Cost · permit · scope
ADU planning guide
Plan an ADU: why every one needs a permit, California's size and setback floors, Los Angeles's 1,200 sq ft cap and Austin's three-unit HOME rules.
Permit complexity
High
Compare quotes per
unit
ADU buying path
Zoning first, floor plan second.
An ADU is a second home on the lot, so the permit is a given. What decides the project is whether the lot qualifies and which size, height and setback lines apply to it.
01 · Qualify
Check the lot
Austin wants an SF-1, SF-2 or SF-3 lot of at least 5,750 square feet. California allows an ADU on any lot zoned for homes with a house on it or planned, and Los Angeles limits them on lots in both a Very High Fire Hazard Severity Zone and a Hillside Area.
02 · Size
Fit the lines
Every California city must allow 800 square feet and 16 feet of height with 4-foot side and rear setbacks, and Los Angeles caps a detached ADU at 1,200 square feet and two stories. Central Austin caps each new unit at 40 percent of the lot or 2,300 square feet.
03 · Connect
Plan the utilities early
In California, an ADU made from existing space cannot be made to take a new utility connection unless it comes with a new house. Austin wants an address for every unit before the building permit application.
Decision matrix
Compare one specification at a time
Detached ADU
The most privacy, on a lot with room
Specify the foundation, framing and roof, the utility runs from the house, and the distance to each lot line.
Garage conversion
Reusing a structure already on the lot
Specify slab, wall and insulation upgrades, the new doors and windows, and the plumbing run to the house.
Junior ADU inside the house
A unit of up to 500 square feet within the home
Specify the separate entrance, the efficiency kitchen, and whether the bathroom is shared or its own.
Quote controls
Keep every bid comparable
- Same floor area, bedroom count and height
- Foundation, framing and roofing itemized
- Water, sewer and electric runs priced with any connection fees
- Plans, permit fees and inspections assigned
- Kitchen and bath fixtures named by model or allowance
Next best action
Calculate, then lock the scope.
Size the foundation in the concrete calculator once the footprint is fixed, and ask bidders to separate the shell, the utility runs and the finishes.
Start with two separate questions
Always: an ADU is a dwelling unit, and no exemption covers one.
California caps the side and rear setbacks a city may require of a new ADU at 4 ft and requires none for a conversion; Austin's rules for a second or third unit set a 15 ft front setback.
What usually decides the scope
- Detached, attached or a conversion
- Lot zoning and area
- Floor area, height and bedrooms
- Setbacks and the walk to transit
- Utility connections and an address
State floors in California
A California city must allow an ADU of at least 800 square feet and 16 feet of height with 4-foot side and rear setbacks, and it cannot require the owner to live on the property.
Units per lot in Austin
Since the HOME amendments, SF-1, SF-2 and SF-3 lots of at least 5,750 square feet may hold up to three units, each with its own address; in central Austin each new unit is capped at 40 percent of the lot or 2,300 square feet, whichever is greater.