Almost every zoning code across the country defines “family” in a traditional way: people who are legally related by blood, marriage or adoption. Sometimes, the definition allows a small number of unrelated people (say, two or three), who are functioning as a “housekeeping unit” to be considered a family.

Such definitions exclude people just as committed to each other as members of “traditional” families, but who don’t satisfy legal conditions. These “families of choice” consist of unrelated adults who decide to share finances, child-rearing responsibilities, home repairs, chores and meals. They include groups of single moms, households that have merged and older adults forging new lives together after t he deaths of their spouses.

The definition of family matters because zoning codes typically have a “one family per housing unit” policy. These policies are most strictly enforced in the neighborhoods with single-unit detached homes — 64 percent of neighborhoods, according to the 2013 American Housing Survey. It’s in these communities where housing affordability tends to be low, and racial segregation high.

Some cities, like Minneapolis, have started making plans to reduce or eliminate the amount of land devoted to single-unit zoning. But other cities, like Plano, Tex. — where more than 4,000 residents have mobilized to overturn similar plans — have taken steps backward. The amount of land devoted to single-unit, detached dwellings is not likely to change greatly in the places that need it the most. Other aspects of zoning, like lot size controls and minimum square footages, would also be hard to override.

But definitions of family appear to be more ripe for change. Four state supreme courts — California, Michigan, New Jersey and New York — have already struck down zoning ordinances that failed to allow “ functional families .” They found that such ordinances violate rights to due process, privacy or both. They also found that communities can still achieve a “residential character” without delving into the specifics of the relationships among residents. And they said that traditional family definitions flunk the “rational basis test” courts use to determine whether a law is constitutional.