What is Accessory Dwelling Unit (ADU) : Know its Unexplored Potential

A very common story in USA is the houses that have a small construction within the property where a family, a couple or a student who is studying at the university lives.

Probably, this extra construction is not regulated by current planning instruments and those who live there have some kind of agreement with the owners of the house where the price and the rules of coexistence are set. 

In the United States (US) this type of housing is known as units of housing accessory or grapes ( accessory dwelling units or ADUs ) and are an important strategy to build affordable housing in cities.

Two houses on the same lot?

By definition, and according to the American Planning Association, Accessory Housing Units (UVAs) are independent homes built on the same lot as the main home and their construction is regulated by local regulations varying in their characteristics. What normally differentiates a UVA from a smaller construction on a lot where a house already exists, is that they have a separate entrance, their own bathroom, kitchen and, at least, one room.

According to the American Association of Retired Persons (AARP), UVAs can be classified as internal, attached, or isolated .

  • Internal : these are units that are part of the main building, where the attic or basement of the house is converted into an independent unit.
  • Attached : these are constructions that are attached to the main house, whether it is a new construction or the conversion of a garage.
  • Isolated : they are independent units from the main house.

Accessory dwelling unit typologies. Adapted from AARP and Regional Planning Association.

History of UVAs

It is difficult to know for sure the origin of this type of housing. However, a document from the US Department of Housing and Urban Development (HUD) places its origins at the beginning of the 20th century . Although popular, the explosion of post-WWII suburban development and the emergence of zoning as a planning tool meant the ban on UVAs. Despite this, and in contravention of these new regulations, UVAs have continued to be built in many cities outside of the existing regulations since they also represent an extra income to the property owner. A study conducted in 2001, for example, estimated that in the city of San Francisco one in five residential buildings included a second illegal unit..

Currently, UVAs have been one of the policies that has been very popular in the US because they are a quick way to build houses and because most cities are zoned for single-family housing . The opportunity to generate housing through UVAs is gigantic.

Are two homes better than one?

Various documents ( HUD , RStreet and AARP ) show the benefits of accessory units for the places where they are implemented. Among the most important are:

  • Increase in the housing stock : the construction of UVAs increases the number of housing units provided on each plot and, therefore, the supply of housing.
  • Extra income : UVAs, being an additional rental home, generate extra resources for families of different income levels. Many times these units are generated by low-income families, so they are a mechanism to improve their economy.
  • Low construction cost : being units that are built on the same lot, or that are generated through a conversion or rehabilitation of spaces in a home, the construction cost is much lower.
  • They generate affordable housing : Being the cheapest UVAs to build, they are generally more affordable housing options for young people or small families.
  • Avoid displacement of families: many older couples build a UVA to move in and rent the main home, allowing them to stay in the communities where they have lived for years and reducing their housing expenses. Other families, on the other hand, use UVAs to allow several generations of a family to live together.

What are the implementation challenges?

Although UVAs are a great strategy for generating new homes, they are not without challenges. Here are some of the most important:

  • Changes in zoning : implementing a UVA strategy in a city is not an easy task, since changes are required in the regulations for its application, such as controls for density and / or quantity of housing per plot and space regulations open on lots with single family homes.
  • Informal UVAs : having a UVA regulation is often not enough. The example of Durango, in Colorado, shows that the real challenge in the city came later, with the need to regulate the existing UVAs and adapt the requirements to meet the demands of the community .
  • Construction and permitting costs : Although cheaper, UVAs are not necessarily easy to build for those interested. However, many of these construction costs are tied to existing regulations. In California, for example, some of the barriers to the construction of UVAs are construction fees and costs relative to the minimum requirements required by regulations .
  • Impact on local infrastructure : the increase in the number of existing units in a community generates impacts on the existing infrastructure network (water, electricity, garbage collection, etc.). However, and according to a document from the Regional Planning Association (RPA) entitled “ Be my neighbor. Housing solutions with potential: UVAs and conversions ”, the increase in value generated by UVAs allows infrastructure improvements to be made when necessary, taking into account that many times the installed capacity is greater and that it is possible to densify without the need for improvements 
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