ADU vs Guest House vs Granny Flat — What's the Difference?

Homeowners often use terms like ADU, guest house, granny flat, in-law suite, and casita interchangeably — but legally, they can mean very different things. The terminology you use when applying for permits directly impacts what you can build, how it can be used, and whether you can rent it out.

ADU (Accessory Dwelling Unit): The Legal Definition

An ADU is a legally defined secondary housing unit on a residential property. It must have its own kitchen, bathroom, and sleeping area. ADUs are fully independent living spaces that can be rented to non-family members. Most zoning codes now specifically reference 'ADUs' with clear size limits, setback requirements, and permit processes.

Guest House: Not Always Rentable

A guest house is typically a separate structure without a full kitchen. Many jurisdictions distinguish between a guest house (no kitchen = not an independent dwelling) and an ADU (full kitchen = independent dwelling). This distinction matters because guest houses often cannot be legally rented out.

Granny Flat, In-Law Suite, Casita: Regional Terms

These are informal terms with no universal legal definition. 'Granny flat' is common in Australia and the US West. 'In-law suite' is popular in the Northeast. 'Casita' is used in the Southwest. When applying for permits, always use your city's official terminology — usually 'ADU' or 'accessory dwelling unit.'

Why Terminology Matters for Your Permit

Using the wrong term on your permit application can cause delays or denial. If your city has an ADU ordinance, file under that program. If you say 'guest house,' you may get classified under different (often more restrictive) zoning rules. Always check your city's specific ADU regulations before applying.