Accessory Dwelling Units (ADUs) Case Studies

Issues Addressed:
Housing Costs Housing Options Multigenerational

Portsmouth, NH

The City of Portsmouth passed a local ADU ordinance in 2017, which allows attached and detached ADUs by Conditional Use permit across many of its zoning districts. ADUs may be no larger than 750 square feet in most cases. Either the primary home or 12 Accessory Dwelling Units (ADUs) the ADU must be occupied by the property owner, and this must be verified annually. Since the law’s passage, 31 ADUs have been permitted and 25 have begun construction. The City’s Planning Board has considered ways to simplify permitting, as one estimate places a price tag of $20,000 to $30,000 in permitting costs alone. Several amendments have been proposed to the ADU law, with the stated aim to make ADU production easier.
  • City of Portsmouth, “Land Use and Zoning Regulations” 
  • Portsmouth Herald, “Accessory dwelling units in Portsmouth: 'Easiest' housing solution or is it too late?” (November 29, 2022)
  • Portsmouth Herald, “As popularity of accessory dwelling units grows, Portsmouth looks to simplify permitting” (July 26, 2022)

Swanzey, NH

The Town of Swanzey first introduced ADUs through zoning in 2009, allowing attached ADUs by Special Exception for owner-occupied single-family homes in certain zoning districts. In 2017, the Town amended the law to respond to the statelevel ADU law. Under the current zoning, attached ADUs are allowed by-right and detached ADUs are allowed by Special Exception. Both types of ADUs are allowed in all zoning districts where single-family homes are permitted. Either the principal unit or the ADU must be owner-occupied. Dimensional rules for ADUs are the same as single-family homes, but there must be at least three parking spaces available on the property.
A photo with a 2.5-story colonial home one the left. A 1.5 story two-car garage with a pitched roof is on the right, attached to the main house by a small connector with a separate entrance. A window in the upper half-story of the garage indicates a living area.
A photograph of a home with an ADU in Swanzey, New Hampshire. A single-family ranch-style home is visible in the mid-ground, and a secondary smaller home is visible in the background behind it to the left. The secondary home has steps up to a porch, a yellow door, a pediment, and columns. A driveway connects the two buildings. Both are surrounded by trees and snow.
Homes in Swanzey with ADUs: attached (top photo, above garage) and detached (bottom photo, upper left). (Images via RE/MAX Town & Country from Redfin and Zillow)

Wellfleet, MA

The Town of Wellfleet, MA on Cape Cod has allowed ADUs since 2002, but through 2021 they were required to be deed-restricted affordable housing for income-eligible renters. (Massachusetts has no statewide ADU requirement.) The Town’s affordability requirements were matched by property tax relief for the homeowners, as well as grants by the local Housing Authority for construction. Still, between 2002 and 2020, only 11 affordable ADUs were permitted in Wellfleet, though hundreds of grandfathered ADUs and illegal ADUs exist there. In 2021, Wellfleet Town Meeting passed amendments to remove affordability requirements for ADUs, hoping the move would bring broader market affordability. ADUs are now permitted by right in most of Wellfleet, as long as they meet the zoning’s specifications. The (attached or detached) ADUs must be between 600 and 1,200 square feet in size and require two parking spaces. The ADU must be rented out on a year-round basis. Renting the ADU seasonally or as a short-term rental results in a daily $300 fine.
A birds-eye photograph of a 1.5-story accessory dwelling unit. The home has wooden shingle walls and slate shingle roof with traditional windows, including a large set of windows in the loft area. The unit sits on a raised foundation over a slab. The home is surrounded by trees and brush.
An ADU in Wellfleet, MA. (Image via Wellfleet Housing)

Lexington, KY

Lexington-Fayette County, KY passed an ADU bylaw in 2021. Following a recommendation in the 2018 Comprehensive Plan, Lexington proposed ADU legislation in 2019. The legislation was considered bold by ADU advocates, largely due to allowance for attached and detached ADUs, a lack of owner-occupancy or parking requirements, and reasonable site planning requirements. The proposal received push-back due to a lack of rental registration (and concerns about code compliance); the potential for overcrowding; a lack of design, owneroccupancy, or parking requirements; fears about short-term rentals; and other frustrations about capacity and neighbors wanting more control on permitting. The final legislation was a compromise by only allowing detached ADUs in existing structures (like garages), adding owner-occupancy requirements, and clearer (but still limited) design standards.

Eugene, OR

In 2017, the State of Oregon mandated most towns and cities to allow ADUs in singlefamily residential zones with reasonable local regulations on siting and design. The City of Eugene already allowed ADUs at that time, but required owner occupancy and other restrictions not related to siting or design. In 2020-2021, Eugene brought its ADU code into compliance with State law. Now Eugene permits ADUs in all residential zoning districts. The ADUs can be a maximum of 800 square feet, they have ADU-specific design standards, they do not require owner-occupancy or additional parking, and they are not restricted by minimum lot sizes or certain other dimensional requirements. Most importantly, the City also pre-approved certain ADU designs, and the ADU designers licensed the designs to property owners for $500. Owners must still work on a site plan and follow other steps of construction, but by providing pre-approved designs, the City has simplified the process and reduced ultimate costs.

State Law

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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