The legal definition, in one paragraph
Across California, Oregon, Washington, Nevada, Arizona and Idaho, an ADU is defined as a self-contained residential unit located on the same parcel as a primary dwelling. It must provide permanent facilities for living, sleeping, cooking and sanitation. That is the whole test. A converted garage with a bed and a mini-fridge is not an ADU; a 400 sq ft studio with a full kitchen, a full bathroom and its own door is.
ADUs are also called granny flats, in-law units, casitas, backyard cottages and secondary suites. These are marketing and regional names for the same legal object — the permit still says accessory dwelling unit.
The five types of ADU
Which type you can build is decided mostly by your lot, not your preference. A feasibility check answers it in a day.
| Type | What it is | Best when |
|---|---|---|
| Detached | A standalone building in the rear or side yard | You have open yard and want maximum rent and privacy |
| Attached | An addition sharing one wall with the house | The yard is tight but the house has an expandable side |
| Garage conversion | An existing garage converted to living space | The garage is sound and you want the lowest cost |
| Interior conversion | A basement, attic or wing walled off with its own entry | Square footage already exists inside the home |
| Junior ADU (JADU) | Up to 500 sq ft carved out of the existing house | You want the cheapest, fastest legal unit |
How big can an ADU be?
State law sets floors that local governments cannot go below. In California, a jurisdiction must allow at least 850 sq ft for a one-bedroom ADU and 1,000 sq ft for two or more bedrooms, and it cannot block an 800 sq ft unit on lot-coverage or floor-area grounds. Oregon and Washington run similar state-level protections; Nevada, Arizona and Idaho vary more by city.
In practice the sizes that get built cluster tightly: 400–600 sq ft for a studio or one-bedroom, 700–850 sq ft for a comfortable one-bedroom with a real living room, and 900–1,200 sq ft for two bedrooms. Above 1,200 sq ft you are usually building a second house, and the permitting posture changes.
What goes into an ADU budget
An all-in budget includes design, permits and fees, site work, utilities, the structure itself and finishes. The structure is rarely the expensive surprise — site work and utility connections are, and both are set by your parcel rather than by the plan you choose.
Homeland builds start at $165 per square foot, and we issue a line-item, all-in number after a free lot review. Conversions of existing space sit at the lower end of any budget because they reuse a foundation; new detached construction sits higher and rents for more.
Why people build them
Three reasons dominate. Rental income: a detached one-bedroom in most of our markets rents for $1,600–$2,600 a month, which services a meaningful share of the construction loan. Family: aging parents or adult children housed close but independent. Value: a permitted, income-producing unit is one of the few improvements appraisers can price directly through the income approach.
The trade-offs are real too — you lose yard, you take on a landlord role unless you hire a manager, and you carry construction risk for several months. Our ROI guide runs those numbers honestly.
What to do next
Before you fall in love with a floor plan, confirm what your parcel allows: setbacks, height limits, utility capacity and whether your lot has an existing structure worth converting. That is a free feasibility check, and it is the single highest-value hour in the whole process.