ADU - Build-outs

What is an ADU

An accessory dwelling unit, usually just called an ADU, is a secondary housing unit on a single-family residential lot. The term “accessory dwelling unit” is a institutional-sounding name, but it’s the most commonly-used term across the country to describe this type of housing. While the full name is a mouthful, the shorthand “ADU” is better.

The fact that it’s a secondary housing unit—rather than a given structural form—is what defines an ADU.

When we’re learning about concepts, it’s natural to want to know what that concept looks like in the flesh. We want to visually embed the design concept in our brains as a tangible object that we can mentally reference. However, ADUs vary in their physical form quite a bit, so allow me to broaden that mental model by exposing you to the range of common ADU types, in order to better understand what they are.

Types of ADUs

Here are images of some of the common structural forms of ADUs (as well as some of the other terms you might hear to describe them).

1) Detached new construction ADUs, also sometimes called backyard cottages, granny flats, laneway houses, or DADUs, depending on the jurisdiction:

1) Detached new construction ADUs, also sometimes called backyard cottages, granny flats, laneway houses, or DADUs, depending on the jurisdiction:

2) Garage conversion ADUs

2) Garage conversion ADUs

3) ADUs above a garage or workshop , or attached to it. In some areas, these may be called garage apartments or carriage houses:

3) ADUs above a garage or workshop, or attached to it. In some areas, these may be called garage apartments or carriage houses:

4) Addition ADUs  or “bump-out ADUs”:

4) Addition ADUs or “bump-out ADUs”:

5) Basement conversion ADUs , also commonly called basement apartments, mother-in-law units, in law units, secondary suites, English basements, accessory apartments, and a host of other names.

5) Basement conversion ADUs, also commonly called basement apartments, mother-in-law units, in law units, secondary suites, English basements, accessory apartments, and a host of other names.

6) Internal ADUs , where part of the primary house besides the basement is converted to an ADU.

6) Internal ADUs, where part of the primary house besides the basement is converted to an ADU.

Why cities care about ADU development

There’s a lot of reasons that municipalities may want to spur ADU development. Here’s a few common reasons:

Economic 

  • ADUs provide flexible dwelling options in a central city neighborhoods, utilizes existing governmental infrastructure (eg. roads, sewers, schools), and reduce the demand for expanding infrastructure in far-lying reaches of a developed metropolitan area.

Environmental

  • ADUs provide housing with a relatively small environmental footprint. New, detached ADUs provide rental housing that is 44% smaller per capita than standard, new single family rental units. And new ADUs overall provide housing that is 33% smaller per capita than standard, new single family units. In a building lifecycle, smaller residential spaces use less energy in construction, deconstruction, and habitation.

Social 

  • ADUs provide more affordable housing options in residential neighborhoods without dramatically changing a neighborhood’s character as much as other new housing forms may.

There’s simply too few permitted ADUs to make a real difference in the housing stock. But, even if they aren’t going to solve all a city’s problems, they may help homeowners solve some of their problems. The most common motivation for ADU development is rental income potential, followed by the prospect of flexible living space for multigenerational households.

What ADUs have in common

While their structural forms vary, ADUs share some common traits and face common design and development challenges. For one thing, the fact that they’re secondary housing units on single family residentially zoned lots places ADUs into a unique category of housing. And ADUs also have some other distinguishing characteristics that help further define, differentiate, and distinguish them from other housing types.

  • ADUs are accessory and adjacent to a primary housing unit.

  • ADUs are significantly smaller than the average US house.

  • ADUs tend to be one of two units owned by one owner on a single family residential lot.

  • ADUs tend to be primarily developed asynchronously from the primary house by homeowner developers.

  • A large range of municipal land use and zoning regulations differentiate ADU types and styles, and dramatically affect their allowed uses

  • Vast numbers of informal ADUs exist compared to permitted ADUs.

These differentiating characteristics make ADUs a distinct type of housing. Till now, there has been a lack of common understanding around the language and best practices of ADU development.

Our team of Designers have completed over 300 ADU Kitchen & Bath projects, and have assisted both homeowners, and developers with the design aspect of ADU Kitchen & Bath build-outs with affordable solutions.