The Numbers That Govern Your ADU Project in San Mateo County
California state law sets the floor for ADU rules — but your parcel determines what actually applies.
Under California Government Code Section 65852.2, detached ADUs must maintain at minimum a 4-foot setback from rear and side property lines. Most detached ADUs are capped at 16 feet in height, and the maximum size for a detached ADU is 1,200 square feet. These are state-mandated minimums and maximums. Local jurisdictions — including San Mateo County and its incorporated cities — can allow more, but cannot restrict less for qualifying ADU projects.
Why These Rules Play Out Differently Across San Mateo County
San Mateo County isn't one jurisdiction — it's 20 incorporated cities plus unincorporated county land, each with its own zoning overlay. The setback number on a general California ADU guide may not be the number that governs your specific parcel.
A property in unincorporated county follows county zoning layered on state law; inside Redwood City it follows Redwood City's municipal code; in Menlo Park, Daly City, or San Mateo it follows each city's own ordinance where it doesn't conflict with state law. AB 68 and later legislation created a strong floor of rights — but lot coverage limits, height allowances above the state minimum, and setback interpretations for attached ADUs all vary by jurisdiction and parcel.
Applying a generic "San Mateo County ADU rule" to your lot can mean designing a unit that doesn't fit your parcel. The rule that governs your neighbor's property three blocks away may not govern yours — if those blocks cross a city boundary.
Setbacks, Height, Size, Lot Coverage & Approval
Government Code Section 65852.2 defines six categories of rules that govern ADU construction. Here's what each means for San Mateo County homeowners.
Setback requirements
Qualifying new detached ADUs keep a minimum 4-foot rear and side setback — superseding more restrictive local rules. Attached ADUs follow the primary dwelling's setbacks, which can be stricter. Front-yard setbacks are set by local code.
Height limits
The baseline for most detached ADUs is 16 feet, rising to 18 feet within one-half mile of a major transit stop or when built above a garage. Several county cities sit within a half-mile of Caltrain — so some parcels qualify for 18 feet.
Maximum ADU size
Detached ADUs cap at 1,200 sq ft; attached at the lesser of 50% of the primary dwelling or 1,200 sq ft; JADUs at 500 sq ft. Local jurisdictions can't impose a lower cap — but setbacks or lot coverage may constrain you below these maximums.
Lot coverage
The percentage of a parcel occupied by all structures combined. Each city sets its own maximum by zone. Near the limit, lot coverage can become the binding constraint on ADU size or placement — even when setbacks are satisfied.
Owner-occupancy
AB 3182 removed local jurisdictions' ability to require owner-occupancy as a condition of ADU approval through 2025. Both the primary dwelling and the ADU may be rented simultaneously.
By-right approval
No public hearing, no neighbor notification, no discretionary vote — the building department reviews only objective standards. Projects needing variances, on historic properties, or with non-conforming lots may not qualify for full ministerial review.
Three Real Parcel Situations Where These Rules Changed the Project
17-foot design qualified
A parcel within one-half mile of a Caltrain station designed at 17 feet for a functional second-floor ceiling. Under the standard 16-foot limit it wouldn't pass; under the transit-proximity provision it qualified — once the measured distance was confirmed before design.
1,200 sf planned, 420 sf allowed
A 6,000 sq ft lot in a 45%-coverage zone was already at 38% between the dwelling and garage — leaving room for a 420-square-foot ADU, not 1,200. The lot coverage calculation, not the state size cap, controlled the outcome.
Attached setback forced a redesign
An addition off the back of the primary dwelling was an attached ADU, so the primary structure's setbacks applied — not the 4-foot detached minimum. A 15-foot rear setback meant the addition had to be redesigned to stay within the existing footprint.
What We Confirm Before Any Design Work Starts
"How big can an ADU be?" has no single answer here — it's a parcel-specific one. It depends on which jurisdiction the property sits in, what zone applies, how much lot coverage is already committed, and whether the configuration is detached, attached, or a conversion. State law gives strong rights; exercising them correctly means knowing exactly which provisions apply to your lot.
Our in-house team is familiar with the building departments across the county — Redwood City, Menlo Park, San Mateo, Daly City, and unincorporated parcels — and we've navigated correction cycles across multiple jurisdictions. We know which cities have adopted local ADU ordinances that expand beyond state minimums, and which haven't yet updated their published codes. That knowledge doesn't cost extra — it's part of how we build.
CA License #1119304 — we operate across San Mateo County.
When Your ADU Project Needs Parcel-Specific Regulatory Review
A regulatory review makes sense at the start of planning — before any design money is committed. A parcel review clarifies which setback, height, size, and lot coverage rules apply to your property, before design rather than during plan check. Reach out if any of these apply:
ADU Projects We Support Across San Mateo County
From Redwood City to Daly City and every city in between. If your property is in San Mateo County, we work there.
Ready to Find Out What Your Parcel Can Support?
We confirm the setback, height, size, and lot coverage rules that govern your parcel before any design work begins. That confirmation shapes the floor plan, prevents redesigns, and means the permit package we submit reflects what your lot can actually support.
See how we apply these rules: ADU Construction · ADU Permitting · ADU Floor Plans