Your City Can't Cap Your ADU Below 850 Square Feet

You've got a backyard, a budget, and a plan for a real one-bedroom ADU — something in the 850- to 900-square-foot range, with a proper kitchen and a bedroom that fits a bed and a closet. Then you open your city's ADU code and hit the ceiling: 800 square feet, or 50 percent of your main house, whichever comes out smaller. Your design just lost a room.

Here's the thing: HCD has told city after city that a cap like that is more restrictive than state law — and has to be fixed.

The floor state law sets

Two parts of the Government Code work together here. Section 66321(b)(2) sets the maximum size a city has to allow: at least 850 square feet of interior livable space for a studio or one-bedroom ADU, and 1,000 square feet for a unit with more than one bedroom. Section 66321(b)(3) then blocks the workaround — no lot-coverage, floor-area-ratio, open-space, front-setback, or minimum-lot-size rule, and no size "based upon a percentage of the proposed or existing primary dwelling," can be written so that it doesn't "permit at least an 800 square foot accessory dwelling unit with four-foot side and rear yard setbacks". A local cap that dips under those numbers is what HCD keeps catching.

Gardena: an 800-square-foot cap on a detached unit

Reviewing Gardena's ADU Ordinance No. 1883, HCD went straight to the size section. The ordinance said a detached single-family ADU "shall not be more than eight hundred square feet." HCD's reply was blunt:

Moreover, "eight hundred square feet" is more restrictive than State ADU Law; therefore, the Ordinance must be amended to precisely reflect state law.

HCD pointed to the statute — Section 66321(b)(2) "authorizes 850 square feet of interior livable space, or 1,000 square feet of interior livable space for an ADU with more than one bedroom" — the sizes the 800-square-foot cap fell below. HCD also flagged that Gardena measured size by "floor area" instead of interior livable space — a separate way the usable number can come out smaller than the law allows.

Verified against HCD's ADU Ordinance Findings letter to the City of Gardena, dated May 8, 2026 (Finding 5, Sections 18.13.050 E.1. and 18.13.060 A.2.).

Same move, different cities

This isn't one city's typo. HCD has flagged the same size problem in ordinance after ordinance, in two recurring flavors.

The flat number below 850:

• San Pablo capped a detached ADU at "eight hundred square feet or less of interior livable space". HCD said the maximum "may be no less than 850 square feet or 1,000 square feet for units with more than one bedroom".
• Palmdale wrote that detached ADUs on multifamily lots "shall be 800 square feet each." HCD told the city to "remove the restriction to 800 square feet".

The percentage of the main house:

• Malibu limited an attached ADU to "50 percent of the existing primary dwelling". HCD: "Restricting the size of attached ADUs to less than 800 square feet directly conflicts with Government Code section 66321, subdivision (b)(3)."
• Needles said an attached ADU's floor area "shall not exceed 50 percent of the existing Living Area". HCD flagged it under the same subdivision.

Different wording, one result HCD reads as illegal: a hard number under 850, or a percentage of the primary dwelling that quietly does the same thing.

What it means for your project

If your ADU is getting squeezed below the state floor — 850 or 1,000 square feet for a flat size cap, or the 800-square-foot unit that a percent-of-primary, lot-coverage, FAR, or open-space rule can't undercut — HCD's position is that the floor controls, not the local number.

Two cautions before you rely on it. First, these are HCD enforcement letters, not court rulings; HCD is stating its interpretation, and a city can adopt findings and push back. Second, the size math turns on unit type and on how "interior livable space" gets measured. So check the specifics with your building department or a land-use attorney before you redraw plans — this isn't legal advice. But the floor is written into statewide statute, and the cap printed in your local code may not be enforceable against your project.

What CrossBeam does with it

When the letter lands on your desk, CrossBeam turns it around the same day: what each item is actually asking, the code or law behind it, and a written response you can hand to your team or your attorney — the back-and-forth that used to eat weeks, done in a couple of hours.