Module 1 · Lesson 1
What Is an ADU?
An ADU (accessory dwelling unit) is a complete second home on your property — with its own kitchen, bathroom, and entrance. Here's what that means in plain English.
An ADU — short for Accessory Dwelling Unit — is a complete, self-contained home that sits on the same property as another house.
The key word is complete. An ADU is not a shed with a bed in it, and it's not a spare room. To count as an ADU, the space needs four things:
- A kitchen — somewhere to cook, with a sink, counter, and space for appliances.
- A bathroom — a toilet, sink, and shower or tub.
- Its own entrance — someone living there shouldn't have to walk through your house.
- Space to live and sleep — a real living area, not just a hallway with a mattress.
If a space has all four, a person could live there full-time without ever needing the main house. That's the test.
It shares your property, not your house
An ADU lives on the same lot as your main home. It might be a small house in the backyard, a converted garage, or an addition attached to the side of your house. It can share walls with your home or stand completely apart — but legally, it's a second dwelling on one property.
That's different from a duplex (two units in one building on a lot zoned for it) or a guest house (which often has no kitchen and can't legally be lived in full-time).
Why California made ADUs easy
For decades, most California cities made it nearly impossible to add a second unit. Starting in 2017, the state passed a series of laws that force cities to approve ADUs that meet basic standards. Cities can't say no just because they don't like the idea.
That's why you're seeing ADUs everywhere: the law is on the homeowner's side now. Most single-family properties in California can legally add at least one ADU — often an ADU and a junior ADU (more on that in the next lesson).
Want the technical version?
California's ADU statutes were consolidated in 2024 into Government Code sections 66310–66342 (formerly 65852.2). The law defines an ADU as an attached or detached residential unit that provides complete independent living facilities — permanent provisions for living, sleeping, eating, cooking, and sanitation — on the same parcel as a proposed or existing primary dwelling. Cities must review compliant applications ministerially (no public hearings, no discretionary review) and the state Department of Housing and Community Development (HCD) can void local ordinances that conflict with state law.
Key takeaways
- An ADU is a complete second home on your property: kitchen, bathroom, its own entrance, and living space.
- It can be detached, attached, or converted from existing space like a garage.
- California state law requires cities to approve ADUs that meet basic standards — approval is not a favor, it's the law.
- Most single-family properties in California qualify for at least one ADU.