Find a Professional
  • ADU Types

Four ways to add a second home to your lot.

Detached, attached, garage conversion, or basement — the right type depends more on your existing structure and budget than on preference. Here’s how they actually compare.
Modern wooden modular attached accessory dwelling unit with large glass panels and a front patio deck set among pine trees.
  • Most private · Most flexible

Detached ADU

A fully standalone structure, usually in the backyard. Offers the most privacy and design freedom of any ADU type, since it isn’t constrained by an existing structure — but it’s also the most expensive, since everything is built from the ground up.

Typical cost

$180K – $400K+

Typical size

500–1,000 SQ FT

Timeline

8–14 MONTHS

Where it wins
  • Maximum privacy for both households
  • Full design freedom, including prefab/modular options
  • Easiest to rent out independently
Where it costs you
  • Highest total cost of the four types
  • Requires new utility runs from the main house
  • Longest permitting and construction timeline
Exterior view of a detached single-story backyard accessory dwelling unit with a front porch and wooden deck.
  • Shared structure · Lower cost than detached

Attached ADU

Built onto the main house, sharing at least one wall, but with its own separate entrance. Splits the difference between a detached unit and a full conversion — new construction, but shorter utility runs and a smaller footprint than going fully standalone.

Typical cost

$120K – $280K

Typical size

500–800 SQ FT

Timeline

6–10 MONTHS

Where it wins
  • Shorter utility runs than a detached unit
  • Often smaller footprint impact on yard space
  • Can share some structural costs with the main house
Where it costs you
  • Less acoustic/visual privacy than detached
  • Still new construction — not a budget option
  • Design constrained by existing house footprint
Interior conversion of a garage and basement into a living space with exposed steel beams and a spiral staircase for 303 ADU.
  • Most budget-friendly · Fastest

Garage conversion ADU

An existing attached or detached garage, converted into living space. Because the structure, foundation, and roof already exist, this is consistently the fastest and most affordable path to an ADU — with the tradeoff of losing covered parking.

Typical cost

$60K – $150K

Typical size

300–500 SQ FT

Timeline

4–7 MONTHS

Where it wins
  • Lowest cost of the four types
  • Fastest permitting — often no new foundation
  • Structure and roof already exist
Where it costs you
  • Loses covered parking or storage
  • May need new insulation, plumbing runs, ceiling height fixes
  • Smallest typical footprint of the four types
Interior remodeling and moving boxes during a basement accessory dwelling unit construction project.
  • No new footprint · Slowest to plan around code

Basement / interior ADU

A self-contained unit carved out of existing basement or interior square footage, with a separate entrance added. Adds no visible footprint to the lot, but often needs the most work on egress windows, ceiling height, and moisture control to meet code.

Typical cost

$75K – $175K

Typical size

400–700 SQ FT

Timeline

5–9 MONTHS

Where it wins
  • No change to the lot's visible footprint
  • Can sidestep some setback/height rules entirely
  • Often the shortest utility runs of any type
Where it costs you
  • Egress window and ceiling-height code requirements
  • Moisture and radon mitigation often required
  • Least private of the four — shares floor/ceiling with main house

Curious what any of these actually cost near you?

Browse vetted professionals
Secret Link