Accessory Dwelling Units

What is an ADU?

An accessory dwelling unit (ADU) is a separate living space within a home. It can be used as a rental property, an in-law suite or just extra living space for the owner. ADUs were once illegal in many cities across the U.S., but now many communities are allowing them to be built on existing lots that already have a single family home.

Accessory Dwelling Units: What They Are & How They Work

Accessory dwelling units (ADUs) are small homes built on the same property as a larger single-family residence. The term "accessory" refers to their secondary nature when compared with the main house.

They generally come in two types: detached units placed behind a single-family home or attached units attached to its side or rear, like a backyard cottage or carriage house. Some jurisdictions may require that they be detached, while others permit both options.

The number of people living in single-family homes has been declining for decades. In 1950, about 70% of Americans lived in such dwellings; today, it's about 50%. Yet the U.S. population keeps growing — by about 2 million people a year now — and our houses aren't getting any bigger.

So what do we do with all those people?

The answer is accessory dwelling units, commonly known as ADUs or granny flats. These are second homes on the property of a primary residence that are smaller than the main house, but not small enough to be called studios or lofts. They can be detached or attached to the main house, but they always share an outdoor space with it (sometimes just a patio). The most common type is a garage converted into a small apartment with its own kitchenette and bathroom. But there's no law against converting other structures like barns or workshops into residences too.

ADUs have long been popular in Europe and Asia (where they're often called "accessory dwellings" instead), but until recently they were rare here because zoning laws didn't allow them in most areas — and even where they were allowed, homeowners worried that if they built one their neighbors would complain about increased traffic.

Check out ADU rules in your area.