FAQS – Accessory Dwelling Units

“ADU” stands for Accessory Dwelling Unit. These are secondary homes or apartments built on an existing single-family residential lot. Other names for ADUs include in-law units, granny flats, and backyard cottages. ADUs can be attached to a home or garage, or they can be a standalone unit. They typically range in size from 500 to 1,200 square feet and have their own kitchen, bathroom, living area, and a separate entrance.

ADUs are legal in California, and as of 2021, they’re now much simpler to build. Many of the previous restrictions on them have been lifted, and permits can be acquired in half the time (60 days instead of 120).

Anyone who owns a single-family home can build an ADU if their home is in a residential or mixed-use zone. For Sonoma and Napa Counties, you can look up your address to determine what you’re eligible to build. The information provides your jurisdiction, recorded use, zone, number of dwellings, approximate lot size, and approximate building footprint for your property. It also summarizes what possible ADU options or ADU combinations could be available to build on your property.

In Sonoma County, ADUs must be a minimum of 275 square feet and a maximum of 1,200 square feet. If the ADU will exceed your home’s floor-area ratio (FAR), the maximum size your ADU can be is 800 square feet.

The cost of an ADU varies depending on the project. You can use this accessory dwelling unit calculator to get an idea of the estimated costs to build an ADU. The costs for an ADU varies by type, site conditions, utility connections, selection of interior and exterior finishes, and added features (solar, new garage with upper ADU unit, extended patio, or decking, etc.

ADUs have the potential to increase the resale value of your property by approximately 20-35%. If you use your ADU as a rental, it will also generate monthly income. The next logical question is, “How much value does an ADU add?”. Unfortunately, it’s impossible to provide a universal answer to this question. The contributory value of additions such as adding an ADU, guest house, or in-law suite, are difficult to measure. But professional appraisers will look for sales comparisons on which to base their appraisals-which are their opinion of value of the property based on the sales comparison data that is available to them at the time of their appraisal. 

Building on site gives you the greatest flexibility to design something that works on your property. Materials for construction are easy to transport to the site because they are in small pieces. It’s easier to work around power poles, trees, and neighbor’s concerns. Prefabricated delivers the project to the site ready to sit on a site-built foundation. If there’s a small deviation with that foundation, it could cause much delay in retrofitting the prefabricated to engage with the foundation. Pre-fabs also have to be built to meet transportation guidelines and you as the client are paying the extra cost of transporting an exceptionally large object on roads and highways.