ADU Laws · 2026
San Mateo County ADU Regulations (2026)
In San Mateo County, you can build a detached ADU of up to 1,200 square feet on nearly any single-family lot, and your city must approve a complete application within 60 days with no public hearing and no neighbor sign-off. That is because California state law sets the floor and overrides any local rule that would be more restrictive. The local layer that still matters here is mostly process and zoning detail, not whether you can build, and on the Peninsula, where land is some of the most expensive in the country, the economics of adding a unit are about as strong as they get.
The short version: State law guarantees you an ADU up to 1,200 sq ft with ministerial (no-hearing) approval in 60 days, 4-foot setbacks, and no owner-occupancy requirement. San Mateo County cities (Redwood City, Menlo Park, San Mateo and the rest) add their own permit process and a few zoning details on top, but they cannot take those state protections away. We confirm exactly how the rules apply to your address in a feasibility study.
State law is the baseline in every San Mateo County city
Whether your property sits in Redwood City, Menlo Park, San Mateo, San Carlos, Belmont, Burlingame, Foster City, San Bruno, Daly City, South San Francisco, East Palo Alto, or unincorporated county land, the same California ADU law applies as the minimum standard. Cities can add detail and run their own permit counter, but they cannot remove these protections.
| Rule | What state law guarantees |
|---|---|
| Maximum size | Up to 1,200 sq ft for a detached new-construction ADU |
| Guaranteed minimum | At least an 800 sq ft / 16 ft tall ADU, regardless of local rules |
| Setbacks | No more than 4 ft required on the side and rear |
| Approval type | Ministerial: no public hearing, no neighbor approval |
| Decision deadline | The city must approve or deny a complete application in 60 days |
| Owner-occupancy | Not required for an ADU (still required for a JADU) |
| Parking | At most 1 space, with broad exemptions (see below) |
| Impact fees | Waived for ADUs under 750 sq ft, proportional above |
This is why an ADU is one of the most predictable construction projects you can take on along the Peninsula. On a standard lot it is a by-right addition, not a discretionary negotiation with the planning department. You can read the full state framework in our California ADU laws 2026 guide and our breakdown of whether you can build an ADU on your property.
Parking is rarely a real obstacle here
A San Mateo County city can ask for at most one parking space per ADU, and the state exemptions are wide enough that the requirement usually falls away. You do not have to add parking if your property is within a half-mile walking distance of public transit, within one block of a car-share, in an on-street permit area, part of an existing or proposed structure, or when the ADU replaces a garage or covered parking. With Caltrain running the length of the Peninsula through Redwood City, San Carlos, San Mateo, Burlingame, and Millbrae, plus BART reaching Daly City and South San Francisco and dense SamTrans bus coverage, a large share of county lots sit close enough to transit that no extra parking is needed. We cover the details in our guide to ADU parking requirements in California.
The local layer that actually matters in San Mateo County
State law decides whether you can build and how fast. Each city decides the texture of the process: which forms, which department, and a handful of zoning details. None of these stop a well-planned ADU, but they shape how the plans are drawn and how smoothly the permit moves.
City permit process and timelines
Larger cities like Redwood City and San Mateo run busy planning counters and have worked to streamline ADU intake, while smaller jurisdictions such as San Carlos, Belmont, and Hillsborough run their own counters with their own quirks. All of them are bound by the same 60-day ministerial clock once your application is complete. The practical difference is mostly how quickly a city deems an application “complete,” which is exactly where having your plans and engineering done right the first time pays off.
Lot coverage, height, and design rules
Cities can apply objective design and development standards, like maximum lot coverage, height limits within the state-protected 16-foot floor, and where the unit sits relative to the main house. These vary from Menlo Park to Burlingame, but because they have to be objective (not subjective taste), they are knowable in advance. We design the placement to fit your specific lot’s coverage and setback math before anything goes to the city.
Coastal and hillside parcels
San Mateo County stretches from the bayfront to the coast, and parcels in places like Half Moon Bay, Pacifica, and the coastside fall partly under Coastal Commission jurisdiction, which can add a coastal development permit step. Hillside lots in the foothills above San Carlos, Belmont, Hillsborough, and Woodside can mean extra foundation engineering and grading. This is one reason factory building works well here: the home is built about 97% in our Salinas factory while the site work and foundation are handled to match the terrain.
Flood, fire, and access considerations
Bayfront and low-lying parcels in Foster City, parts of Redwood City, and East Palo Alto can sit in flood zones that affect foundation height and engineering, while wildland-interface lots near the coastal range can carry fire-access and defensible-space rules. One thing that does not change: fire sprinklers are not required for the ADU unless they are already required for your main house under state law.
