How much does it cost to build an ADU in California?

Why a small unit prices high per square foot#

An accessory dwelling unit is a complete home compressed into a small footprint. It needs a kitchen, a bathroom, a water heater, heating and cooling, an electrical panel, insulation, waterproofing and finishes. Those items cost close to what they cost in a large house.

That is the whole mechanism. At a given finish level the fixed content of a dwelling is roughly fixed, and a small unit has very little area to spread it across. Build the same rooms to the same standard on a larger footprint and the per square foot number falls. The comparison only holds like for like, though. A high end custom home finished to a far higher standard runs well above any of the ADU numbers below, because finish level moves the figure at least as hard as size does.

Mobilization works the same way. Fencing, a dumpster, temporary power, a portable toilet, supervision and the general conditions of running a jobsite are priced by the week, not by the square foot. A small project carries nearly the same weekly overhead as a large one and has far fewer square feet to spread it across.

The third reason is that very little is shared with the existing house. Even an attached unit generally needs its own systems, its own point of entry and, in many jurisdictions, its own metering or at least a clean separation of service. Anything that has to be independent has to be built twice.

What the three types actually run#

These are general Southern California market ranges for a completed unit, including the contractor's overhead but excluding land, financing and furniture. Treat them as a planning band rather than a quote.

Type	General all-in range	What moves the number
Garage or interior conversion	$150 to $350 per square foot	Slab condition, ceiling height, whether the foundation needs work
Attached addition	$250 to $500 per square foot	New foundation, tie-in to the existing structure, roof and wall interface
Detached new construction	$350 to $700 per square foot	Site work, trenching distance, access for equipment, finish level

The ranges overlap because the type of unit is a weaker predictor of cost than the condition of the site. A conversion on a failing slab can cost more than a detached unit on a clean flat pad with utilities at the property line.

Garage or interior conversion#

The cheapest path when the shell cooperates. The savings come from a foundation, walls and roof that already exist. The risk is that a slab poured for parking was never intended to be lived on, so drainage, moisture protection, floor height and the footing under the perimeter all have to be verified rather than assumed.

Attached addition#

Sits between the other two. There is new foundation and new framing, but the unit shares a wall and often a roofline. The complications are at the seam: matching the existing structure, cutting into an old roof, and dealing with whatever is discovered inside the wall that was opened.

Detached new construction#

The most expensive per square foot and the most predictable, because nothing is inherited. Everything is new and everything is designed. Cost is driven less by the building than by what it takes to reach it, which is the part most owners have not priced.

The lines that get left out of the first budget#

Small residential projects tend to run over in the same five places.

Utility connections. Water, sewer, gas and power all have to reach the new unit. On a deep lot that means a long trench, and a trench crossing a driveway, a mature root system or a finished hardscape is expensive in a way that has nothing to do with the building itself.

Electrical service. Adding a dwelling means adding a range, a water heater and a condenser to a service sized for one household. Where the existing panel will not carry that, the budget picks up a panel upgrade, and sometimes a service upgrade that the utility prices and schedules on its own calendar.

Sewer capacity. The existing lateral was sized for one house. Adding a second dwelling can require a new lateral, an upsized one, or a capacity study. On a septic system the question is larger: the leach field may need expansion or replacement, and a percolation test is the only way to know.

Site work and access. The line nobody prices. When ready mix has to be pumped over a roof instead of chuted off the back of the truck, the foundation gets more expensive before a single form is set. When a compact excavator will not fit past the corner of the garage, the dig becomes hand work. Anything that cannot be delivered to the work area gets carried to it, and carrying is paid by the hour.

Fees and soft costs. Architecture, structural engineering, energy compliance documentation, a survey where one is required, plan check and permit fees, school fees where they apply, and utility connection charges. State policy has narrowed which impact fees can be charged on units below a general size threshold, but that threshold, and how each agency applies it, varies locally and changes. Ask the local agency what applies to a unit of the size you are proposing rather than relying on a figure quoted anywhere else, including here.

What to price before you commit to a design#

Get a load calculation on the existing electrical service. An electrician can do this quickly. It converts the single largest budget surprise into a known line before the drawings are finished.

Find the sewer lateral and confirm its condition. A camera inspection is inexpensive relative to what it tells you. On septic, commission the soils and percolation work early rather than after design.

Measure the side yard at its tightest point before you draw. That single dimension decides what equipment reaches the work area, which in turn sets how the foundation, the framing and the material handling get priced. Have a builder look at it with you rather than reading it off a plot plan.

Ask the planning counter what applies to your parcel. Not what applies generally in California, but what the current local ordinance requires for your zone, your lot and the unit size you are considering. The answer varies between neighboring cities.

Price the finishes at the level you actually want. A kitchen and a bathroom carry disproportionate weight in a small unit. The difference between builder grade and the level of custom cabinetry most owners picture is a meaningful share of the total on a footprint this small.

Planning around a band instead of a number#

The honest answer to what an ADU costs is that the site decides more than the design does. Trenching distance, equipment access, slab condition and service capacity can move a bid by a third, and not one of them appears anywhere on the floor plan.

The practical approach is to spend a small amount early on the three unknowns that move the number most: the electrical service, the sewer or septic capacity, and the physical route into the back yard. Each of those can be investigated for a fraction of what it costs to discover mid-construction, and each one converts a guess into a line item.

Plan around the upper half of the range for your unit type until those three answers exist. If they come back clean, the budget improves. That direction is easier to live with than the alternative, and it is how the residential development side of a small project stays predictable.