Quick Answer: A detached ADU in Milpitas costs $300,000 to $520,000 all-in for 600 to 800 square feet in 2026. A garage conversion runs $130,000 to $215,000. City and school fees add $14,000 to $34,000 on top, and utility connections another $28,000 to $65,000 for a detached unit. The cost driver homeowners underestimate most is not finishes: it is the distance from the main house to the new unit, because every foot of that is trenching.
Milpitas has become one of the more active ADU markets on the east side of the valley, helped by lot sizes that are generous by Bay Area standards and a city that processes these projects routinely. The costs are still Bay Area costs, and the gap between the headline construction number and the real all-in number is where most budgets go wrong.
What does an ADU cost in Milpitas?
| Type | Size | All-in cost 2026 |
|---|---|---|
| Garage conversion | 380 – 450 sq ft | $130,000 – $215,000 |
| Attached ADU | 500 – 650 sq ft | $235,000 – $380,000 |
| Detached ADU, 1 bedroom | 600 – 700 sq ft | $300,000 – $440,000 |
| Detached ADU, 2 bedroom | 750 – 1,000 sq ft | $380,000 – $560,000 |
| Two-storey ADU or above-garage | 700 – 900 sq ft | $420,000 – $620,000 |
These are all-in figures including permits, fees, utilities and site work, not construction only. A contractor quoting $280 per square foot is quoting the building, and the building is roughly 60 to 70 percent of what you will spend.
Why is a garage conversion so much cheaper?
Because the expensive parts already exist. You have a slab, walls, a roof, and utilities within a few feet.
| Element | Garage conversion | Detached ADU |
|---|---|---|
| Foundation | Existing slab, sometimes needs levelling | $26,000 – $52,000 |
| Framing, roof, exterior | Existing | $78,000 – $145,000 |
| Utility connections | $9,000 – $20,000 | $28,000 – $65,000 |
| Insulation and interior | $32,000 – $58,000 | $45,000 – $85,000 |
| Kitchen and bathroom | $40,000 – $70,000 | $52,000 – $95,000 |
| Permits and fees | $9,000 – $20,000 | $14,000 – $34,000 |
The trade-off is size and quality of space. A single garage converts to about 380 square feet, which is a studio. It also usually means giving up covered parking, which matters to some resale buyers and not to others.
The cost nobody budgets for: trenching
A detached ADU needs water, sewer, electrical and often gas run from the main house or the street to the new building. That trench is the single most variable line item in a Milpitas ADU.
- Unit 20 feet from the house, clear path: $28,000 – $38,000
- Unit 60 feet back, across a driveway or patio: $42,000 – $58,000
- Long run with concrete removal and replacement: $55,000 – $80,000
- Sewer needs a pump because the ADU sits below the main line: add $8,000 – $18,000
Practical implication: placing the ADU closer to the house saves real money, often $20,000 or more, and that is a design decision made in the first week. Pushing the unit to the back corner of the lot for privacy is a legitimate choice, but it should be a priced one.
What are the city fees?
Budget $14,000 to $34,000 for a detached ADU, filed with the Milpitas Building Division at 455 East Calaveras Boulevard.
| Fee | Typical range |
|---|---|
| Building permit (valuation-based) | $3,500 – $9,000 |
| Plan check | $2,800 – $7,000 |
| School impact fees | ~$5.17 per sq ft over 500 sq ft |
| Plumbing, electrical, mechanical | $700 – $1,800 |
| Utility connection and meter fees | $2,500 – $9,000 |
| Energy compliance review | $300 – $800 |
One useful detail: school impact fees only apply to area above 500 square feet. A 500 square foot ADU avoids them entirely, and a 750 square foot unit pays on 250 square feet, roughly $1,300. That threshold is worth knowing when you are deciding between a 500 and a 550 square foot plan.
What drives the number up?
- Distance from the main house. Covered above, and the most underestimated.
- Electrical service capacity. Many 1970s and 1980s Milpitas homes are on 100 or 125-amp service. An ADU plus the existing house usually needs 200 amps. Add $5,000 to $10,000.
- Sewer lateral condition. If the existing lateral is old clay or undersized, the city may require replacement before allowing a second connection. $8,000 to $25,000.
- Two storeys. More expensive per foot and brings setback and privacy review that a single storey does not.
- Grade. Parts of Milpitas near the foothills slope enough to need retaining and stepped footings.
How long does it take?
| Stage | Garage conversion | Detached ADU |
|---|---|---|
| Design and engineering | 4 – 8 weeks | 8 – 14 weeks |
| Plan check | 4 – 10 weeks | 8 – 16 weeks |
| Construction | 3 – 5 months | 7 – 12 months |
| Total | 6 – 10 months | 13 – 22 months |
State law caps ADU plan review at 60 days once a complete application is submitted, which helps. What it does not cap is the time your set spends incomplete, which is why a thorough first submittal is worth more than any expediting.
Does an ADU pay for itself in Milpitas?
The rental math works better here than in most of the county. A one-bedroom detached ADU in Milpitas rents for a figure that services a meaningful share of the construction loan, helped by proximity to major employers and the BART extension.
On resale, a permitted ADU is valued by appraisers as real additional living area, and in Milpitas’s market that is a genuine uplift rather than a rounding error. A garage conversion is valued more conservatively, particularly if it eliminated the only covered parking.
The honest framing: a garage conversion is the better return on capital, and a detached unit is the better asset. Which one is right depends on whether you are optimising for payback period or for what the property is worth in ten years.
Getting a real number for your lot
An accurate Milpitas ADU estimate needs four things measured: the distance from the house to the proposed location, the electrical panel capacity, the condition and size of the sewer lateral, and the grade. Those four move the total by six figures between the best and worst case, and none of them are visible in a per-square-foot quote.
DevArt8 Builders builds ADUs throughout Milpitas, San Jose, Fremont and the South Bay, and prices trenching, service upgrades and lateral work as explicit line items rather than allowances. See our Milpitas service area page for project detail.
Frequently Asked Questions
How much does an ADU cost in Milpitas?
$300,000 to $440,000 all-in for a 600 to 700 square foot detached one-bedroom unit in 2026, and $130,000 to $215,000 for a garage conversion. Those are all-in figures including permits, fees, utilities and site work. Construction alone is only about 60 to 70 percent of the total.
Why is a garage conversion so much cheaper than a detached ADU?
Because the slab, walls, roof and nearby utilities already exist. A detached unit adds $26,000 to $52,000 in foundation, $78,000 to $145,000 in shell, and $28,000 to $65,000 in utility trenching that a conversion largely avoids. The trade-off is size, since a single garage converts to roughly 380 square feet.
What is the most underestimated ADU cost?
Utility trenching. Running water, sewer, electrical and gas to a detached unit costs $28,000 for a short clear run and up to $80,000 for a long run crossing concrete. Placing the ADU nearer the main house can save $20,000 or more, and that is a first-week design decision.
Do I pay school fees on a Milpitas ADU?
Only on area above 500 square feet, at roughly $5.17 per square foot. A 500 square foot ADU avoids them entirely; a 750 square foot unit pays on 250 square feet, about $1,300. Worth knowing when choosing between a 500 and a 550 square foot plan.
How long does an ADU take to build in Milpitas?
Thirteen to twenty-two months total for a detached unit and six to ten months for a garage conversion. State law caps ADU plan review at 60 days once a complete application is filed, so a thorough first submittal saves more time than any expediting service.