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San Mateo ADU builders and construction

Compare builders for a detached ADU, garage conversion or prefab installation. Filter the list by project type, then check service areas, licenses and quote scope. This page covers the City of San Mateo, not all of San Mateo County.

Checked against the city's ADU pages on Oct 18, 2026.

Can I build one?
Yes, no fixed size cap Up to 800 sq ft is exempt from FAR. A larger ADU must fit the zone's FAR and be smaller than the main house. No parking required.[1]
What will the city charge?
$2,832 New ADU building permit (flat fee). School and utility fees extra. See every fee[2]
Who builds here?
10 licensed builders Each lists San Mateo on its own site and holds an active CSLB license. See the list
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Licensed ADU builders in San Mateo

Ranked by a public score anyone can check: license, years licensed, ADU types and cities shown on their own sites, a local office, and published prices. Nobody pays to move up. Quote requests go to our team with the builder you picked. How we rank

These builders name San Mateo on their own websites. A service-area listing is not proof of a completed City of San Mateo ADU. We have not verified local completed projects for this list. Ask whether the company serves your address and can show a similar permitted project.

Directory records checked Oct 18, 2026. Use each builder’s service-area and CSLB links below to recheck the records before you hire.

Before you hire a San Mateo ADU builder

  1. 1. Confirm the address and a comparable local project

    Ask whether the builder works in the City of San Mateo. A San Mateo County service area alone does not answer that question. Request a permit number for a similar detached ADU or garage conversion and ask the city how to check it.

    City of San Mateo ADU resources

  2. 2. Match the contract to the CSLB record

    Open each builder’s CSLB link and match the legal name and license number to the proposed contract. Review the current status, classification, bond and workers’ compensation record. Use the directory’s original check date to judge how old the record is.

    Check the current CSLB record

  3. 3. Assign responsibility for plans and site work

    For a custom or preapproved plan, ask who handles the site plan, permit corrections and utility design. For prefab, ask who provides the foundation, delivery access and installation. A plan in the city gallery still needs review for your lot.

    Review San Mateo’s preapproved plan resources

  4. 4. Compare bids for the same ADU scope

    Use the same size and project type for each bid. Ask for separate lines for design, permits, site work, utilities and construction. List exclusions and allowances in writing. Keep city, school and utility charges separate from the construction price.

    Review San Mateo ADU costs and fee lines

What San Mateo allows

  • Detached ADU, max size[1]

    No fixed cap. Up to 800 sq ft is exempt from FAR; above that the zone's FAR applies. Must be smaller than the main house, unless the house is under 800 sq ft

  • Attached ADU, max size[1]

    Same as detached

  • Junior ADU (inside the house)[1]

    Up to 500 sq ft, one per lot

  • Height[1]

    Detached: 18 ft to the top plate, 24 ft to the peak. Attached: 24 ft and 32 ft

  • Side and rear setbacks[1]

    4 ft side and rear. Front setback applies only over 800 sq ft

  • Daylight plane[1]

    ADUs over the state-exempt size must fit a plane starting at 12 ft and sloping in at 45 degrees from side and rear lines

  • Units on a single-family lot[1]

    1 attached or conversion ADU + 1 detached new ADU + 1 JADU

  • Parking[1]

    None. No replacement parking when you convert or remove a garage or parking space

  • Owner must live on site[1]

    No for an ADU. For a JADU only if it shares a bathroom with the house
  • ADUs and JADUs can't be rented for less than 30 days or sold separately from the main home.[1]
  • An ADU that doesn't meet every standard can apply for discretionary review by the Zoning Administrator, with public notice.[1]
  • A new site-built detached ADU needs solar panels under the state Energy Code. They can go on the ADU or the main house.[1]
ADU rules in San Mateo drawn on a typical lot Detached ADU, FAR-exempt size: up to 800 square feet, 4 feet from side and rear lot lines, 24 feet tall. Parking: None required. Rear lot line Street Main house ADU up to 800 sq ft 24 ft tall 4 ft 4 ft Parking: none required
Detached ADU, FAR-exempt size. Not to scale.

