Detached vs Attached ADU

The detached-versus-attached distinction describes how an accessory dwelling unit physically relates to the primary residence. A detached ADU — sometimes called a DADU or backyard cottage — is a standalone structure separate from the main house. An attached ADU shares at least one wall with the primary residence, more like an addition with its own entrance, kitchen, and bathroom.

The practical differences matter for cost, privacy, and construction. Detached ADUs usually offer the most privacy and design flexibility but typically cost the most, because they are new construction from the ground up with their own foundation and utility connections. Attached ADUs can be less expensive when they extend existing structure and tie into nearby utilities, though they offer less separation and are constrained by the shape of the existing house.

Garage conversions are a common middle path: converting an existing detached garage produces a detached-style unit, while converting an attached garage yields an attached one — often at lower cost because the shell already exists, subject to egress, ceiling-height, and utility considerations. Which form a project counts as can also affect the applicable setback, size, and parking rules, so the distinction is more than cosmetic.

Neither form is universally better; the right choice depends on lot size, budget, how much privacy the occupant needs, and local zoning and setback rules. Detached and attached ADUs both require a building permit and are distinct from a junior ADU, which is carved from a home's existing interior. Setback, size, and parking rules vary by jurisdiction and can change, so confirm what your city allows before you design.