Cross and Bacher have also been examining “green living” possibilities for the cottages, such as installing rain barrels to mitigate runoff and putting in solar panels to lower the total electrical demand for the properties.

The District is following nearby Montgomery and Arlington counties, in Maryland and Virginia, respectively, which loosened restrictions on accessory dwellings in recent years. The key to homeowners’ taking advantage of the zoning change, Cort said, is for the rules to suit the neighborhoods without being too restrictive. In Arlington, she said, the rules are so restrictive that few accessory dwellings have been built.

Portland and Seattle also have been tweaking their rules to encourage building; after Portland removed “development charges” amounting to about $11,000 per home in 2010, according to an article in the Seattle Times newspaper, the city began permitting, on average, one unit per day.

To Cort, the D.C. Office of Planning has struck the right balance.

“It does have a lot of restrictions, but it’s not nearly as restrictive as in other jurisdictions,” she said. “We think this is great for homeowners, for the diversification of neighborhoods and for housing affordability.”