So Many Types of ADUs, So Many Varying Costs

Author: Renée J. Schomp, Director, Napa Sonoma ADU Center

Accessory dwelling units are known by many different names -- granny flat, second unit, backyard cottage, in-law apartment, garage conversion, carriage house -- that it can get pretty confusing what is what. Do the same ADU rules apply to all of these different forms?

The answer is YES -- an ADU is simply a secondary dwelling unit with complete independent living facilities for one or more people. So whether or not you’re creating a stick-build, ground-up detached unit on your parcel or converting already-existing interior space within your primary residence, ADU rules apply. The same goes for “Prefab” ADUs, too. (And don’t forget Junior ADUs!) The cost of building these different types of ADUs can vary significantly.

In short, there are five different types of ADUs that encompass all of those different names listed above:

DETACHED

Free-standing structure, such as a backyard cottage

GARAGE APARTMENT

Converted former garage space

ATTACHED

Shares at least one wall with the primary home

ABOVE GARAGE

Unit built above garage

INTERIOR CONVERSION

Built from existing converted space (e.g., an attic or a basement)

When many homeowners first think of building an accessory dwelling unit, they envision a separate, detached dwelling in their backyard. This is a terrific option, and it’s also helpful to keep in mind that it is often the most expensive type of ADU to build.

ADUs range in cost significantly depending on a number of factors including size, type, materials, fees, and parcel characteristics. On average at time of publication, a small interior conversion can cost under $50K if you are converting already finished space and adding a small kitchen. 

Converting unfinished space, like an attic, basement, garage or other existing space is more and depends on the condition of what is there now, but might cost $100,000-$150,000 for a midsized ADU.

Detached ADUs can frequently run closer to $200-300K and it's possible to see large ADUs with high end finishes that run closer to $350K and up. 

As a nonprofit organization that does not provide design/build services ourselves, our goal is to serve as a neutral third party to help you assess realistic ADU costs for your project. We recommend you use our ADU Calculator to assess the rough cost of your ADU project. And, keep in mind that due to wildfire rebuilds and the impacts of COVID-19 on the supply chain, costs for materials and labor are frequently changing.

And, we always remind homeowners that they may be surprised by some of the creative ways they can build an ADU without creating a brand new detached unit. Many folks have unused space, including multiple unused bedrooms, in their primary residence that can be converted into a separate unit. This is a great solution especially for homeowners looking to downsize and cut costs for retirement so that they can afford to age in place. Learn more about how one of your neighbors in Sebastopol, Marian, did this for just $10,000!