San Anselmo ADU contractor — a few builds in, here’s what the town taught us.

Step one in San Anselmo is a code check: the town’s ADU chapter (Municipal Code Title 10, Chapter 6) sorts projects into those exempt from an ADU Permit — which apply directly to the Building Department, online, with zero planning fees — and those that need an administrative ADU Permit from Planning first. The non-exempt track brings the town’s objective design standards into play: building design, landscaping, entrances, window and door treatment, exterior lighting, even skylights, with extra layers for historic-register properties and heritage trees. Still ministerial, still no hearings — but a real review with real standards.

Timelines are honest here. The state’s 60-day clock governs ADU decisions, the town’s own materials describe four to six weeks for full building-permit review with two to three weeks for resubmittals, and Ross Valley Fire targets ten business days for its plan review. San Anselmo also runs a pre-approved detached ADU program: plans already approved for the town get site-specific review in 30 days.

If your ADU replaces a detached garage, note the paperwork detail that trips people: the garage needs its own demolition permit, and a converted detached garage is processed as an ADU, not a JADU.

San Anselmo’s defining constraint is water. Much of the town — including downtown — sits in FEMA flood zones along San Anselmo Creek, which has flooded seriously for a century; the 2005 New Year’s Eve flood put four feet of water down San Anselmo Avenue. The town takes floodplain rules accordingly seriously, and ADUs are fully subject to them.

Practically, that means three things. In the AE zone, there’s a cumulative “substantial improvement” rule: if all work over a five-year window reaches 50% of the structure’s value, the entire project must meet full FEMA standards — so an ADU on a property that recently had a kitchen remodel can inherit flood-compliance obligations nobody expected. Below the design flood elevation, only flood-resistant materials are allowed, with elevation certificates at submittal and before framing inspection. And any building within 15 feet of the creek needs a watercourse permit, potentially with state and federal resource agencies involved.

None of this makes flood-zone ADUs impossible — it makes them projects you design from the flood map outward. The height rule even adapts: in the flood zone, a detached ADU’s height is measured from the base flood elevation plus a foot of freeboard, so elevating the unit doesn’t cost you your ceiling height.

On the hill sides of town, the numbers to know are 25 and 100. Grading beyond 25 cubic yards triggers a grading permit with construction-management and erosion-control plans; beyond 100 cubic yards, the project goes to the Planning Commission — the one door in the ADU process that leads to discretionary review. We run cut-and-fill estimates early on sloped lots specifically to stay on the right side of those lines. Steeper properties also need contoured plans and, typically, a soils report and stamped structural calculations.

Fire is Ross Valley Fire Department’s domain, and two of its rules matter most: sprinklers are required in new residential buildings in San Anselmo (state ADU law limits sprinkler mandates where the main house doesn’t have them — RVFD makes the call per project, so we confirm early), and substantial remodels in the wildland-urban interface need a vegetation management plan. If the new unit lands more than 350 feet from a hydrant, hydrant work can enter the scope — rare, but worth checking on the long flag lots.

Town fees stack like this: a valuation-based building permit, plan check at 65% of it (covering two review rounds — additional rounds bill hourly, one more reason clean submittals pay), a 1.5% road impact fee that estimators routinely miss, and a 10% general-plan surcharge on planning fees. We price the full permit stack into every proposal rather than leaning on published averages. School district fees apply on added square footage; state law spares ADUs under 500 square feet.

The sewer bill is where design choices show up directly. Ross Valley Sanitary District charges a $513 ADU permit across the board, but capacity charges depend on type: conversions within existing structures pay none, while new construction pays $335 per drainage fixture unit — meaning every additional sink, shower, and laundry hookup literally moves the fee. A new detached ADU connects through your existing private lateral rather than a new main connection, which keeps costs contained. Water is Marin Water’s call, and we confirm connection requirements per address.

Two town-wide requirements to budget from the start: utilities to new buildings go underground, and any project with a new structure needs a property-line survey. An ADU deed-restricted as affordable housing for 55 years gets its building permit fee waived entirely — niche, but real for the right owner.