ADU Basics

Learn the fundamentals of accessory dwelling units (ADUs) including what they are, how they work, and why homeowners across California are building them. Our ADU basics guides cover everything from terminology to timelines.

An accessory dwelling unit is a secondary housing unit on a single-family residential lot. Also known as granny flats, in-law suites, or backyard cottages, ADUs provide flexible living space for family members, rental income, or home office use. Understanding ADU basics helps you make informed decisions about design, permitting, and construction.