City fees for an ADU in San Mateo

Fee lines for an ADU in San Mateo
ADU building permit (flat fee) $2,831.57 new ADU; $1,455.43 manufactured, prefab or pre-approved; $1,454.02 JADU[2] ADU remodel: $2,031.12.
Valuation-based add-ons General plan maintenance 0.59%, technology 0.15%, park and rec tax $0.30 per $100 of valuation[2] The schedule does not say whether these apply on top of the flat ADU fee.
Transportation and child care fees $7,339.55 transportation and $4,677.29 child care per single-family unit[2][3] The schedule has no ADU line. State law: no impact fees under 750 sq ft; above that, a share in proportion to the main home.
School fees $5.02 per sq ft (San Mateo-Foster City SD and San Mateo Union HSD combined)[4][3] San Mateo Union HSD collects it. Under state law, ADUs of 500 sq ft or less pay no school fees.
Wastewater capacity charge (City of San Mateo) $5.87 per sq ft of ADU[2][1] At that rate a 750 sq ft ADU pays $4,402.50. Detached ADUs may need a new connection; JADUs don't.
School fee rates for districts in San Mateo (1)

San Mateo Union HSD collects one combined $5.02 fee for itself and San Mateo-Foster City SD. Some edge areas may be in other districts; we did not check.[4]

  • San Mateo-Foster City School District[4]Combined fee with San Mateo Union HSD. Pay it once, to SMUHSD. $5.02/sq ft
Total cost in San Mateo, fees plus building

How the permit works

  1. 1

    Check the standards

    Read the single-family ADU handout. A compliant ADU needs only a building permit, with no public notice.[1][5]

  2. 2

    Pick a custom design or a pre-approved plan

    Pre-approved plans from the city gallery follow a two-phase process and pay a lower permit fee.[6][7][2]

  3. 3

    Apply online

    Submit your building permit on the city's online permit portal.[8]

  4. 4

    Pay fees and get the permit

    The city charges its flat ADU permit fee and any capacity charges. Pay school fees to San Mateo Union HSD.[2][4]

State deadline: State law gives the city 15 business days to say if your application is complete and 60 days to approve or deny a complete one.[3]

Apply at: City of San Mateo online permit portal

Preapproved plans

San Mateo has a pre-approved ADU plans gallery. The permit fee for a pre-approved, prefab or manufactured ADU is $1,455.43, about half the $2,831.57 for a custom new ADU.[6][7][2]

  • City of San Mateo ADU Plans Gallery Designs listed on the city's ADU Accelerator site

We could not read the designer list on the gallery site. You buy plans from the designer.

Common questions

How big can an ADU be in San Mateo?

The code sets no fixed maximum. Up to 800 sq ft is exempt from FAR; a larger ADU must fit the zone's FAR and be smaller than the main house. A junior ADU is up to 500 sq ft.[1]

Do I need parking for an ADU in San Mateo?

No. San Mateo requires no parking for any ADU or JADU, and no replacement parking if you convert a garage.[1]

How much is an ADU permit in San Mateo?

The flat permit fee is $2,831.57 for a new ADU, or $1,455.43 for a pre-approved or prefab one. Add the $5.87 per sq ft wastewater charge and $5.02 per sq ft in school fees over 500 sq ft.[2][4]

How tall can a detached ADU be in San Mateo?

18 ft to the top plate and 24 ft to the roof peak, measured from existing grade.[1]

Do I have to live on the property?

Not for an ADU. For a junior ADU, only if it shares a bathroom with the main house.[1]

Nearby cities

What we couldn't confirm

  • No city-published review timeline for ADUs.
  • The fee schedule does not say how transportation and child care fees apply to ADUs over 750 sq ft, or whether valuation add-ons stack on the flat ADU fee.
  • Water connection costs from the water company not recorded.
  • Pre-approved gallery designer list could not be read.
  • School district boundaries at the city's edges not checked.

Sources

Every fact on this page links to one of these. We read each one on the date shown.

  1. 1. City of San Mateo Standards Checked Oct 18, 2026
  2. 2. City of San Mateo ADU flat fees Checked Oct 18, 2026
  3. 3. California Department of Housing and Community Development (HCD) All state ADU/JADU rules used in state_baseline.md (sizes, heights, setbacks, parking, owner-occupancy, JADU, Checked Oct 18, 2026
  4. 4. San Mateo Union HSD San Mateo/Foster City $5.02 Checked Oct 18, 2026
  5. 5. City of San Mateo Ministerial Checked Oct 18, 2026
  6. 6. City of San Mateo (ADU Accelerator gallery) City of San Mateo pre-approved ADU plans gallery (JS-rendered Checked Oct 18, 2026
  7. 7. City of San Mateo Two-phase process Checked Oct 18, 2026
  8. 8. City of San Mateo EnerGov online permit portal Checked Oct 18, 2026