A project of the Solano County Planning Collaborative

ADU 101

Let’s start from the beginning

What is an ADU?

Accessory dwelling units (ADUs), also known as “granny flats,” “in-law units,” or “backyard cottages,” come in many shapes and sizes but are always a self-contained home that is usually smaller than the main house and legally part of the same property.

They must have a kitchen, bathroom, and place to sleep, and can range from small studios to larger homes with multiple bedrooms. 

Types of ADUs

ADUs do not always fit neatly into one category – like an attached ADU made from converted space, for instance.


New freestanding structure separate from the main home, like a backyard cottage


New structure (may include some converted space) sharing at least one wall with the main home


Converted existing space in the main home or on the property (bedroom, basement, detached garage)


A smaller conversion ADU within the main home or attached garage

  • Up to 500 square feet
  • Efficiency or full kitchen
  • Can share bathroom with main home (if accessible from JADU)
  • Owner must live on the property


Any of the above (except JADU) that is part of a multi-family building like a duplex, triplex, or apartment building (existing or proposed)

Not an ADU

RVs, tiny homes on wheels, yurts, and storage structures are NOT accessory dwelling units

Types of ADU Construction

Prefab, modular, and panelized are partially or mostly built in a factory, then shipped to your site and assembled or placed on a foundation. Some companies provide a “turnkey” service that helps with permitting and all on-site construction, from laying the foundation to utility hookups. If not, you’ll need to hire additional professionals to help.

Using a prefab design can save time and make clearer what your completed project will look and feel like, but they also typically offer less customization, can be difficult to permit and finance, and often cost the same as site-built ADUs. People are often surprised at the cost, but transportation, crane, foundation, and sales tax can end up being 20-40% of the total cost. Some companies do offer a smaller price point.

To explore prefab or modular ADU options, you will want to get in touch with companies directly. They will visit, evaluate your property, and make recommendations. You can find prefab contractors by researching online.

Site-built is a traditionally constructed ADU built from scratch on your property (AKA “stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.

Project Timing

Building an ADU is an investment of time as well as money. Most projects take 12-24 months to complete, with interior conversions being the fastest.

Why Build an ADU?

Building small has a big impact on your property, your family, and your community.

ADUs often provide homes for the local workforce and small families who have difficulty finding local housing that meets their needs. For homeowners, there are many reasons to build an ADU — rental income, safe and convenient housing for relatives, flexibility over time, and many others.

Here are some of the reasons why people build ADUs:

Housing Family

Close but independent living for adult children, aging relatives, or loved ones with special needs.

A Home As You Age

A nearby home for caregivers or for aging adults who want to rent their main home and live in their ADU.

Rental income

A steady source of additional income for retirement or a rainy day. Can come from the ADU or the main home.


An extra set of eyes and hands on your property can be helpful for safety, when you travel, upkeep, and more.

Property Investment

Not only will you receive rental income in the short term, but your sale price and property value can increase too.

Community Support

Provide housing to families, workers, and others who might struggle to find something local.


Here are a few of the most frequently asked questions about basic ADU details. 

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