Nationwide, cities and small towns are experiencing higher demand for housing in mixed-use downtown and suburban neighborhoods where residents are close to shops, restaurants, and offices according to a 2013 report by the Urban Land Institute. These communities are experiencing pressure on housing driving prices higher because of the growing demand for walkable urban neighborhoods. More communities are addressing the increased demand for housing in traditional neighborhoods by amending regulations to allow accessory dwelling units.

Accessory dwelling units (ADUs) are also known as granny flats, mother-in-law apartments or carriage houses. Regardless of the name, this definition from the Village of Beulah, Michigan zoning ordinance of 2003 gets at the key concept – “An incidental and subordinate dwelling unit which provides living quarters for one individual or a family that is on the same lot, but is separate from the primary dwelling unit….” A typical standard for accessory dwellings is to require one of the two dwelling units – either the principal dwelling or the accessory dwelling – to be occupied by the property owner in order to avoid an absentee landlord situation in which structures sometimes fall into disrepair. Accessory dwelling units might be in an accessory building, such as a converted garage or new construction, or the accessory dwelling might be attached or part of the principal dwelling, such as a converted living space, attached garage, or an addition.

Accessory dwelling units have been touted as an affordable housing strategy in communities where a brisk real estate market is driving families and talented workers out of the community. The relatively smaller size of such units makes them more affordable. What’s more, such dwellings also provide additional income for property owners, making the principal residence also more affordable. Although, it is important to note that accessory dwellings will not solve the affordable housing problem alone.

In addition to making housing more affordable, communities have looked to ADUs as a way to increase density in neighborhoods as a counter measure to falling household size over time. In order to improve business for local establishments in and around traditional neighborhoods, ADUs not only subtly increase population density; they increase the customer base near a community business district.

Any community that has considered changes to zoning to allow accessory dwelling units knows the topic is not without objection. Resistance to accessory dwelling units often relate to concerns about overcrowding, declining structural conditions, and increasing traffic. In practice, communities can offset these concerns with reasonable regulations.

For instance, accessory dwellings may not be appropriate on every residential parcel. Sometimes local regulations include a minimum parcel size to be permitted (although, if this minimum size is too big it may defeat the purpose of increasing density in a traditional neighborhood). Also, regulations typically require provision of one additional off-street parking space accessible from the existing driveway or alley. To help preserve neighborhood character, entry doors are often required to be screened from the street or off the side of a structure. So, depending on the size of the parcel, the size and orientation of the existing, principal dwelling, the driveway or alley configuration, and other special circumstances, accessory dwellings may not be appropriate on every single-family residential lot in a community.

There are numerous resources on ADUs for communities considering this housing option, some of those include:

• Accessory Dwelling Units: Model State Act and Local Ordinance by the AARP
• Accessory Dwelling Units: Case Study by the U.S. Department of Housing and Urban Development
• Accessory Dwelling Units – Quick Notes by the American Planning Association
• A Guide to Building a Backyard Cottage by the City of Seattle
• AccessoryDwellings.Org

Additionally, example zoning regulations from Michigan communities that allow ADUs include:

• Clark Township (Mackinac County) allows as a permitted use in any zoning district that permits single family dwellings
• Hamburg Township (Livingston County) allows as a permitted use in its Village Center and Village Residential districts
• Village of Beulah (Benzie County) allows as a permitted use in its One Family Residential District
• City of Traverse City allows up to 10 per year in its Single-Family Dwelling Districts
• City of Niles allows as a special land use in its Low-Density and Medium-Density Residential districts
• City of Manistee allows as a special use in its Medium-Density and High-Density Residential districts
• City of Grand Rapids allows as a special land use in all residential districts

If interested in learning more about accessory dwelling units, contact a Michigan State University Extension land use educator.