An accessory dwelling unit, usually just called an ADU, is a secondary housing unit on a single-family residential lot.

ADUs vary in their physical form quite a bit, so allow me to broaden that by exposing you to the range of common ADU types, 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 in-laws suite, granny flats or guest house.
  2. Garage conversion ADUs
  3. ADUs above a garage or workshop or attached to it. In some areas, these may be called garage apartments.
  4. Addition ADUs or “attached ADUs”
  5. Basement conversion ADUs, also commonly called basement apartments.
  6. Internal ADUs, where part of the primary house besides the basement is converted to an ADU.

What are the benefits of ADUs?

  • ADUs are an affordable type of home to construct in California because they do not require paying for land, major new infrastructure, structured parking, or elevators.
  • ADUs can provide a source of income for homeowners.
  • ADUs are built with cost-effective wood frame construction, which is significantly less costly than homes in new multifamily infill buildings.
  • ADUs allow extended families to be near one another while maintaining privacy.
  • ADUs can provide as much living space as many newly-built apartments and condominiums, and they’re suited well for couples, small families, friends, young people, and seniors.
  • ADUs give homeowners the flexibility to share independent living areas with family members and others, allowing seniors to age in place as they require more care.

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 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.

The demographics driving demand for ADU.

Why are some cities so eager to improve their ADU codes?

Well, for starters, most households in the United States are now 1 and 2-person households. Yet, most of our legacy housing stock, and even our new residential housing stock, is designed for families of 4 or 5 people. That may have made sense 70 years ago. But, things have changed.

3 bedroom and 4-bedroom homes no longer match the demographic realities of the United States: 1-2-person households now represent 62% of the country’s households. Only 38% of the nation’s households have more than 3 or more people in them.

Close to 2/3rds of the population in the US are living in 1-2-person households!

Year by year, 1-2-person households are forced into eating up the single-family housing stock (housing pellets, if you will) that was designed for nuclear families, not because they want or need to live in big homes, but because there simply isn’t enough houses built in residential areas that were designed for 1-2-person households.

Among other demographic factors at play, single person households have become extremely common in major cities, representing more than ⅓ of the households of many cities.

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 neighborhood, utilizes existing governmental infrastructure (e.g. 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.

The process of getting an ADU

  1. Site/property assessment to verify ADU size potential.
  2. Explore the different type of ADU options and decide on the best suite your needs in terms of cost and return on investment.
  3. Design of an ADU: size, location, access, interior and exterior features.
  4. Issue Architectural plans.
  5. Issue Structural engineer plans (if needed based on type of ADU).
  6. Get bids for constructions and hire a contractor.
  7. Submit for city review and approval.
  8. Pull construction permit once approved the city.
  9. Conduct construction as plans calls for.
  10. Go through city inspections during construction.
  11. Issue certificate of occupancy once construction is completed.