How this plays out city by city
The state baseline is identical everywhere, but the day-to-day feel differs by jurisdiction. Here is the practical picture in the places we build most across the county.
| Area | What tends to drive the local review |
|---|---|
| Redwood City | Streamlined intake, transit-based parking exemptions, busy counter |
| Menlo Park | Objective design standards, premium neighborhoods, infill lots |
| San Mateo / Burlingame | Flat infill lots, strong Caltrain access, straightforward path on many parcels |
| San Carlos / Belmont | Foothill slope, lot coverage limits, smaller-staff counters |
| Coastside (Half Moon Bay, Pacifica) | Coastal permit overlay, slope, longer review on some parcels |
We deliver across this whole area from our Salinas factory. You can see local detail on our Redwood City, Menlo Park, and San Mateo city pages.
Why the ADU math is so strong in San Mateo County
This is where San Mateo County stands apart. Land on the Peninsula is among the most valuable in the nation, which means an ADU does two powerful things at once: it adds a high-rent rental unit and it adds resale value backed by Peninsula land prices. The all-inclusive cost of the home is roughly the same whether you build it in Redwood City or in a lower-cost county, but the income and appraisal upside in this market are typically much larger. That is the core reason ADUs pencil out so well here. For the numbers, see our guides on how much rent an ADU can earn and ADU appraisal and resale value.
What it costs and how long it takes
A Framework First ADU runs $199,000 to $599,000 all-inclusive, and the words “all-inclusive” are the part that matters. That single number includes a permitting budget along with plans, the foundation, the factory build, delivery and crane set, utility hookups, appliances, and the final inspection, with the exact permit cost for your specific address confirmed during the feasibility study since it’s the city, not us, that sets it.
We build 14 models from 405 to 1,200 sq ft in three styles: Classic Craftsman (our standard), Urban Farmhouse, and Ultra Modern. A few popular points on the lineup:
| Model | Size | Layout | All-inclusive price* |
|---|---|---|---|
| Four Fifty | 450 sq ft | Studio, 1 bath | $199,000 |
| Six Sixty | 660 sq ft | 1 bed, 1 bath | $309,000 |
| Nine Forty-Five 3/2 | 945 sq ft | 3 bed, 2 bath | $479,000 |
| Twelve Hundred U | 1,200 sq ft | 3 bed, 2 bath | $599,000 |
*Includes a permitting budget, not final permit costs. Permits and property-specific site work (septic, retaining walls, etc.) vary by property and are confirmed in your feasibility study.
Most projects deliver in 4 to 9 months, because the home is being built in the factory while permits and the foundation move in parallel. See the full lineup on our models page and run your own numbers with the ADU cost calculator.
Frequently asked questions
Do I need owner-occupancy to build an ADU in San Mateo County?
No. State law has permanently suspended the owner-occupancy requirement for standard ADUs, so you can build one and rent out both the ADU and the main house. The exception is a JADU (a junior ADU up to 500 sq ft created inside the walls of an existing home), which still requires the owner to live on the property. For the difference, see our JADU vs ADU guide.
How fast can I get approved in Redwood City or another San Mateo city?
Once your application is complete, state law requires the city to approve or deny it within 60 days, with no public hearing. The real variable is how quickly a city deems your application complete, which comes down to having complete, correct plans and engineering. Our team handles the submittal and the back-and-forth so the clock starts as early as possible.
Are there extra rules for coastal or bayfront lots?
Sometimes. Parcels on the coastside near Half Moon Bay and Pacifica can require a coastal development permit, and low-lying bayfront lots can carry flood-zone engineering. Neither rules out an ADU, but both can add a step or shape the foundation design. The exact requirements vary by parcel and are confirmed in the feasibility study.
Can I build an ADU plus a JADU on the same lot?
Yes. State law allows a single-family lot to have the primary home plus one detached ADU plus one JADU. That means up to three households on a lot that today holds one, which is a meaningful play in a land-constrained market like the Peninsula. We map out which combination fits your parcel in the feasibility study.
Who handles the permits, the city, and the inspections?
Our team does. Permitting, plan check, city coordination, delivery, and the final inspection are all part of our process, with a permitting budget built into your all-inclusive price and the exact permit cost for your parcel confirmed during the feasibility study. You are not standing in line at the planning counter or chasing an inspector. You can see the full sequence on our process page.
Every lot in San Mateo County is a little different, and the only way to know exactly which city rules, coverage limits, coastal overlays, or slope considerations touch your address is to look at the parcel itself. Start your feasibility study and our team will tell you what is possible on your property, what the permit path looks like, and what your all-inclusive number would be.