance (Ordinance 1330) is not compliant with state ADU law, and noted the city's stated intent to process ADU applications under state law instead. Read as of Nov 5, 2026.
• Deed restriction — two layers, both true today. The city requires a recorded deed restriction for an ADU in practice. HCD's position is that state law does not allow a city to require one for an ADU. (A JADU is different: state law itself requires a deed restriction.) Ask the city in writing which it will apply to your application.
• Permit wait on the record: a median of 186 days from application to issuance, across 12 standalone ADU permits. Fees on the record: a median of $17,812.56 in total recorded fees on those 12 permits — per unit permitted, not per lot.
• Guide: Mill Valley ADU rules · Mill Valley ADU cost.

San Rafael

• The ordinance of record (SRMC §14.16.285, Ordinance 2002, adopted December 6, 2021) caps a detached ADU at 1,000 sq ft. The state's 800 sq ft unit is unaffected by local lot-coverage and floor-area limits — and that is the unit AB 956 doubles.
• Ordinance status: HCD sent the city a Letter of Technical Assistance on February 27, 2026 stating its 2021 ordinance "may be outdated and out of compliance". Read as of Nov 5, 2026.
• Fees: under 750 sq ft, the city waives a published list of fees — impact fees, general plan maintenance, street maintenance and bedroom taxes. From 2027, SB 1117 changes the impact-fee basis for units over 750 sq ft.
• Guide: San Rafael ADU rules.

Unincorporated Marin County

• Your permit is a county permit. Septic and slope are the first two questions on most lots.
• Permit wait on the record: a median of 205 days from application to issuance, across 189 permits.
• Fee waiver: up to $2,500 per ADU and $1,500 per JADU through December 31, 2026 — see the next section.
• Guide: Marin County ADU fee waiver.

Novato, Tiburon, Larkspur, San Anselmo and the other towns

We have not published a verified guide for these towns yet, and we do not fill the gap with guesses. AB 956 applies there as state law. For the local layer, ask the planning counter one question in writing: "How will you process an application for two detached ADUs under Government Code §66323(a)(2) from January 1, 2027?"

Two clocks

The fee waiver ends the day before the two-ADU right begins

If you are in unincorporated Marin, two dates sit side by side:

• December 31, 2026 — the county fee waiver's window closes. Up to $2,500 per ADU, $1,500 per JADU; an additional $5,000 or $10,000 is reimbursed later for a deed-restricted unit.
• January 1, 2027 — the two-detached right, and the SB 1117 fee basis, begin.

As published, those windows do not overlap. Two honest unknowns:

1. What must happen by December 31 is not confirmed. The county's published programme does not say whether the qualifying event is an application, an issued permit or something else. Ask Marin County CDA in writing.
2. Whether a first ADU started under the waiver can be followed by a second detached unit in 2027 is the County's call. Ask that in the same letter.

If the Board of Supervisors extends the waiver, both answers may change. It extended the programme once, on March 11, 2025. That is not a promise it will again.

Inside an incorporated town, none of this applies: the county waiver was never available there.

Be ready

What to do in 2026

1. Screen the lot first. Slope, septic or sewer, fire zone, and what is already built. If your lot is a one-unit lot, finding out now saves the design fee on a second building.
2. Decide the JADU trade on purpose. Option A or Option B above is a family decision before it is a drawing.
3. Design to the known envelope. 800 sq ft and 4-ft setbacks per unit are fixed in the law now. Design work done against them in 2026 is a head start.
4. In unincorporated Marin, write to CDA with the two questions above before December.
5. Price it with the same bid structure for both units — four buckets, your lot's drivers as separate lines. The Marin builders guide has the list.

Questions, answered

Two ADUs in Marin — FAQ

Keep going

Related guides

Check your lot

Is yours a two-ADU lot?

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