Types of ADUs

1. Detached ADU (backyard cottage, DADU)

A freestanding small house in the yard. The most expensive option (it's a complete building), but also the most valuable: maximum privacy for both households, the strongest rental appeal, and the cleanest separation of utilities and entrances.

• Cost: $150k–$400k+ all-in
• Best for: larger lots, rental income, multigenerational living
• Watch for: setbacks, height limits, utility trenching distance, lot coverage caps

2. Garage conversion

The workhorse of the ADU boom, especially in California. The shell already exists, which cuts cost substantially, but garages were never built to be lived in, so insulation, windows, plumbing, and often the slab all need work. You also lose the garage.

• Cost: $80k–$180k
• Best for: standard lots where the garage sits well relative to the house
• Watch for: parking replacement requirements, slab condition, ceiling height

3. Attached ADU (addition)

New construction attached to the main house: a wing with its own entrance. Shares a wall and often utilities, so it's cheaper than detached, but design has to handle sound separation and the connection to the existing structure.

• Cost: $120k–$300k
• Best for: lots without room for a detached unit; aging-in-place setups
• Watch for: size caps tied to a percentage of the main home's area

4. Basement or interior conversion

Carving a unit out of existing space: a basement, an attic, or a section of the main floor. No new footprint, so approvals are often simpler, but code requirements for egress windows, ceiling height (typically 7 ft minimum), and moisture control decide feasibility.

• Cost: $90k–$200k
• Best for: homes with tall, dry basements; cold climates where yards are precious
• Watch for: egress, ceiling height, radon, waterproofing

5. Junior ADU (JADU)

A California-specific category worth knowing even elsewhere: a unit of 500 sq ft or less created within the existing home (often a converted bedroom + en-suite), allowed an efficiency kitchen and, in some cases, a shared bathroom. Cheapest possible path to a legal second unit, and in California you can typically add a JADU and a detached ADU on the same lot.

• Cost: $40k–$125k
• Best for: house-hacking a spare bedroom; maximizing unit count
• Watch for: owner-occupancy requirements (JADUs usually retain them)

Prefab vs. site-built (a build method, not a type)

Any detached ADU can be prefab/modular (built in a factory and craned in) or site-built. Prefab buys price certainty on the box itself and a faster above-ground schedule; it does not eliminate site work, foundations, utilities, or permits, which is where budgets usually slip. Some cities maintain pre-approved plan programsthat fast-track permits for standard designs; check your city's planning pages.

Quick chooser

Whatever the type, the first step is the same: read your city's official rules; they determine which of these are legal on your lot before any designer can.