Garage conversion · checked Oct 18, 2026
Garage conversion ADU fees in Milpitas: $3,816–$4,519 for a 400 sq ft garage
That’s $940 less than building the same 400 sq ft as a new detached ADU in Milpitas, and $1,292 less at 550 sq ft, the size you reach with the 150 sq ft expansion state law allows. Both sizes are under 750 sq ft, so there are no impact fees either way.
What changes
Why a conversion costs less in Milpitas
- Water connection fee: $525 for a new detached unit at 550 sq ft, $0 for a conversion. State law bars a new utility connection fee or capacity charge for an ADU inside an existing structure (Gov. Code §66311.5(d)).
- Sewer connection fee: $525 for a new detached unit at 550 sq ft, $0 for a conversion. State law bars a new utility connection fee or capacity charge for an ADU inside an existing structure (Gov. Code §66311.5(d)).
- Treatment plant fee (new sewer connection): $242 for a new detached unit at 550 sq ft, $0 for a conversion. State law bars a new utility connection fee or capacity charge for an ADU inside an existing structure (Gov. Code §66311.5(d)).
Line by line: converted garage vs new detached ADU
2,000 sq ft main home (example assumption), law in force today. Garage sizes are examples: 400 sq ft is a typical two-car garage; 550 sq ft adds the 150 sq ft expansion.
| Fee | Garage, 400 sq ft | New detached, 400 sq ft | Garage + 150, 550 sq ft | New detached, 550 sq ft |
|---|---|---|---|---|
| Building permit & plan check | ||||
| Building, planning and land development permit fees (City estimate table) | Est.$3,107 | Est.$3,107 | Est.$4,451 | Est.$4,451 |
| New address request permit | $709 | $709 | $709 | $709 |
| Fire sprinkler review | May apply$703 | May apply$703 | May apply$703 | May apply$703 |
| Impact fees | ||||
| Park in-lieu fee | Exempt | Exempt | Exempt | Exempt |
| School district fee | ||||
| School fee (statewide cap shown) | Exempt | Exempt | Up to$2,959 | Up to$2,959 |
| Utility connections | ||||
| Water connection fee | Exempt | May apply$382 | Exempt | May apply$525 |
| Sewer connection fee | Exempt | May apply$382 | Exempt | May apply$525 |
| Treatment plant fee (new sewer connection) | Exempt | May apply$176 | Exempt | May apply$242 |
| Modeled total | $3,816–$4,519 | $3,816–$5,459 | $5,160–$8,822 | $5,160–$10,114 |
State law
What California guarantees for a garage conversion
A city must approve one ADU inside an existing accessory structure such as a garage, can’t require a setback for it, and can’t make you replace the parking you lose.
Gov. Code §66323(a)(1)
One accessory dwelling unit and one junior accessory dwelling unit per lot with a proposed or existing single-family dwelling if all of the following apply:
Gov. Code §66323(a)(1)(A)
may include an expansion of not more than 150 square feet beyond the same physical dimensions as the existing accessory structure
Gov. Code §66314(d)(7)
No setback shall be required for an existing living area or accessory structure or a structure constructed in the same location and to the same dimensions as an existing structure that is converted to an accessory dwelling unit or to a portion of an accessory dwelling unit
Gov. Code §66314(d)(11)
When a garage, carport, covered parking structure, or uncovered parking space is demolished in conjunction with the construction of an accessory dwelling unit or converted to an accessory dwelling unit, the local agency shall not require that those offstreet parking spaces be replaced.
Quoted from Government Code, Title 7, Division 1, Chapter 13 (Accessory Dwelling Units), incl. §66311.5 (opens in new tab), checked Oct 18, 2026.
FAQ
Questions about garage conversion in Milpitas
How much are the fees to convert a garage into an ADU in Milpitas?
Our model of Milpitas’s official fee schedules comes to $3,816–$4,519 for a 400 sq ft garage conversion and $5,160–$8,822 at 550 sq ft, with a 2,000 sq ft main home. A new detached ADU of the same size would be $3,816–$5,459 and $5,160–$10,114.
Is a garage conversion cheaper than a new ADU in Milpitas?
In fees, yes: $940 less at 400 sq ft and $1,292 less at 550 sq ft. Water connection fee is lower for a conversion. Sewer connection fee is lower for a conversion. Treatment plant fee (new sewer connection) is lower for a conversion.
Do I have to replace the garage parking?
No. When a garage is converted to an ADU, state law bars the city from requiring replacement off-street parking (Gov. Code §66314(d)(11)).
Does a garage conversion pay impact or school fees?
No impact fees: the unit is under 750 sq ft. School fees apply only above 500 sq ft; at 550 sq ft the statewide cap would be up to $2,959, and the district may charge less.
Not modeled yet
What these totals leave out
- The City's estimate table is based on the FY 2025-26 schedule and is labeled a minimum; Milpitas adopted a new user and regulatory fee schedule effective November 2, 2026 that we haven't modeled line by line yet.
- ADUs in the Milpitas Metro (TASP) area may owe the area's development impact fee ($51,828 per residential unit in FY 2026-27); whether and how it applies to an ADU isn't stated, so it isn't included.
- School district rate not captured; shown at the statewide cap.
- Local size, height and parking rules not captured yet; the state minimums apply.
More
Keep going
Sources
Official sources
Every figure comes from these documents; each line’s quoted text is on the Milpitas page.
- City of Milpitas, Office of Building SafetyAccessory Dwelling Unit (ADU) Permit Fee Estimate (handout dated June 11, 2026; based on the FY 2025-26 fee schedule) (opens in new tab)
- City of MilpitasFY 26-27 Planning Permit Fees and Development Impact Fees (approved June 2, 2026, effective August 3, 2026) (opens in new tab)
- California State Allocation Board (Department of General Services, OPSC)State Allocation Board item: 2026 adjustment to the Level I developer fee (meeting of January 28, 2026) (opens in new tab)
- California Legislative Information (leginfo.legislature.ca.gov)Government Code, Title 7, Division 1, Chapter 13 (Accessory Dwelling Units), incl. §66311.5 (opens in new tab)
- HCD via data.ca.govHousing Element Annual Progress Report (APR) data, Table A2: ADU building permits by year (opens in new tab)