Accessory Dwelling Units (ADUs)

Issues Addressed:
Housing Costs Housing Options Multigenerational
A diagram illustrating attached and detached Accessory Dwelling Units. There are two attached versions: one where a garage or addition has been turned into an ADU with a separate side entrance and one in which an attic has been turned into an ADU with stairs leading to a separate upstairs entrance. There is one detached ADU diagram, showing a small out-building with its own entrance and pathway behind a larger home.

What is it?

Accessory Dwelling Units (ADUs) are secondary homes or apartments on an existing single family lot. They can be apartments within the primary home (such as in a basement or attic), attached to the primary home, or in an accessory building (like a converted garage). ADUs are sometimes called “in-law apartments” or “granny flats.” In New Hampshire, ADUs are defined under state law. An ADU is not simply an extra room, but an “independent living facility for one or more persons, including provisions for sleeping, eating, cooking, and sanitation…” (RSA 674 § 71).

Attached ADUs (where the ADU is within or connected to the primary structure) are allowed by default in all New Hampshire communities. Municipalities can adopt more specific zoning rules related to ADUs, but they must be allowed in all singlefamily housing and additional zoning rules cannot be applied to discourage them. Municipalities can also allow detached ADUs, but they are not required to do so.

How can it help?

ADUs can…

  • Provide an age-friendly housing option for older adults, as they can down-size while aging in place.
  • Provide less expensive housing options for younger adults, single parents, and others.
  • Accommodate multi-generational families and other household structures that are not well-served by conventional development.
  • Reduce demand on assisted living facilities by providing homes to caregivers for elderly homeowners.
  • Create more housing without changing the architectural character of an existing neighborhood.
  • Create more housing affordability and options without the need for additional water and sewer infrastructure.
  • Provide more units with fewer building materials than conventional development and with no additional land costs.
  • Create income-support opportunities for homeowners in expensive housing markets.
  • Generate increased tax revenue (over the long term) for municipalities.

Getting Started

  1. If your community has a zoning ordinance without ADU provisions, attached ADUs are allowed by-right under State law in all zoning districts that permit single-family dwellings.
  2. If your community already has local zoning provisions for ADUs, you may still want to amend the zoning bylaw to incentivize more ADU production or ADUs that are more aligned with the community’s needs and desires.
  3. Review your community’s single-family districts. Assess demographics, architectural character, typical lot size, typical home size (by square feet and bedrooms), presence of existing outbuildings (like garages or sheds), presence and capacity of existing utilities, parking need and capacity. Assess local knowledge of any unpermitted ADUs, how they are configured, and what issues they create. Use these assessments to determine what constraints and opportunities exist for ADU production.
  4. Hold a community engagement process with property owners, potential ADU tenants, and relevant boards (e.g. Planning and Zoning boards). Use this process to determine the community’s general housing needs and desires and/or ADUspecific needs and desires.
    1. The community should consider how ADUs are permitted, whether detached ADUs are allowed, square footage rules, parking requirements, design regulations or guidelines, and more.
    2. Engagement will not only help craft locally appropriate ADU zoning, but generate awareness and interest in ADUs among property owners.
  5. Draft amendments to the zoning bylaw to either add local ADU rules or amend existing rules. Amendments can encourage ADU production that meets the needs and desires of the community.
  6. The community can also consider pre-permitting site plans and ADU designs based on local conditions and could consider public financing (e.g. low-interest loans) for ADU construction.
  7. The municipality can promote ADUs to residents and homeowners.
A photo of a single-story ADU with blue wooden cladding, slate shingles, and traditional windows. The ADU has two massings shaped as small L, with a single door on the longer massing. In the corner of an L sits two white Adirondack chairs. Visible on the right edge of the photo is a another building with 1.5 stories and natural wooden cladding. The homes are surrounded by trees.
An ADU in Florence, MA. (Image via Backyard ADUs.)

