Step 02 · Understand
Which type is open to you is decided by your lot and your jurisdiction, not by which is cheapest. The differences that matter are foundation, utilities and what already exists on the site.
The short answer
There are six ADU types in common use: detached, attached, garage conversion, above-garage, basement or interior conversion, and prefabricated. They differ mainly in whether you are building a new foundation and new utility runs, or reusing ones that already exist — which is the largest single swing in what an ADU costs.
What moves it
Ignore the marketing categories. Three questions sort every ADU type:
A conversion reuses a slab. New construction pours one. This is the largest single line item that changes between types.
A detached unit in the rear yard needs water, sewer and power run to it. An interior conversion is often meters from existing service.
An above-garage unit loads a structure that was built to hold a car and a roof. A garage conversion inherits whatever the slab actually is.
01
A separate structure, usually in the rear yard. The most flexible type and the one with the clearest legal path in California: Gov. Code § 66323(a) requires ministerial approval of one detached new-construction ADU with four-foot side and rear setbacks on a lot with a proposed or existing single-family dwelling.
It is also the type that pays for everything itself — its own foundation, its own utility runs, its own roof. Nothing is inherited.
02
Reuses a slab and usually three walls. In Los Angeles this is roughly half of all ADU permits — see our permit analysis, linked below.
Whether it is genuinely cheaper depends entirely on the slab. A garage slab was poured to hold a car, not to carry a habitable floor with plumbing under it; where it has to come out, the saving goes with it.
California removes the objection people brace for: under § 66314(d)(11) the agency cannot require you to replace the parking a converted garage was providing.
03
Shares a wall with the primary house and has its own entrance. Cheaper utility runs than detached, and a taller permitted envelope in California — § 66321(b)(4)(D) sets 25 feet, or the primary dwelling’s limit if lower, for an attached unit.
The trade is privacy and separation, which is often the actual reason the ADU is being built.
04
A second story over existing parking. Buys floor area without spending yard — valuable on a small lot.
It asks the most of the structure beneath it. The existing garage was almost certainly not engineered to carry a dwelling, so a structural upgrade is the norm rather than the exception.
05
Carving a unit out of space inside the existing envelope. Often the cheapest route where the space is already conditioned and has adequate ceiling height and egress.
Egress and light are what usually decide it. A bedroom needs a compliant escape opening, and adding one to a basement wall is an excavation job.
06
Built in a factory and set on a prepared foundation. Compresses the on-site schedule; it does not remove the site work, the utilities or the permit.
Which standard the unit is built to matters more than the word "prefab" — a HUD-code manufactured home and a factory-built modular unit are different legal objects with different placement rules.
Go deeper
ResearchOriginal analysis
StatisticsSourced figures
ToolsRun your numbers
Usually an interior or garage conversion, because the foundation and often the utilities already exist. But that is a tendency, not a rule: a garage slab that has to be removed, or a basement that needs egress excavated, can cost more than a small detached unit. The cost follows the site conditions, not the category.
No. Your jurisdiction decides what is permitted on your lot. California requires ministerial approval of certain configurations statewide under Gov. Code § 66323, but the detail — setbacks, height, lot coverage, whether a second unit is allowed at all — is local. Check the jurisdiction record for your city before choosing.
You lose the garage. In California you do not have to replace the parking space: Gov. Code § 66314(d)(11) prohibits the agency from requiring replacement of off-street parking when a garage is converted to, or demolished for, an ADU. Outside California this varies and you must check locally.
Plan your project
Three short steps: the ADU you want, your home, and how to reach you. Nothing goes to a contractor unless you agree.
A straight answer: our ADU professional matching is still being built, so this is not a quote service, and we cannot tell you whether your lot qualifies. We can help you work out which questions to ask and what a thorough proposal should contain. Your planning department decides what is permitted at your address.
Get ADU Options
Tell us about the ADU you are planning and where you are. We will follow up about connecting you with contractors serving your area.