Types of ADUs
Detached ADU
A standalone backyard cottage. Local rules must allow at least 850–1,000 sq ft, with a default maximum of 1,200 sq ft. Height is 16 ft by default, or 18 ft near transit. These units give the most privacy and rental value, but cost the most.
Typical SoCal planning range: $350–550 per sq ft all-in
Attached ADU
Built onto the main home, sharing a wall. Size is up to 50% of the primary home, with a minimum allowance of 850 sq ft, and height up to 25 ft. They can share foundation and utility runs.
Typical SoCal planning range: $325–500 per sq ft
Garage conversion
Converting an existing garage. These have no setback requirement, no unit-size cap for conversions, and no replacement parking. They’re often the cheapest full ADU.
Typical SoCal planning range: $200–350 per sq ft (two-car garage roughly $90k–160k)
Junior ADU (JADU)
Up to 500 sq ft inside the existing home or attached garage, with an efficiency kitchen. Since 2026, owner occupancy is required only with a shared bathroom, and rentals must be longer than 30 days.
Typical SoCal planning range: Roughly $50k–150k total
Prefab / modular
Factory-built modular or panelized units, set on a site-built foundation. The build time is shorter, but site work, cranes and permits still apply.
Typical SoCal planning range: $325–550 per sq ft installed
Two-story / above-garage
A second floor over a garage, or a two-story detached unit. Gets more living space on a small footprint. It must fit state height allowances (16–25 ft depending on type and location).
Typical SoCal planning range: $400–600 per sq ft
Ranges come from our cost estimator sources and are for planning only. Rules are summarized from California ADU law.