Second Unit Idea Book & Workbook

The San Mateo County Second Unit Center is part of the Home For All  “Action Plan” to create more housing in the 20 cities in San Mateo County.  Among other links, there is a cost estimate calculator on their website.

By definition, a “second unit” or accessory dwelling unit, also called a Granny Flat, garage conversion, or whatever the configuration, second units are required to have a kitchen, bathroom, and a place for sleeping. San Mateo County also is planning a new pilot program to help local homeowners to build their ADUs called the “One Stop Shop”.

The San Mateo County Second Unit Center has provided some excellent materials to support those interested in adding a second unit to their property, and have given permission to post these here for your use:

Idea Book

Granny flat, in-law suite, accessory dwelling unit (ADU), converted garage, backyard cottage, basement apartment - whatever you call it, get inspired to build one in San Mateo County!


A tool for homeowners considering building a second unit in San Mateo County, California. While the county’s focus is local, the ideas are great for ADUs anywhere.