ADU fees in San Mateo: $8,702–$20,090 in modeled fees for 1,000 sq ft, plus valuation-based permits
San Mateo charges a flat ADU building permit fee: $2,831.57 for new construction in FY 2026-27. A new ADU that's separate from the house also pays a wastewater capacity charge of $5.87 per sq ft. Percentage fees on construction valuation are added on top.
8 fee lines · 3 official sources · checked Oct 18, 2026 · Sources · Calculator
Totals leave out valuation-based building fees. Enter a construction valuation in the calculator to include them.
Modeled fees by ADU size
Detached ADU, 2,000 sq ft main home (example assumption), law in force today. Bars: solid = fees that apply, hatched = fees that may apply.
Each fee with its official source (numbers link to the source list). Est. = computed by us from the official rate; May apply = high total only; Up to = statewide cap, district may charge less.
Fee
499 sq ft
750 sq ft
1,000 sq ft
1,200 sq ft
Building permit & plan check
ADU building permit, new construction (flat)1Remodel/conversion ADU: $2,031.12. Eligible manufactured, prefab or preapproved ADU: $1,455.43. JADU: $1,454.02.
$2,832
$2,832
$2,832
$2,832
General plan maintenance fee (0.59% of valuation)1
Park plan check and inspection (0.05% of valuation)1On permits over $5,000 valuation; you can apply for an exemption if there's no park plan check or inspection requirement.
Needs valuation
Needs valuation
Needs valuation
Needs valuation
Impact fees
Transportation improvement fee1$7,339.55 per single-family dwelling. The schedule doesn't list an ADU rate; if it's charged, the City's code requires it to be proportional to the main home's size, which is how we compute it. High total only.
Exempt
Exempt
May apply$3,670
May apply$4,404
Child care development fee1$4,677.29 per single-family dwelling; no ADU rate is listed. Computed proportionally as above. High total only.
Wastewater capacity charge (new detached ADU)1Per sq ft of ADU. San Mateo's code requires a capacity charge for an ADU that is separate from an existing structure; conversions pay none.
$2,929
$4,402
$5,870
$7,044
Modeled total
$5,761
$7,234–$11,269
$8,702–$20,090
$9,876–$23,542
Exemptions
San Mateo's code: no impact fees for ADUs of 750 sq ft or less; above that, proportional to the main home's size. 2
Rules
What you can build in San Mateo
Maximum size
State floor: cities must allow at least 850 sq ft (1,000 sq ft with 2+ bedrooms); detached ADUs can be capped at 1,200 sq ft. Local rule not captured yet.5
Height
State floor: 16 ft detached (18 ft near transit or on a multifamily lot, +2 ft for roof pitch); 25 ft attached. Local rule not captured yet.5
Setbacks
State limit: no more than 4 ft side and rear. Local rule not captured yet.5
Parking
State limit: at most 1 space per ADU or bedroom, and none within ½ mile of transit and in other cases. Local rule not captured yet.5
Owner occupancy
State law: cities can't require the owner to live on site for an ADU (JADUs: owner on site if sanitation is shared). Local rule not captured yet.5
Pre-approved plans
State law: every city must run a program to pre-approve ADU plans (Gov. Code 65852.27). Local rule not captured yet.5
State law
Why a 750 sq ft ADU pays no impact fees
California bars impact fees on ADUs of 750 sq ft or less of interior livable space. Above that, they must be proportional to the main home’s size; we assume 2,000 sq ft unless you enter yours. School fees start at 500 sq ft. From January 1, 2027, SB 1117 limits impact fees to the area over 750 sq ft. How the state rules work →
ADU permits issued
San Mateo issued 151 ADU building permits in 2025
589 from 2018 to 2025, as the jurisdiction reported to the state in its Housing Element Annual Progress Reports (Table A2, ADU rows; may include JADUs). “–” means no ADU rows were reported that year.
Valuation-based fees (general plan maintenance, technology, park plan check, state SMIP and BSASRF) depend on construction valuation; enter one in the calculator.
The park in-lieu fee in the schedule applies to parcel maps, subdivisions and other multifamily projects, so it isn't counted for an ADU.
Whether the transportation and child care fees apply to ADUs isn't stated; they're counted in the high total only.
School district rate not captured; shown at the statewide cap.
Local size, height and parking rules not captured yet; the state minimums apply.
The Governor approved SB 1117 (Chapter 809, Statutes of 2026) on September 29, 2026. It amends Gov. Code 66311.5: on lots with up to two ADUs, impact fees on an ADU over 750 sq ft must still be proportional to the primary home, and may only be charged on the area over 750 sq ft. Regular 2026 statutes take effect January 1, 2027. Until then, fees on a larger ADU are figured on its full size. Our calculator has a 2027 setting that shows the difference.
The Department of Housing and Community Development updated its ADU Handbook to reflect the laws that took effect January 1, 2026. Its proportional-fee example: a 1,000 sq ft ADU next to a 2,000 sq ft home pays 50% of the fee a new home would pay. Our models use 2,000 sq ft as the example primary-home size unless you enter your own.
On January 28, 2026 the State Allocation Board raised the statewide maximum Level 1 developer fee for residential construction from $5.17 to $5.38 per sq ft (+4.06%). School districts collect it on ADUs of 500 sq ft or more, at their own adopted rate up to this cap (higher Level 2/3 fees need extra findings). Where we haven't confirmed a district's rate, we use $5.38 as the upper bound.
SB 543 (Chapter 520, Statutes of 2025) took effect January 1, 2026. It restates the impact-fee rule in Gov. Code 66311.5: no impact fees on an ADU of 750 sq ft of interior livable space or less, or a JADU of 500 sq ft or less; above that, fees must be proportional to the primary home. It also caps JADUs at 500 sq ft of interior livable space. Many city documents still cite the older section numbers (65852.2 or 66324).
For a detached ADU with a 2,000 sq ft main home (an example assumption), our model of San Mateo’s official fee schedules comes to $7,234–$11,269 at 750 sq ft and $8,702–$20,090 at 1,000 sq ft. These totals leave out valuation-based building fees. Ranges show fees that may or may not apply to your project.
Does San Mateo charge impact fees on a 750 sq ft ADU?
No. California Government Code section 66311.5 bars impact fees on an ADU of 750 sq ft of interior livable space or less (500 sq ft or less for a JADU). Above 750 sq ft, impact fees must be proportional to the size of the main home. At 1,000 sq ft, the impact fees we model for San Mateo come to at most $6,008.
Do school fees apply to an ADU?
Only at 500 sq ft or more. School districts charge their adopted developer fee per sq ft, capped at the State Allocation Board’s Level 1 maximum ($5.38 per sq ft since January 28, 2026) unless they qualify for higher levels.
For a Conversion ADU, payment of a connection fee or capacity charge is not required. For an ADU that is separate from an existing structure, the City will require payment of a connection fee or capacity charge that is proportionate to the ADU's burden.
Any impact fees charged for an ADU over 750 square feet shall be charged proportionately in relation to the square footage of the primary residential dwelling unit.
No impact fees shall be charged for ADUs 750 square feet in size or smaller.
A local agency, special district, or water corporation shall not impose any impact fee upon the development of an accessory dwelling unit that has 750 square feet of interior livable space or less