A growing number of families want to live in a home along with elderly parents or “boomerang” grown children – but they may run into problems with the local zoning board.

Some 18% of Americans now live in a home with more than one adult generation, and that figure is growing. In many cases, what families want is a home with an “in-law” apartment – one that has a separate entrance, separate kitchen, and separate utilities. The idea is that the family can live together, but the elderly parents or grown children can nevertheless have a measure of independence.

Home builders say there is strong demand for this type of arrangement. The problem is that such structures are often banned by zoning laws in residential neighborhoods.

These zoning rules were typically adopted many years ago to deal with different problems. City planners wanted to relieve neighborhoods full of traditional families in single-family homes from the congestion that can come from multiple apartments and rooming houses. As a result, zoning laws often prohibit houses from having multiple kitchens, and there can be steep fees for separate water and sewer hookups or electrical meters.

To get around this, some builders are designing new homes with separate scaled-down kitchenettes – featuring a hot plate, microwave, sink and refrigerator. To avoid acknowledging that there’s a separate kitchen, these areas may be labeled in the plans as “service bars” or “convenience centers.”

Some zoning boards have begun taking a second look at their rules, and making it easier for multiple generations to live under one roof. But others are worried that, in the age of Airbnb, allowing houses with separate apartments will ultimately lead to an influx of short-term renters – and dramatically change the character of many neighborhoods.