Building an ADU in your backyard is an attractive option for many people. It can allow for extra storage, multi-generational housing, and an income opportunity if you decide to rent it out. However, you can’t just build one as big or as little as you want. You need to be aware of the different ADU sizes that you can and cannot build.

Maximum ADU Size Limit

Regulations surrounding size limits vary from region to region. The house down the street might be able to build a big ADU but you might not be able to. It all depends on what zone you fall within.

However, generally speaking, the maximum size of an ADU in California is 1,200 square feet for a detached unit. If your unit has one or fewer rooms, it may be limited to 800 square feet. If it has two or more bedrooms, you may be limited to 1,000 square feet. It is all dependent upon your local government’s laws and regulations, but these are the average limits.

For an attached, the general rule is that it cannot exceed 50% of the square footage of the home. So, how big it can be is completely dependent upon how big your house is.

Minimum ADU Size Limits

Since ADUs are intended to be lived in, there are indeed minimum size limits you should be aware of. In most areas, an ADU must have a living area of at least 150 square feet. The kitchen itself must be at least 50 square feet and the bathroom must be 30 square feet.

There is, however, a way to build a smaller ADU. You will need to apply for a discretionary permit. This is very difficult to obtain, but if you can make a good case as to why you should be allowed to build a smaller ADU, it is possible to get.

Determining Your ADU Size

You will need to take into consideration what you will be using your ADU for when determining how big it should be. The best size for your ADU is probably below 750 square feet because then it will be exempt from bulk and massing development limits.


ADUs aren’t for everyone. Just as owning a small apartment doesn’t necessarily lead to a life without material wants, small house owners can feel entitled to every amenity they can get. But most people who own ADUs love the freedom afforded by their size. Some want the added space for guests on occasion, while others may want the opportunity to earn some extra income from another home within their own. But most of all, ADUs offer an escape hatch from expensive city living for those who intend to stay put for awhile and are willing to invest in the process.