An ADU is a second home on a lot that already has a house.

An ADU (accessory dwelling unit) is a permitted, self-contained second home on a lot that already has a house: its own kitchen, its own full bath, and its own entrance. California state ADU law recognizes four typologies: detached (standalone, 500–1,200 sqft), attached (built off the primary residence), Junior ADU (converted inside the existing footprint), and garage conversion (an existing garage rebuilt as living space).

ADU stands for accessory dwelling unit, and the ADU meaning is in the words: a dwelling — a full home with its own kitchen, bath and entrance — that is accessory to the main house on the same lot. State law and city permit offices use the term; homeowners say granny flat, casita, backyard cottage or in-law suite for the same thing.

ADU home and ADU describe the same thing: an accessory dwelling unit. People say home because the unit really is one — full-time living space with its own kitchen or kitchenette, its own full bath, and its own entrance, rather than a converted room, an office, or a shed. Avorino builds all four typologies that way.

Cities set their own ADU size limits, but state law caps how small a limit they may set. A local agency may not impose "a maximum square footage requirement for either an attached or detached accessory dwelling unit that is less than either of the following: (A) Eight hundred fifty square feet of interior livable space. (B) One thousand square feet of interior livable space for an accessory dwelling unit that provides more than one bedroom" (Gov. Code §66321(b)(2)). Above that floor the ceiling is the city’s, and the detached ADUs Avorino builds in Orange County typically run 500–1,200 sq ft.

A Junior ADU is a conversion inside the existing footprint of the primary house: typically an unused interior wing or bedroom, 300–500 sqft, one bedroom, with a kitchenette and full bath. It's the tightest-scope path in Orange County, typically $135K–$220K+ over 4–6+ months, because no new foundation or exterior structure is involved.

A garage conversion turns an existing garage into permitted living space: insulation, a conditioned envelope, and a full kitchen and bath, usually 350–600 sqft as a studio or one bedroom. In Orange County it typically runs $85K–$170K+ over 4–6+ months, and it's the fastest move when the existing garage frame is sound and you're ready to give up the parking.

A detached ADU is a standalone unit on your property, typically 500 – 1,200 sqft at $245K – $440K+ over 5–8+ mo. It carries the most rental upside and the longest schedule, and it needs the room on the lot. An attached ADU is an addition off the primary residence: it shares a structural wall, and often a utility service line, at typically 450 – 1,000 sqft and $200K – $350K+ over 5–8+ mo. Attached is the answer when the lot is tight on setbacks but the house has a wall to extend off.

Detached ADUs, attached ADUs, Junior ADUs, and garage conversions, every permitted typology under California state ADU law.

Detached ADUs typically run $245K–$440K+. Attached ADUs $200K–$350K+. Junior ADUs $135K–$220K+. Garage conversions $85K–$170K+. Use the Avorino cost estimator for a calibrated range on your specific project.

Detached: 5–8+ months. Attached: 5–8+ months. Junior ADU: 4–6+ months. Garage conversion: 4–6+ months. Windows vary with city, plan-check timing, and finish-level decisions.