What is the difference between an accessory dwelling unit (ADU), granny flat and mother-in-law unit?

Accessory Dwelling Unit (ADU), Granny Flat, and Mother-in-Law Unit are essentially different terms used to describe the same concept: a smaller, secondary housing unit on the same property as a primary residence. The terms are often used interchangeably, but they can sometimes carry slightly different connotations based on regional usage or specific features. Here’s a brief explanation of each:

1. Accessory Dwelling Unit (ADU): This is a general term that encompasses any secondary housing unit on a residential property. ADUs can be built in various ways, such as being attached to the main house (like an above-garage apartment), detached (a separate building), or a conversion of existing space (like a basement). They typically have their own kitchen, bathroom, and separate entrance.
2. Granny Flat: This term is often used to refer to a type of ADU that is intended for use by aging parents, hence the name “granny”. The idea is to provide a living space that allows for independence while still being close to family for support. However, it is just another term for an ADU and does not have to be used exclusively by grandparents. It can also be detached or attached to the main house.
3. Mother-in-Law Unit: This term is very similar to a Granny Flat and is another name for an ADU. It traditionally refers to a space that is separate from the main house and is used to provide privacy and independence for in-laws when they visit or live with the family.

In essence, the differences between these terms are minor and more about their usage in different contexts or regions. In practice, they all refer to a secondary housing unit on a property. The design, size, and specific features of these units can vary widely depending on local regulations, the property’s size, and the homeowner’s preferences.