Why Your ADU Only Needs Four-Foot Side and Rear Setbacks

You've got the backyard and the plan: a detached ADU tucked near the back fence, four feet off the side and rear lines — the tight footprint that makes the whole thing pencil out. Then you open your city's ADU code and hit the line: an ADU has to meet the same yard setbacks as everything else in the zone — five feet, ten feet, sometimes more. Your buildable box just shrank.

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

What state law actually caps

Two statutes do the work. Government Code § 66314(d)(7) says that a setback of no more than four feet from the side and rear lot lines may be required for a new-construction ADU — and no setback at all for a structure converted from, or rebuilt to the same footprint as, an existing structure. Government Code § 66321(b)(3) then blocks the front-setback squeeze: no lot-coverage, floor-area-ratio, open-space, front-setback, or minimum-lot-size rule can be written so that it does not "permit at least an 800 square foot accessory dwelling unit with four-foot side and rear yard setbacks."

So the zone's five- or ten-foot side-and-rear setback doesn't ride along onto your ADU, and a front setback can't be set so deep it blocks an 800-square-foot unit.

San Pablo: the copied line

Reviewing San Pablo's draft ADU ordinance, HCD went straight to the yard section. Section 17.60.070 F.2.a. told homeowners an ADU "must comply with all requirements relating to yards (front setbacks, side, and rear) and building height that are generally applicable to residential construction in the zone in which the property is located." HCD's position was blunt: § 66321(b)(3) bars any setback rule "that does not permit at least an 800 square foot accessory dwelling unit with four-foot side and rear yard setbacks." HCD told the city to "remove any set back greater than four-foot side and rear yard."

Verified against HCD's ADU Ordinance Findings letter to the City of San Pablo, dated May 20, 2026 (Updated Draft Ordinance Review, Finding 4, Section 17.60.070 F.2.a.).

Same line, different cities

This isn't one city's drafting slip. HCD has flagged the same move in ordinance after ordinance.

• Belmont required an ADU to "conform to all requirements of the underlying residential zoning district ... including height, setback, floor area ratio, and historic preservation requirements." HCD measured that against § 66315, which bars standards beyond those § 66314 allows.
• Chowchilla set the front yard "as established in the applicable zone district." HCD's reply: § 66321(b)(3) "prohibits front setback requirements" for either kind of ADU that don't allow an 800-square-foot unit "with four-foot side and rear setbacks."
• San Diego proposed five-foot setbacks for ADUs inside Very High Fire Hazard Severity Zones. HCD's position: § 66314(d)(7) "requires only four-foot side and rear setbacks for all ADUs," and fire separation distance "cannot be conflated with setbacks."

Different wording, one pattern HCD keeps catching: the zone's bigger setback quietly bolted onto an ADU where the statute caps the side and rear at four feet.

What it means for your project

If your city is holding your ADU to the zone's five-, ten-, or fifteen-foot setback, HCD's position is that the four-foot side-and-rear cap controls — and a front setback can't be written so as to block an 800-square-foot unit.

Two cautions before you lean on it. First, these are HCD letters — HCD's reading of the statute, not a court ruling; a city can adopt findings and push back. Second, the details matter: converted structures, fire-separation rules under the building code, and units approved under § 66323 each carry their own wrinkles. So confirm the specifics with your building department or a land-use attorney before you redraw plans — this isn't legal advice. But the four-foot floor is written into statewide statute, and the zone-setback line in your local code may not hold 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.