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What an ADU costs in San Mateo

Two numbers, kept apart. City fees come from the city's own fee pages. The build price is our estimate from the last owner survey and today's published prices. How we got it

To compare construction bids, use the San Mateo ADU builder filters to match project types, then review the license, local project and quote checks before making a shortlist.

Official City fees

$2,832

New ADU building permit (flat fee). [1]

Our estimate Building it

$350k–465k

750 sq ft detached, design and construction. Middle of the range: $405k.[2][3][4][5]

All in, before school and utility fees: about $353k to $468k for a 750 sq ft detached ADU. Adjust it for your lot

10 licensed builders list San Mateo on their own sites. Your quote request goes to the Bay Area ADU Builders team.

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Estimate your ADU

Change the type and size. City fees come from San Mateo's own averages; the building cost is our estimate.

Type
School districts
Building it (our estimate)
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City permit fees (city average)
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School fees (rate × the whole ADU once it's over 500 sq ft, capped at $5.38)
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Likely total
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Building (middle) City fees School fees

Not included: water and sewer connection, and site surprises like a long utility run or a crane. See the fee list below.

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Licensed builders who list San Mateo

All 10

The top three by our public score for San Mateo. Some publish prices, which is the quickest check on the estimate above.

Every fee San Mateo can charge

ADU building permit (flat fee) $2,831.57 new ADU; $1,455.43 manufactured, prefab or pre-approved; $1,454.02 JADU[1] 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[1] 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[1][6] 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)[7][6] 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[1][8] At that rate a 750 sq ft ADU pays $4,402.50. Detached ADUs may need a new connection; JADUs don't.

How we estimate the building cost

The last survey of what Bay Area owners actually paid is from UC Berkeley in 2021: a median of $329 per sq ft for ADUs permitted in 2018 and 2019.[2] Construction prices in California have risen about 60% since then by the state's own cost index (2018-2019 average 6,719 to September 2026: 10,717).[3] We check that against two current numbers and use the spread as the range.

  • UC Berkeley 2021 owner survey, Bay Area median for ADUs built 2018-19 ($329 per sq ft), raised by the California Construction Cost Index (x1.595)[2][3] $525/sq ft
  • San Mateo County ADU Resource Center rule of thumb for design, construction and fees[4] $450 to 600/sq ft
  • Abodu published all-in price, 800 sq ft 2 bed / 2 bath ($424,800)[5] $531/sq ft

Other ADU types

  • Attached ADU: Detached band scaled by the survey's addition-to-detached ratio ($250 vs $300 per sq ft statewide)[2] $375 to 500/sq ft
  • Garage conversion: UC Berkeley 2021 owner survey, statewide median for garage ADUs ($189 per sq ft), raised by the California Construction Cost Index[2][3] $302/sq ft
  • Garage conversion: Same, with the survey's Bay Area premium over the statewide median (x1.32)[2][3] $397/sq ft
  • Garage conversion: SYMBiHOM published garage conversion price, $150K to $225K (per sq ft at a 400 sq ft two-car garage)[9] $375 to 560/sq ft
  • Junior ADU: UC Berkeley 2021 owner survey, statewide median for converting part of the main home ($173 per sq ft), raised by the California Construction Cost Index[2][3] $276/sq ft
  • Junior ADU: Same, with the survey's Bay Area premium (x1.32)[2][3] $363/sq ft

Smaller units cost more per foot. We scale by size the way Abodu's own price list does. The survey counted design and permits in its figure, so a little of the city fee may overlap.

Prefab prices you can check

Abodu published all-in prices (home + typical installation).[5]

  • Studio, 385 sq ft$298,800
  • 1 bed, 500 sq ft$324,800
  • 2 bed / 1 bath, 625 sq ft$358,800
  • 2 bed / 2 bath, 800 sq ft$424,800
  • 3 bed / 2 bath, 1,070 sq ft$464,800
  • 3 bed / 3 bath, 1,195 sq ft$498,800

Not included: Survey package ($6,000), trenching past 50 ft, crane past 100 ft, demolition, site engineering and sales tax.

What other sites say

  • 9Builders: $150,000 to $350,000 in San Jose[10]
  • IMKAT Homes: $205,000 to $415,000 by size[11]
  • ADU Builders Group: $450 to $475 per sq ft[12]
  • SYMBiHOM: $250,000 to $500,000 for a 500 to 1,200 sq ft backyard ADU[9]

None of these pages cites where its number comes from.

Common questions

How much does it cost to build an ADU in San Mateo?

For a 750 sq ft detached ADU, our estimate is $350,000 to $465,000 to build, plus about $2,832 in city fees by the city's own numbers. School and utility fees can add more.[1][2][3][4]

What are the ADU permit fees in San Mateo?

The city's own estimate is $2,832 (new adu building permit (flat fee)). School and utility fees are separate.[1]

Do ADUs under 750 sq ft pay impact fees?

No. State law bars impact fees on ADUs under 750 sq ft. Larger ADUs pay in proportion to the main home. School fees follow a different rule, and districts disagree about it, so ask yours.[6]

Is a garage conversion cheaper than a new ADU?

Usually. In the 2021 UC Berkeley survey, garage ADUs had a median cost of $90,000 statewide against $180,000 for detached ones. Raised to 2026 prices, a 400 sq ft garage conversion lands around $120k to $225k by our estimate.[2][3][9]

San Mateo ADU rules and builders Compare fees across cities

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 ADU flat fees Checked Oct 18, 2026
  2. 2. UC Berkeley Center for Community Innovation Statewide median $150,000/$250sf Checked Oct 18, 2026 read from an archived copy of this official page
  3. 3. California Department of General Services California Construction Cost Index (ENR BCI average of San Francisco and Los Angeles) Checked Oct 18, 2026
  4. 4. San Mateo County ADU Resource Center $450-$600/sf all-in placeholder Checked Oct 18, 2026
  5. 5. Abodu Home From $234,800 Checked Oct 18, 2026
  6. 6. 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
  7. 7. San Mateo Union HSD San Mateo/Foster City $5.02 Checked Oct 18, 2026
  8. 8. City of San Mateo Standards Checked Oct 18, 2026
  9. 9. SYMBiHOM Garage conversion $150K-$225K Checked Oct 18, 2026
  10. 10. 9Builders FAQ: SJ ADU $150k-$350k, rents $1,800-$3,000/mo, unsourced Checked Oct 18, 2026
  11. 11. IMKAT Size-tier cost table $205k-$415k, unsourced Checked Oct 18, 2026
  12. 12. ADU Builders Group $450-$475/sq ft Checked Oct 18, 2026