"How much does an ADU cost" is a loaded question. The same 800 square foot unit can cost $220,000 or $450,000 depending on finish level, whether the lot is flat or sloped, how far the sewer and electrical service are, whether the city wants a separate meter, fire sprinklers, soils conditions, the contractor's backlog, and a dozen smaller items. The ranges below are real 2026 numbers from Southern California and Bay Area projects, but they are ranges, not quotes. Nobody can price your ADU without seeing your lot.
With that said: an ADU in California costs between $80,000 and $600,000, and the spread comes from the type of unit, the city, the finishes, and what is already on the lot. Below are the line items most owners miss.
Cost by ADU type
| ADU type | All-in range | Per sq ft | Notes |
|---|---|---|---|
| JADU (inside the house) | $60,000 to $140,000 | $150 to $300 | Cheapest unit; reuses the envelope |
| Garage conversion | $90,000 to $180,000 | $200 to $350 | Slab, insulation, and plumbing drive the range |
| Prefab or pre-approved plan, detached | $180,000 to $300,000 | $300 to $430 | Factory cost plus site work, crane, utilities |
| Site-built detached, 800 to 1,200 sq ft | $280,000 to $450,000 | $350 to $500 | Most common investor build |
| Two-story or over-garage | $400,000 to $600,000+ | $400 to $550 | Structural and fire separation add cost |
ADU cost by size
| Size | Typical use | Site-built detached | Garage conversion / JADU |
|---|---|---|---|
| 400 sq ft | Studio | $170,000 to $240,000 | $80,000 to $140,000 |
| 500 sq ft | Studio or JADU | $190,000 to $260,000 | $90,000 to $150,000 |
| 800 sq ft | 1 or 2 bed, the state-exempt size | $280,000 to $380,000 | n/a |
| 1,000 sq ft | 2 bed / 2 bath | $330,000 to $430,000 | n/a |
| 1,200 sq ft | 2 or 3 bed, LA maximum | $380,000 to $480,000 | n/a |
Per square foot falls as size rises because the kitchen, bathroom, and utility connections cost about the same at any size. That is why Parcy's "Maximize value" toggle usually improves return on cost.
Soft costs and fees
- Design and engineering: $8,000 to $30,000. Pre-approved plans (required of every city under AB 1332) cut this sharply.
- Permits and plan check: $5,000 to $15,000.
- Impact fees: waived for ADUs under 750 square feet; proportional above that. School fees waived under 500 square feet from 2026.
- Utilities: a separate electrical service, sewer lateral, or water meter can add $10,000 to $40,000. Converted and JADU units generally do not need separate connections.
- Site work: grading, retaining walls, and tree removal are the surprises on hillside lots.
Financing an ADU
Most owners use a HELOC or cash-out refinance on the main home. Renovation and construction loans that count projected ADU rent toward qualifying income are now common, and the CalHFA ADU Grant has periodically offered up to $40,000 for pre-development costs when funded. For multifamily owners, a construction loan sized on stabilized NOI with the new ADUs is the standard structure, and the refinance after lease-up is what returns the capital.
What moves the number most
- Finish level: builder-grade versus custom kitchens, tile, and windows can swing $60 to $120 per square foot on their own.
- Site conditions: slope, retaining walls, soils reports, and tree protection.
- Utilities: distance to the sewer lateral and whether the panel needs an upgrade or a new service.
- Jurisdiction: plan check fees, sprinkler rules, and inspection timelines vary city to city.
- Contractor pricing: the same scope can bid 25% apart in the same month.
Return on cost
Parcy's core metric is return on cost: value added divided by what you spend. A $320,000 detached ADU renting for $3,200 a month adds roughly $450,000 of value under the income approach at a 5.5% cap rate, a 40% return on cost before financing. Run your own lot with the app.
Frequently asked questions
What is the cheapest type of ADU to build?
A JADU or a garage conversion, because both reuse the existing structure. Expect $60,000 to $180,000.
Are ADUs exempt from impact fees in California?
ADUs under 750 square feet are exempt from impact fees statewide. Larger units pay fees proportional to the main house.
Informational only. Not legal, architectural, or investment advice. Confirm current rules with your local planning department.