Triple-pane, six-foot gaps, coverage caps: extras a 66323 ADU doesn't owe

You're designing a detached ADU for the back yard — call it 700 square feet, four-foot setbacks, by the book. Then the city's review comes back with extras. The windows near the street have to be tempered triple-pane glass. You have to leave a six-foot gap between the new unit and the house. And before any of it, you have to prove a "regular" ADU couldn't be built on the lot first. Each one costs money, eats buildable space, or adds a hoop that can stall the permit.

Here's the thing: if your unit qualifies as a state-exemption ADU under Government Code § 66323, HCD's position is that those add-ons can't be applied to it at all.

One section, one closed list of standards

Section 66323 is the by-right track. A qualifying unit — for example, a new detached ADU up to 800 square feet of livable space — must be approved ministerially, and the city's power to pile on standards is capped by subdivision (b). HCD quoted the rule verbatim in its letter to Sausalito:

"A local agency shall not impose any objective development or design standard that is not authorized by this section upon any unit that meets the requirements of any of paragraphs (1) to (4), inclusive, of subdivision (a)."

That's a short, closed list. Anything not on it — window specs, a structure-separation gap, vision clearance, a lot-coverage cap, a prove-it-first gate — can't ride along on a 66323 unit. Across three 2026 letters, HCD told three cities exactly that.

Paramount: triple-pane windows and a six-foot gap

Paramount's ordinance required, for windows within 10 feet of the front line, "tempered triple-pane glass windows with a Sound Transmission Class rating of 29 or more." It also required "A minimum six-foot separation… between any detached ADU and the existing single-family dwelling." On the windows, HCD was blunt:

"the City must amend the Ordinance to clarify that tempered triple-pane glass windows are not required for ADUs created under Government Code section 66323."

Verified against HCD's Paramount ADU Ordinance Findings Letter, dated July 1, 2026.

Now the nuance that matters most: HCD did not call the six-foot gap illegal across the board. On the separation rule it wrote that if the gap is codified in the local building code, "the City is permitted to impose it only on ADUs constructed pursuant to Government Code 66314" — the regular, local-standards pathway. The same add-on a 66323 unit doesn't owe can still be lawful against a 66314 ADU.

Sausalito: a coverage cap that can't reach a 66323 unit

Sausalito flagged a proposed detached ADU — under 800 square feet of livable space, so a 66323(a)(2) unit — as incomplete partly for missing a building-coverage diagram. HCD said the city may ask for the diagram to judge completeness, but drew a hard line on using it:

"Maximum building coverage is not one of the development standards authorized for ADUs under Government Code section 66323."

Once the application is complete, HCD wrote, the city "must take care not to apply development standards prohibited by Government Code section 66323, such as maximum building coverage, to an ADU which is exempt from those standards, including the proposed detached ADU." That finding is in HCD's Sausalito ADU Technical Assistance Letter, dated August 12, 2026.

San Carlos: the prove-it-can't-be-a-regular-ADU gate

San Carlos went a step further and built a gate: before an applicant could use the exemption standards, "The applicant must also demonstrate that an accessory dwelling unit cannot be constructed in accordance with applicable development standards." HCD rejected the gate in its San Carlos ADU Ordinance Findings Letter, dated May 6, 2026:

"local agencies must ministerially approve units constructed under Government Code section 66323 regardless of the ability or inability to meet objective design standards."

Translation: a city can't make you prove a 66314 ADU is impossible before it will honor your 66323 unit.

What it means for your lot

If the extras line — triple-pane glass, a separation gap, vision clearance, a coverage cap, a prove-no-alternative test — lands on a unit that qualifies under § 66323, HCD's position is that the barrier may not be enforceable against that unit. Two cautions, stated plainly. First, these letters are HCD's interpretive position, not settled law — courts are the final arbiter, and a city can answer with its own findings instead of amending. Second, the very standard that can't touch your 66323 unit may still be lawful on a 66314 ADU. Read your own ordinance against § 66323, and verify your specific lot with your jurisdiction and your own counsel before you rely on any of it. None of this is legal advice.

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.