A Danville ADU builder’s guide to the town’s brand-new rulebook.

In November 2025 Danville repealed and replaced its ADU chapter outright after the state housing department flagged the old one. The practical upshot for homeowners: the current rules are cleaner and more generous than what most contractor websites still describe, and staff apply them as written. Review is ministerial and staff-level only — the town approves or denies within 60 days of a complete application, with no neighbor hearings.

The process itself is small-town straightforward: an optional sit-down with Planning first (worth taking — it’s how easements and grading questions surface early), then a building-permit application through the town’s online portal. One detail that surprises people: if any setback in your design is under five feet, the town requires a lot survey, so budget for one on tight side yards.

A single-family lot can hold more than one unit: an ADU plus a JADU, or — the full state combination — an interior conversion ADU, a detached ADU up to 800 square feet, and a JADU. Owner-occupancy is not required for a standard ADU, rentals must run 30 days or longer, and the town has also offered free pre-checked “Garden Cottage” plan sets for detached units; the program’s plan downloads were in flux when we last checked, so ask Planning about current availability.

Detached ADUs cap at 1,200 square feet in most of town and 2,000 square feet in the estate districts zoned for 40,000-square-foot-plus lots — the R-40, R-65, and R-100 neighborhoods where Danville’s largest parcels sit. Attached ADUs follow the same caps plus a half-the-main-house rule that can never push you below 850 square feet (1,000 for two-plus bedrooms). There’s no bedroom limit and no minimum size.

Then come the bonuses. A detached ADU can include a two-car garage up to 20 by 22 feet that does not count toward its square footage, and a non-conditioned basement up to half the unit’s conditioned area doesn’t count either. On a big lot, that pencils out to a genuinely substantial second home. Setbacks are 4 feet at the sides and rear — measured to the eaves, not the walls, which catches people — with the normal front setback for your district. Conversions of existing structures skip the size, height, and setback rules entirely and can add up to 150 square feet for entry and egress.

Design standards are simple by East Bay standards: the ADU uses the same paint colors, siding, and roof materials as the main house. Units within 10 feet of a side or rear line can’t have raised decks or balconies facing that line. And the 800-square-foot unit is the escape hatch — at or under 800 square feet, an ADU is exempt from every local development standard beyond the short state list, including the matching rule.

Three agencies sign off alongside the town: San Ramon Valley Fire reviews the plans, EBMUD verifies water supply, and Central San handles sewer. Central San’s numbers are homeowner-friendly — a capacity fee of $3.63 per square foot charged on at most 1,200 square feet, so under $3,000 for an 800-square-foot unit against $8,539 for a new house. Its one hard piping rule: a detached ADU’s sewer line connects to your lateral downstream of the main cleanout, never back through the house plumbing. Central San also reviews on paper only and quotes 10 to 25 business days with no expedite option, so we start that clock early.

School fees go to San Ramon Valley Unified at $5.17 per square foot, with units of 500 square feet or less exempt — and as of July 2026 the district requires a signed certificate of compliance from the town before you can even book the payment appointment. State law bars impact fees entirely on ADUs under 750 square feet, and larger units pay proportionately.

The timing hook: the Senate District 7 ADU Accelerator rebate — up to $7,500 for a standard unit, $15,000 for one deed-restricted affordable, nothing for units over 1,000 square feet — runs only through September 30, 2026. If your project is close to permit-ready, it’s worth asking the town whether you can still qualify before the window closes.

Danville’s flat valley floor builds are straightforward; its hillsides are where experience earns its keep. Any ADU on a slope or needing grading routes through the town’s Engineering Division, and the real work is geotechnical — our Danville ADU went in on a drilled-pier raised foundation, coordinated with soils and structural engineers, precisely because the lot demanded it. If your site slopes, plan for engineering from day one rather than treating it as a surprise in plan check.

Fire is the other overlay. The 2025 state fire-hazard maps put parts of Danville — especially toward Las Trampas on the west side — in high and very-high severity zones, and the town confirms new ADUs there must meet the building code’s fire-hardening standards. Sprinklers follow the main house: required in the ADU only if the primary home has them. Check your parcel on San Ramon Valley Fire’s zone map before designing.

Finally, the question we hear most in Danville: “my HOA says no.” The town’s own handout answers it — homeowners associations may not prohibit ADUs, and both state law and Danville’s ordinance void CC&R provisions that effectively prohibit or unreasonably restrict them. HOAs can hold reasonable input; they cannot veto the project.