Considerations

  • Attached ADUs are allowed by-right by default under state law in all zoning districts that permit single-family dwellings. Local zoning may allow them explicitly either by-right, by special exception, or by conditional use permit. Compared to by-right permissions with clear requirements, discretionary permissions are more likely to limit ADU production by typical homeowners and encourage unpermitted ADU production by bad-faith actors. 10 Accessory Dwelling Units (ADUs)
  • Detached ADUs are not allowed by-right by default under state law. They may be allowed explicitly in local zoning, either by-right, by special exception, or by conditional use permit.
  • ADU construction typically costs between $50,000 and $200,000. Most homeowners or homebuyers will require bank financing for construction. That financing may be backed by equity in the existing home or the expected increase in property values.
  • Local law can require owneroccupancy and principal place of residence of either the primary home or the ADU. Owneroccupancy requirements may limit the number of ADUs produced, however, as they discourage ADU construction on seasonal housing and rental properties. Owner-occupancy requirements also limit the availability of financing for ADU construction, as many lenders fear such restrictions limit their options in the event of a foreclosure.
  • Local law cannot require that residents of both units be relatives.
  • ADUs can be counted toward a municipality’s Workforce Housing requirement.
  • In most cases, ADUs will not be deed-restricted affordable housing. However, some municipalities offer ADU construction financing in exchange for affordability restrictions on the ADU.
  • Dimensional requirements for single-family homes with attached ADUs must not differ from single-family homes generally. Dimensional requirements are allowed to differ for single-family homes with detached ADUs.
  • Local zoning may add rules on additional parking for ADUs.
  • Local zoning may add minimum and maximum square footage rules for ADUs, but must allow ADUs of at least 750 square feet. Local zoning may not limit ADUs to one bedroom.
  • Property owners must demonstrate the adequacy of water and wastewater utilities on-site, but local law may not require a separate water and wastewater facilities for the ADU.

State Law

Case Studies

  • Portsmouth, NH
  • Swanzey, NH
  • Wellfleet, MA
  • Lexington, KY
  • Eugene, OR
    • Read more

Resources

Related Tools

A diagram of an idealized age-friendly neighborhood. The neighborhood includes a mix of housing types, including detached single-family homes of various sizes, mixed-use buildings, and a cottage court. The diagram includes the following labels pointing to aspects of the drawing: "Nearby retail, community center, and open space;" "Diverse housing options, including apartments and starter homes;" "Single-story housing layouts with shared amenities;" "Sidewalks and other accessible infrastructure;" and "Accessory dwelling units for multi-generational households."
Age Friendly Neighborhoods
Three axonometric diagrams connected by one-way arrows. First, a small home on a small plot of land labeled "Historic, compact development pattern." Second, a larger home on a larger plot of land with arrows specifying setbacks, etc. labeled "Development mandated under zoning." Third and finally, a home and land parcel the same size as the first drawing, with arrows indicating setbacks, etc. and labeled "Reduced requirements."
Reduced Zoning and Subdivision Requirements
A diagram of a large house labeled "Owner-occupied home" and a smaller unit behind it labeled "Short-term rental." Beneath it is a list reading: "Registered locally, lodging uses permitted, fees for inspection, meals and rooms tax, max occupancy, max days rented."
Short-Term Rental Regulations
A diagram with two versions of the same plot of land, one over the other. On the top version, the land contains only one single-family home with many wings and a large garage and a long driveway. This version is labeled with a large 'X' and "Regulations cannot mandate unaffordable housing everywhere." The version below contains two modestly sized single-family homes at the front of the site, and a driveway leading back to a cottage court, containing six small single-family homes around a central green. This version is labeled with a large checkmark and "Workforce housing must be allowed in a majority of residential areas."
Workforce Housing Ordinance
A subdivision of eight single-family homes surrounding a central green. The plot of land is labeled "Site plan that allows for community septic, prioritizing shared open space." Underground pipes lead from each home into a large underground box labeled "Joint septic tank." A pipe leads from that box into a smaller underground box labeled "Pre-treatment system." A pipe extends from that box and branches into many pipes with holes in them under a green area labeled "Community open space that doubles as a leach field."
Wastewater System Alternatives
A diagram showing the front facades of housing types and given labels: small single-family, townhomes (attached single-family), cottage court (grouped small single-family), duplex, stacked duplex, triplex, stacked triplex, four-plex, six-pack, courtyard apartments.
Missing Middle Housing Types