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ADU 101

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 studios small studios under 500 square feet to larger homes with multiple bedrooms.

ADU Types

Detached ADU

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

Pros

Cons

Project Timeline

Building an ADU is an investment of time as well as money. Most projects take one to two years to complete, with interior conversions being the fastest.

Project Cost

The cost to build an ADU typically ranges from $30,000 for a simple interior conversion JADU, to $400,000+ for a large detached ADU with high-end finishes on a hillside lot. See more about costs in our ADU Workbook and on the Budgeting & Finance page. Our ADU Calculator is a great place to start when developing a budget.

Where to start?

The best place to start is with an ADU Feasibility Consult. You’ll meet with an ADU expert and receive a customized ADU Feasibility Report about your property and your local ADU rules.

Related Blog posts

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FAQs

Find answers to the most frequently asked questions about ADU basics.

When you need an ADU, it’s already too late.

It’s better to plan ahead of time and rent it out until a family member needs it.”

Sonoma Resident

Benefits for Owners and the Community

Building and renting an ADU is truly a community service — 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 to build an ADU:

Housing Friends & Relatives

Space for adult children, aging relatives, or loved ones with special needs.

Financial Security

A steady source of rental income and increased property value.

A home as you age

A home for a caregiver or an accessible and comfortable option for aging adults.

Safety

An extra set of eyes on the property for safety, travel, and more.

Environmental benefits

Small-scale living and reduced commutes for local workers.

Let's ADU!

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