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What an ADU costs in Campbell

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

Official City fees

Line by line

The city doesn't publish a total for an ADU, so we list each charge.See every 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 $350k to $465k for a 750 sq ft detached ADU. Adjust it for your lot

9 licensed builders list Campbell 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 Campbell'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 Campbell

All 9

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

Every fee Campbell can charge

Building permit, by valuation 2.00% of the first $500,000 of valuation ($237 minimum)[1] Plan check adds 33% of the permit fee. Fire plan check adds $119 or 10%, whichever is greater. FY 2027 schedule, effective July 1, 2026.
Valuation and size add-ons General Plan 0.16%, technology 0.04%, training 0.02% and roadway 0.32% of valuation, plus $0.50 per sq ft construction license tax[1]
Planning plan check $259 detached; $517 attached; $647 conversion[1]
Park in-lieu fee $0 under 750 sq ft; up to $9,328 at 750 sq ft and up; $0 for a JADU[1] Figured under state law and capped at that amount. A new single-family home pays $33,025.
School fees $1.61 to $3.79 per sq ft by district[6][7][8][9] Campbell Union SD charges $3.79 and Campbell Union HSD $1.61. Parts of the city may be in Moreland or Cambrian districts instead. Ask your districts before you budget.
Sewer connection (West Valley Sanitation District) $6,727 max connection + $1,024 max treatment plant capacity[10] Smaller ADUs pay $511.16 and $73.53 per drainage fixture unit. The district's fee tables are labeled FY2018/19 to FY2020/21.

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)[11] $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[12]
  • IMKAT Homes: $205,000 to $415,000 by size[13]
  • ADU Builders Group: $450 to $475 per sq ft[14]
  • SYMBiHOM: $250,000 to $500,000 for a 500 to 1,200 sq ft backyard ADU[11]

None of these pages cites where its number comes from.

Common questions

How much does it cost to build an ADU in Campbell?

For a 750 sq ft detached ADU, our estimate is $350,000 to $465,000 to build. School and utility fees can add more.[1][2][3][4]

What are the ADU permit fees in Campbell?

The city doesn't publish a total. No published total; see fee lines The fee lines on this page list each charge.[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.[15]

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][11]

Campbell 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 Campbell Valuation-based building permit % 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. Campbell Union SD Residential & ADU $3.79/sq ft Checked Oct 18, 2026
  7. 7. Campbell Union HSD $1.61/sq ft eff Checked Oct 18, 2026
  8. 8. Moreland School District Residential $3.35/sf Checked Oct 18, 2026
  9. 9. Cambrian School District Eff Checked Oct 18, 2026
  10. 10. West Valley Sanitation District ADU connection $6,727 max/$511.16 per DFU Checked Oct 18, 2026
  11. 11. SYMBiHOM Garage conversion $150K-$225K Checked Oct 18, 2026
  12. 12. 9Builders FAQ: SJ ADU $150k-$350k, rents $1,800-$3,000/mo, unsourced Checked Oct 18, 2026
  13. 13. IMKAT Size-tier cost table $205k-$415k, unsourced Checked Oct 18, 2026
  14. 14. ADU Builders Group $450-$475/sq ft Checked Oct 18, 2026
  15. 15. 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