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Featured Image Accessory Dwelling Units. What Are They

Accessory Dwelling Units. What Are They?

Accessory Dwelling Units (ADUs) have been known by many names: granny flats, in-law units, backyard cottages, secondary units and more. No matter what you call them, ADUs are an innovative, affordable, and effective option for adding much-needed housing in California.

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

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Detached new construction ADUs

also sometimes called backyard cottages, granny flats, laneway houses, or DADUs, depending on the jurisdiction: As the name suggests these ADUS are built brand new from the ground up.

Garage conversion ADUs

Garage conversion ADUs

The price for a garage conversion ADU is substantially less expensive than a new detached or attached ADU. Since the garage already exists, a conversion saves money on the foundation, framing, and roofing portions of the construction project.

ADUs above a garage or workshop

ADUs above a garage or workshop

Garage and workshops often have ample space in the rafters for a comfortable ADU. The ADU can be added to an existing garage as a remodel or included in a new build design.

Addition ADUs or “bump-out ADUs”

Addition ADUs or “bump-out ADUs”

Attached ADUs, also known as addition ADUs or ‘bump-out ADUs’, People add a room (and usually a bathroom) so an extra individual could live inside the house. Attached ADUs sometimes have their own entrance, so people living there have an extra sense of privacy and increased ease of access.

Basement conversion ADUs_

Basement conversion ADUs

One of the ways to implement an ADU on your property is to convert your basement. The basement may be currently collecting dust as a storage area, but there is much potential for space if you want to renovate it.

Internal ADUs

Internal ADUs

is created when a portion of an existing home — an entire floor, part of a floor, or an attic or basement — is partitioned off and renovated to become a separate residence.

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.

This site and the book Backdoor Revolution, will help change that by providing some clarity about ADUs and how average homeowners develop them.

ADUs Are Good for People and Places

  • ADUs can generate rental income to help homeowners cover mortgage payments or simply make ends meet. The income provided by an ADU tenant can be especially important for older people on fixed incomes.
  • Since the land on which an ADU is built already belongs to the homeowner, the expense to build a secondary residence is for the new structure only. The lot is, in a sense, free.
  • ADUs are typically owned and managed by homeowners who live on the premises. Such landlords are less likely to raise the rent once a valued tenant has moved in. Many ADUs are created for family members to reside in for free or at a discounted rate.
  • Although market rate rents for ADUs tend to be slightly more than for similarly sized apartments, they often represent the only affordable rental choices in single-family neighborhoods, which typically contain no studio or one bedroom housing options at all.
  • Some municipalities are boosting ADUs as part of affordable housing and anti-displacement strategies. Santa Cruz, California (see opposite), is among the cities with programs to help lower-income households build ADUs or reside in them at reliably affordable rents.
  • An individual’s housing needs change over time, and an ADU’s use can be adapted for different household types, income levels, employment situations and stages of life.
  • ADUs offer young people entry-level housing choices.
  • ADUs enable families to expand beyond their primary home.
  • ADUs provide empty nesters and others with the option of moving into a smaller space while renting out their larger house or letting an adult child and his or her family reside in it.

Generally measuring between 600 and 1,000 square feet, ADUs work well for the one and two-bedroom homes needed by today’s smaller, childless households, which now account for nearly two thirds of all households in the United States.

  • ADUs require fewer resources to build and maintain than full-sized homes.
  • ADUs use significantly less energy for heating and cooling. (Of all the ADU types, internal ones tend to have the lowest building and operating costs.)
  • ADUs offer a way to include smaller, relatively affordable homes in established neighborhoods with minimal visual impact and without adding to an area’s sprawl.
  • ADUs provide a more dispersed and incremental way of adding homes to a neighborhood than other options, such as multistory apartment buildings. As a result, it’s often easier to get community support for ADUs than for other housing types.
what is an adu


A guide to Accessory Dwelling Units and how they expand housing options for people of all ages. Go Here

The ADU Grant provides up to $40,000 towards pre-development and non-reoccurring closing costs associated with the construction of the ADU. Predevelopment costs include site prep, architectural designs, permits, soil tests, impact fees, property survey, and energy reports. Go Here

This guide includes model state and local legislation to encourage the development of accessory dwelling units (ADUs). It includes extensive commentary, explaining the rationale for model provisions and the justifications for avoiding certain common practices, such as owner-occupancy restrictions and discretionary reviews and use permits.  It serves as an update to earlier models published under the same title in 2000.

It presents two versions of a model state act: (1) an “optimal” act that limits the authority of local governmens to prohibit ADUs and (2) a “minimal” act that explicitly authorizes local governments to permit ADUs.

The model local ordinance includes seven major sections:

  • General provisions
  • Standards
  • Utility connections and building codes
  • ADU application and review procedures
  • Fees
  • Legalizing ADUs

Go Here

homeowners a step-by-step guide to building an Auxiliary Dwelling Unit or a Tandem House on a residential lot. An ADU (also known as a granny flat, carriage house, cottage, in-law apartment, home office, or rental property) or Tandem House can capitalize on the increasing demand for single-family homes, by allowing more residents to live on a single-family lot at a reasonable price.


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