What are ADUs?

An accessory dwelling unit (ADU) is a self-contained housing unit on the same property of a residential property that is the principal dwelling. ADUs can be within or attached to a principal dwelling or be a separate detached structure on the same property.

To promote the creation of ADUs, municipalities should amend their existing zoning statute to provide for greater flexibility regarding ADUs. Provisions within the statute can specify restrictions such as building size, design guidelines, modifications, lot sizes, number of inhabitants, allowable rents, or owner occupancy but the goal should be to allow ADUs as-of-right once criteria are met.

Benefits of ADUs:

  • ADUs provide flexible living situations such as an option for residents who are looking to age in place, want to downsize but stay in their community, have a dependent family member, or for single-person households.
  • ADUs are an efficient use of existing sewer and water infrastructure.
  • ADUs are easily integrated into existing neighborhoods.
  • ADUs are increase housing units by utilizing the existing housing stock or accessory structures.
  • ADUs are low-cost additional units due to their smaller size, existing land, and infrastructure and are therefore a valuable source of affordable rental housing.
  • ADUs are a source of rental income for homeowners, which helps cover their mortgage and other housing costs.

Minimum Criteria for ADU regulations:

  • Allow ADUs in all residential zones without a special permit. Instead of requiring special permits, clearly outline the requirements of an ADU and if those requirements are met, then the ADU is permitted as of right. This removes the uncertainty of needing a special permit where the decision criteria are typically not clearly defined.
  • Allow ADUs in accessory structures such as garages.
  • Actively promote the creation of ADUs by providing educational materials and/or technical assistance to residents who are interested in creating an ADU.

Other optional best practices to ensure an ADU ordinance provides the maximum benefit to a community and results in the creation of more housing units:

  • Examine the development review process to find ways that the process can be streamlined to encourage homeowners to use ADU ordinance.
  • Require that ADUs are rented at rates affordable to households earning 80% or less of AMI.
  • Reduce off-street parking requirements.
  • Only allow a single ADU created within or on a single-family lot.
  • Limit the gross floor area of ADUs to 1,000 square feet.
  • Do not allow the enlargement of an ADU beyond the 1,000 square feet.
  • An ADU may not be occupied by more than three people and may not have more than two bedrooms.
  • Use tax incentives to encourage the development of ADUs for disabled and handicapped individuals and persons with limited mobility.


Lexington, MA – The Town of Lexington, a mostly built-out suburb of Boston with a strong housing market, updated its zoning laws in 2005 to encourage the development of more ADUs. The purpose of the ADU laws are to increase the number of dwelling units, increase the range of housing choices, encourage seniors and young adults to live in the town, and as a way to create both economically and energy efficient use of the existing housing supply.

State of New Hampshire – As of 2017, any municipality in New Hampshire must allow internal and attached accessory dwelling units in all zoning districts where single-family dwellings are permitted.

State of Connecticut – In 2021, the State of Connecticut General Assembly adopted bill HB6107. Under this new law, statewide zoning reforms were put into place including, among other things, the legalization of ADUs as of right.

San Francisco, CA – The city passed legislation in 2014 to allow new accessory dwelling units and created a process to legalize ADUs that had been constructed without permits. In order that ADUs remain affordable, the city mandates that any units which receive a zoning waiver are subject to rent control with a regulatory agreement containing terms of rental.

State of California – In 2021, California enacted statewide zoning laws that mandate cities and counties create a plan and incentives to promote new ADUs. The law also requires that new ADUs are rented at prices affordable to very-low to moderate-income households.

Below are links of examples of model Accessory Dwelling Unit ordinances which can be used as a base for the creation of local ADU ordinance. It is recognized that there is no single ordinance that can be added to community regulations without some tailoring, therefore revisions to the text within this ordinance is encouraged. There may also be a need to examine local development review process to find ways that the process can be streamlined to encourage homeowners to use the accessory dwelling unit ordinance.

ADU Model Ordinance

Town of Marbletown Draft Accessory Apartment Law

Town of Rochester ADU Local Law