A granny flat is a self-contained secondary home built on the same lot as your main house: its own kitchen, its own bathroom, its own front door, and legally, its own permit. The name suggests it's just for grandma. In practice it's a rental unit, a home office, an adult kid's first apartment, or exactly what the name says, all built from the same set of drawings and the same permit process.
Here's the part nobody tells you up front: "granny flat" isn't a legal term anywhere in the United States. It's slang. Your city's building department has never issued a "granny flat permit" in its life. What you're actually applying for is an accessory dwelling unit (ADU), and getting that translation right, before you talk to a contractor or a city planner, is the difference between a smooth conversation and one where everyone nods politely while quietly Googling what you mean.
Granny Flat, ADU, In-Law Suite, Casita: The Terminology Map
Five different words show up in five different Google searches, and they almost all point at the same building department form. Knowing which word your city, your lender, and your contractor actually use saves you from three separate conversations that all should have been one.
