• Brownfield Redevelopment
     • Green Building Standards
     • Accessory Dwelling Units

     • Historic Preservation
     • Main Street Program

     • Alt. Energy Ordinances
     • Stormwater Management

     • Smart Growth Code Fixes
     • Form-Based Codes
     • LEED-ND
     • Transit Oriented Dev.
     • Zoning Ordinances
     • Scenario Planning

     • Agricultural Security Areas
     • Clustered Subdivisions
     • Official Map Ordinance

     • Access Management
     • Complete Streets Policy
     • Impact Fees
     • Traffic Calming
     • Walkability

Accessory Dwelling Units

An accessory dwelling unit is an additional living space located on the same property lot as an original, larger residence.  Accessory dwelling units can take many forms and can be created in different ways. Examples include: using spare space above a garage, retrofitting a basement or other spare room with essential living amenities, building a new addition, or new construction elsewhere on the lot. Accessory dwelling units can offer affordable accommodation to elderly relatives or children returning home, or simply be rented to help pay down a mortgage and other associated housing costs.

In terms of physical amenities, accessory dwelling units generally contain a kitchen, a bathroom, and some form of living space and/or bedroom.  All the daily life functions can be carried out without encountering anyone from the larger dwelling unit as it is designed to be self-contained. The form and layout of the unit and the amount of privacy offered depend on available space and intended use. Accessory housing can be a unique opportunity for homeowners to express their creativity in design and construction while also serving a very useful purpose.

Today, with changing demographic and economic conditions, zoning changes necessary to accommodate accessory dwellings should be seriously considered.  Typical post-war nuclear families are now sharing neighborhoods with extended families in which both parents are working, the millennial generation delays family creation, and in which grandparents prefer to stay in place. This changing family pattern requires some accommodation at the zoning level. For extended families to live together under one roof, or on the same property, requires accessory dwellings be permitted uses. By renting out accessory apartments, or having relatives contribute to housing costs, long-time residents are offered a chance to stay in a neighborhood when rising property taxes might otherwise prove prohibitive to their goal of aging in place.

The concept of accessory dwellings is not new, and once was a fairly common situation in this country.  Three or four generations could live under one roof or next door, and extra help in child rearing and elder care was readily available.  However, most current zoning codes do not allow for their creation. Changes to the municipal zoning code would have to be adopted for particular areas in which it is agreed accessory housing may meet a housing need and be an enhancement to neighborhood character. What zoning mechanism is employed to facilitate this will depend on the specifics of your community.


  • Allows the elderly to age in place
  • Provides close help for child rearing and/or elder care
  • Provides additional money to pay property taxes
  • Provides options for more affordable housing
  • Provides proximity
  • Pride of place


  • Is the neighborhood friendly for the elderly?
  • Is there adequate parking?
  • Is the design compatible or complimentary to the existing visual tone of the neighborhood?
  • Is there adequate infrastructure to support another dwelling unit?

Practical Tips

Encouraging the creation of accessory housing can be achieved through public outreach efforts in which examples are provided and discussed. Since accessory dwellings have been disallowed by zoning in many places, expect a period of adjustment with the concept and practice. Allowing the construction of accessory dwellings is a function of changes made to the zoning code by the municipality. There are mainly two ways to allow the construction of accessory dwelling units. One is through a homeowner’s request for a “special use” permit, allowing them to construct the accessory dwelling.  The other is to allow accessory dwellings “by right.” Allowing accessory dwellings by right is preferable because it eliminates delays in processing permit requests. Creating accessory dwellings by right encourages municipalities to write the sort of controls and conditions they would like to see into the zoning code from the outset. Some neighborhoods may better lend themselves to accessory apartments than others.


In its zoning code, Buckingham Township, Bucks County, includes a section on accessory dwelling units where it defines and permits accessory units as long as a set of “conditions” in the zoning code is followed concerning the units’ construction and location. Bucks County has many rural areas as well as more densely developed areas due to its proximity to Philadelphia. The range of type of accessory dwellings is equally as diverse.

Regional examples can be found commonly in southeastern Pennsylvania, as many historic homes with carriage houses located to the rear of the property have been converted into apartments. Likewise, many Amish homes feature additions in which grandparents live together with the family, but in a separate dwelling, establishing privacy while maintaining physical proximity. Most accessory dwellings units, however, are not visible from the street and are built within a dwelling. This might be preferable where visual cohesion is a community priority.