ADU construction: cost, permits and timelines
A detached ADU in Southern California runs $220,000 to $385,000 for a 600–1,000 sq ft unit and takes seven to eleven months. A garage conversion starts near $95,000. A junior ADU inside the existing house is the cheapest path of all.
Those are our real numbers, not a range scraped off a national average. Below is what each type costs, what moves the number up or down, how long each phase actually takes, and what happens at the permit counter in your city.
We design it, engineer it, submit it, answer the correction letters and build it. One licensed contractor from the first site visit to the certificate of occupancy.
What an ADU costs to build
Priced by unit type. The per-square-foot figure falls as the unit gets bigger — the kitchen, bathroom and utility connections cost roughly the same whether the unit is 500 sq ft or 1,000.
| Type | Typical size | Cost range | Per sq ft | Timeline |
|---|---|---|---|---|
| Junior ADU (JADU) | Up to 500 sq ft | $90,000 – $150,000 | $210 – $320 | 3 – 5 months |
| Garage conversion | 350 – 500 sq ft | $95,000 – $165,000 | $210 – $330 | 4 – 6 months |
| Prefab / modular | 400 – 800 sq ft | $150,000 – $280,000 | $230 – $360 | 5 – 8 months |
| Attached ADU | 500 – 800 sq ft | $170,000 – $290,000 | $255 – $375 | 6 – 9 months |
| Detached ADU | 600 – 1,000 sq ft | $220,000 – $385,000 | $290 – $430 | 7 – 11 months |
The four types, and which one fits your lot
Most homeowners arrive knowing they want “an ADU” without knowing there are four different things that word covers. They are not interchangeable, and the right one usually comes down to your lot, not your preference.
Detached ADU
A standalone building in the yard with its own entrance, kitchen and bathroom. The most expensive and the most valuable — it rents as a real home and appraises as one. Needs enough open yard to meet setbacks and a route for equipment to get back there.
Attached ADU
Built onto the existing house, sharing one wall but with a separate entrance and full kitchen. Cheaper than detached because one wall and part of the foundation already exist. Good on narrow lots where a standalone unit will not fit the setbacks.
Junior ADU (JADU)
Carved out of the existing house, capped at 500 sq ft, with an efficiency kitchen. The fastest and cheapest route. It can share a bathroom with the main house, which is what keeps the cost down. Often a converted bedroom with an exterior door added.
Garage conversion
Turning an existing garage into permitted living space. The slab and walls are already there, so you are paying for insulation, systems and finishes rather than structure. Whether your city still requires replacement parking is the question that decides the budget.
What we actually build
A 750 sq ft detached unit on a 25′ × 30′ footprint — the size that fits most standard lots while still working as a genuine two-bedroom home.
What moves the number
Two identical 750 sq ft units on two different lots can be $80,000 apart. Here is where that gap comes from, roughly in order of how much it matters.
| Variable | What it does | Typical swing |
|---|---|---|
| Electrical service | If the main panel cannot carry another dwelling, you need a sub-panel and often a service upgrade from the utility. | $6,000 – $12,000 |
| Sewer and water runs | Distance from the unit back to the existing laterals. Long trenches through finished hardscape get expensive fast. | $4,000 – $18,000 |
| Site access | Whether an excavator and a concrete truck can reach the back yard, or everything comes through a side gate by hand. | Up to 15% |
| Slope and soils | A flat pad needs a standard footing. A slope may need retaining, deeper footings or a geotechnical report first. | Up to 18% |
| Finish level | Cabinets, counters, flooring and fixtures. The single largest thing you actually control. | Up to 30% |
| Design review | Cities with objective design standards or a historic overlay add drawing time and a review cycle. | $4,000 – $9,000 |
How long an ADU takes
The part that surprises people: design and plan check take about as long as building the thing. Nobody is on your lot for the first three to five months.
- 1 – 2 weeks
Feasibility and site check
We confirm the unit fits your parcel under setbacks, height and coverage before you spend anything on design.
- 4 – 8 weeks
Design and drawings
Floor plan, elevations, selections. This is where you make the decisions that set the budget.
- 2 – 3 weeks
Structural and Title 24
Engineering and the energy compliance package, produced alongside the architectural set.
- 4 – 8 weeks
Plan check submittal
Submitted to your city. Then you wait for the first round of comments.
- 2 – 6 weeks
Corrections and resubmittal
We answer the correction letter and resubmit. Most sets take one round; some take two.
- 4 – 6 weeks
Foundation and framing
Permit issued, ground broken. The unit goes from a hole to a weather-tight shell.
- 3 – 4 weeks
Rough systems and inspections
Electrical, plumbing and mechanical roughed in, then inspected before anything gets covered.
- 6 – 10 weeks
Insulation, drywall, finishes
The longest build phase. Insulation, drywall, cabinets, flooring, fixtures, paint.
- 1 – 2 weeks
Final inspection and sign-off
Final walk-through with you, punch list, then final inspection and the certificate.
What the price includes
The line that separates an honest bid from a low one is usually what got left out of it. Here is ours.
Included in every ADU we quote
- Architectural drawings and the full permit set
- Structural engineering and Title 24 compliance
- Permit submittal and every correction letter answered
- Foundation, framing, roofing and exterior finish
- Electrical, plumbing and mechanical, roughed and finished
- Insulation, drywall, paint, flooring and trim
- Kitchen and bathroom cabinets, counters and fixtures
- All inspections through final sign-off
- Workmanship warranty in writing
Quoted separately, because it varies
- City permit and impact fees, paid at issuance
- School fees where the city levies them
- Utility service upgrades charged by the utility
- Geotechnical report, if your city or slope requires one
- Landscaping and hardscape beyond the unit
- Solar, where a new dwelling triggers it
- Appliances
- Furniture and window coverings
The permit process, honestly
Every ADU needs a permit. There is no version of this where you skip it — an unpermitted unit cannot be legally rented, will not appraise, and becomes the buyer's problem when you sell.
We submit, not you
The drawings go in under our licence. When the correction letter comes back, it comes to us and we answer it. You never stand at a counter.
State law sets the floor
California law requires cities to review a conforming ADU application ministerially — no public hearing, no neighbour vote. Your city fills in the rest.
Your city fills in the rest
Setbacks, height, objective design standards, fire-zone requirements and how the utility handles a new meter. This is where projects actually stall, and it differs across all 134 cities we cover.
We do not publish statute numbers we have not checked. California ADU law has changed nearly every year since 2017 and city ordinances move on their own schedule. Requirements vary by jurisdiction — confirm current rules with your building department before relying on anything you read here.
Where we build ADUs
Every city has its own page with that city's setbacks, height limits, permit process and current plan-check timeline.