ADU
What an ADU Really Costs in Southern California
Almost every ADU conversation starts with a number someone heard at a dinner party. The honest answer is a range with four variables attached, and once you know the variables you can predict your own number closely enough to make a decision.
August 12, 2026 · By Plantek West
The ranges, stated plainly
Across Los Angeles and Orange County, garage conversions typically run roughly $150 to $275 per square foot. Attached ADUs typically run roughly $200 to $350. Detached new construction typically runs roughly $250 to $450 per square foot of finished space.
Those ranges include construction. They do not include design and permitting fees, city permit and impact fees, utility connection charges or furnishings. Budget those separately, because they are real and they are frequently omitted from the numbers people quote each other.
The four variables that decide where you land
Two identical 750 sq ft units on two different lots can differ by a hundred thousand dollars. It is almost never the finishes.
- Utility distance. The sewer, water, gas and electrical run from the existing service to the new unit. Twenty feet is cheap. Eighty feet across a mature garden with a driveway crossing is not.
- Site access. If a mini excavator and a concrete pump can reach the rear yard, the foundation is ordinary. If everything moves by wheelbarrow through a side gate, labour cost rises for the entire structural phase.
- Grade. A flat pad pours cheaply. A slope brings retaining walls, drainage design and frequently a soils report.
- Finish level. The same envelope can be delivered as a durable rental or as a guest house you would happily live in. That gap is usually 20 to 30 percent of construction cost.
The fees people forget
School fees are charged per square foot of new habitable area. Water and sewer capacity charges vary widely by district and can differ substantially between neighbouring cities. Some cities waive impact fees for units under 750 sq ft, which is one reason so many ADUs land at 749.
If a proposal you receive has no line for these, it is not a budget. Ask for them by name.
How to get a number you can actually rely on
Pay for a feasibility review before design. It tests the lot, the utilities, the setbacks and the objective standards of your city, and it produces a cost range grounded in your specific site rather than an average. If the answer is that the project does not pencil, you have learned that for a small fee instead of after a design.
Questions on this topic
Is a garage conversion always the cheapest option?
Usually, and not always. If the slab must be replaced, the framing needs substantial upgrade and the waste line needs a pump to reach the sewer, a conversion can approach new-build cost with less design freedom. Those three items should be checked before you commit.
Does an ADU pay for itself?
That depends on your local rent, your financing cost and how long you hold the property. We are happy to model it honestly with you, including the case where the answer is no.
Keep reading
ADU
ADU rules city by city: how LA and Orange County departments differ
State law sets the floor for ADUs. What still varies is coastal review, objective design standards, fee schedules and how many correction cycles a department runs.
Permitting
What plan check corrections actually cost you
Permit fees are published. Correction cycles are not, and they are usually the larger number. Here is what triggers them and how to avoid most of them.
Commercial
Tenant improvement cost per square foot in LA and Orange County
Fit-out ranges by use type, how to read a work letter before you sign, and the three base building conditions that decide the budget.
