California ADU Law Changes for 2027: What Changes, What Ends, What Stays

Before and after

Today vs. January 1, 2027

AB 956

Two detached ADUs: what the law does and does not do

What it does. From January 1, 2027, a single-family lot may have two detached, new-construction ADUs by right. Each may be up to 800 sq ft, placed with 4-ft side and rear setbacks. The picture is the main house plus two separate backyard buildings.

What it trades. If two detached ADUs are built, the city is not required to also approve a JADU on that lot. So the real choice is not "two detached plus a junior unit". It is two detached homes, or a combination that keeps a JADU. Decide that on purpose, before drawings lock it in. The junior ADU guide covers what a JADU is and when it is the better unit.

What it does not do.

• It does not make two buildings fit. On a narrow, shallow, steep or septic-constrained lot, the arithmetic of the lot is still the blocker.
• It does not apply to conversions. The right is for two detached, new-construction units.
• It does not rewrite your town's published guidance on the day. As of November 2026, San Francisco Planning's Director's Bulletin No. 3 still states the one-detached rule, and Marin towns' ordinances pre-date the change. State law takes effect regardless — but expect local handouts to lag, and check the date on anything you are quoted.

The planning number. For Marin County, our planning range for two detached ADUs is $550k–$900k (as of Nov 2026 — a planning range, an estimate, not a bid). Shared site work softens the second unit's cost; it does not halve it. We publish no San Francisco-wide range, and Marin figures do not apply to San Francisco.

Local detail: two ADUs in Marin from 2027 · two detached ADUs in San Francisco from 2027.

SB 1117

The fee change, in square feet

From 2027, impact fees on an ADU over 750 sq ft will be charged only on the floor area above 750 sq ft, for up to two ADUs per lot.

• An 800 sq ft ADU will pay impact fees on 50 sq ft.
• A 760 sq ft ADU will pay on 10 sq ft.
• An ADU at or under 750 sq ft has nothing above the line for this to apply to — and an ADU under 750 sq ft is already exempt from impact fees under current state law.

We are not turning that into a dollar figure. Impact fees are set by each city, county and district, and a tidy example number would be invented. Two honest cautions instead:

1. Impact fees are one category. Plan-check, permit, utility-connection and school fees are separate, and this law does not remove them.
2. The size decision changes. Today, staying under 750 sq ft is often the most valuable measurement in an ADU design. From 2027, going to 800 sq ft will cost impact fees on 50 sq ft rather than on the whole unit. If you are designing now for a 2027 application, design to the 2027 rule.

What ends

December 31, 2026: Marin County's fee waiver

If your property is in unincorporated Marin County, the county waives up to $2,500 per ADU and $1,500 per JADU, with an additional $5,000 (moderate-income) or $10,000 (low-income) reimbursed later if you record a deed restriction. The programme window runs through December 31, 2026.

Two things to know before you plan around it:

• Scope. It does not apply inside an incorporated town. A Mill Valley or San Rafael address is not in the programme — and a town name in your mailing address does not settle which side of the line you are on.
• The deadline's trigger is not confirmed. The county's published programme states the amounts, the tiers and the end date. It does not state which event must happen by December 31 — an application, an issued permit, or something else. Ask Marin County CDA in writing before you rely on it.

Note the sequence: the waiver's window closes on December 31, 2026, and the two-detached right opens on January 1, 2027. As published, the two do not overlap. Whether a first ADU started under the waiver can be followed by a second detached unit in 2027 is a question for the County, not for a web page.

Full detail: Marin County ADU fee waiver.

Already in force

Changes you may have missed that are already law

JADU owner-occupancy (since January 1, 2026). State law now requires owner-occupancy for a JADU only where the JADU shares a bathroom with the main home. A JADU with its own bathroom does not carry that requirement. A recorded deed restriction is still required for every JADU.

Selling an ADU separately (AB 1033). Only where a city has opted in. San Francisco has — under Ordinance 113-25, effective August 10, 2025 — with a strict test: the ADU application was submitted on or after May 1, 2025; it was permitted under the state programme (Planning Code §207.2); the lot has four units or fewer; one ADU per lot; and JADUs are excluded. See selling a San Francisco ADU separately.

San Francisco ceiling heights (from December 5, 2026). Ordinance 234-26 lowers the Housing Code minimum in existing buildings to 7 ft for habitable rooms and 6 ft 8 in for other rooms. For many San Francisco basements and garage levels, that is the difference between excavating and not. See San Francisco basement conversions.

What stays

What 2027 does not change

• The 800 sq ft unit. State law already guarantees a detached ADU of up to 800 sq ft with 4-ft side and rear setbacks, regardless of local lot-coverage, floor-area or open-space limits. That stays. AB 956 adds a second one.
• The 60-day decision clock. A complete ADU application still gets a ministerial decision on the state's 60-day clock. (The calendar from first application to issued permit has run longer in practice, because corrections and resubmittals sit outside that clock: Marin permit records show a median of 186 days from application to issuance in Mill Valley across 12 permits, and 205 days in unincorporated Marin across 189.)
• No short-term rentals. An ADU or JADU may not be rented for under 30 days.
• JADU size. A JADU is up to 500 sq ft, inside the single-family home.
• Your town's own layer. Size caps above 800 sq ft, design review above the ministerial class, and local fee schedules are still local. Mill Valley's general cap is 1,000 sq ft; San Rafael's detached cap is 1,000 sq ft under Ordinance 2002.

Money, honestly

One programme people still search for is closed

The CalHFA ADU Grant Program is closed. People still search for a 2026 round; as of November 2026 there is not one open. If a lender or builder offers to "get you the CalHFA grant", ask them to show you the open application on calhfa.ca.gov.

What is real in 2026–2027: the under-750 sq ft impact-fee exemption (now), the SB 1117 change (from January 1, 2027), and — in unincorporated Marin only — the county waiver (through December 31, 2026).

Be ready

What to do before January 1

1. Find out what your lot can take today. The 2027 right adds a second detached unit; it does not change whether the first one fits. Run the check below.
2. Decide one unit or two — and the JADU trade — on purpose. That is a household decision before it is a design decision.
3. If you are in unincorporated Marin, settle the waiver question in writing with Marin County CDA now, not in the last week of December.
4. Design to the rule you will apply under. 2026 application, 2026 rules. 2027 application, 800 sq ft and the above-750 fee basis.
5. Check the date on everything. City handouts, builder blogs and older guides will describe the one-detached rule for months after it changes.

Questions, answered

California ADU law 2027 — FAQ

Keep going

Related guides

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