In December, Minneapolis became the first American city to decide to eliminate single-family residential districts by permitting triplexes in all the city's residential zones.

Minneapolis is not alone in pursuing a change: Other cities — including Seattle and Portland — are contemplating more dense development in their single-family districts. Legislation in California has contemplated state pre-emption of local single-family zoning around train stations.

California also recently required the permitting of accessory dwelling units (i.e., "in-law" units) in most of the state's single-family districts.

All these efforts are controversial, but perhaps inevitable: In Minneapolis, 60 percent of the city's area was designated single-family residential. Many U.S. cities are similarly zoned. If cities want to address housing affordability, racial segregation or climate change in any meaningful way, the single-family district has got to give.

Receiving little attention, however, is the fact that changing the zoning does not ensure the end of the single-family district. Since the Industrial Revolution, this country has had two overlapping systems of land control: one public, implemented through zoning; and one private, implemented through the "restrictive covenant."

Until the Industrial Revolution, courts disfavored restrictive covenants. But rapidly increasing urbanism and industrialism needed a legal tool to control change. American courts responded by making restrictive covenants easier to use.

By the late 1860s, when Frederick Law Olmsted developed the Chicago suburb of Riverside, Ill., he utilized restrictive covenants to do work now typical of zoning, such as mandatory setbacks. By the early 20th century, whole cities — like Beverly Hills — and neighborhoods within cities — like Country Club in Kansas City, Mo. — were regulated solely by private restrictive covenants that, among their most controversial restrictions, forbade sale to African-Americans.

Racially restrictive covenants were made unenforceable by the Supreme Court's 1948 decision in Shelley vs. Kraemer. But by then, the public system of zoning, which took off after it was held constitutional in the Supreme Court's 1926 decision in Euclid vs. Ambler, provided a public alternative to the covenant.

A city could zone out multifamily housing and when mixed with federal mortgage policy that prevented minorities from getting mortgages for single-family homes, create de facto segregation.

Now that Minneapolis and other cities are changing the public regulations, private regulation may well return in force.

Some 20 percent of Americans already live in a community governed by restrictive covenants, such as Covenants, Conditions and Restrictions (CC&Rs), where the most common requirement is retention of the single-family residential use. If Minneapolis does not address the private restrictive covenant, it may simply see neighborhoods record restrictive covenants to maintain the single-family nature of the neighborhood by private agreement when no longer mandated by public regulation.

Reformers have several options, all controversial. States with enabling statutes that govern "common interest communities" could prohibit enforcement of restrictive covenants that limit a lot to fewer than three units, or permit local governments to opt in to such a rule.

Local governments in states that permit "home rule" could do the same within their jurisdictions.

State constitutions could be amended to prohibit single-family districts, whether created by public regulation or private agreement.

State courts could also take a stand. In the 2018 case of Tarr vs. Timberwood Park Owners Association, the Texas Supreme Court interpreted the term "single-family residential" in CC&Rs to include the commercial use of homes through short-term rental platforms like Airbnb. Similar decisions favoring more intense use of single-family homes governed by restrictive covenants could become more common in state courts.

Finally, many common interest communities are created at the time of construction solely to permit developers of phased projects to keep control over the community until the developer has sold the last of its lots. Many such communities likely have no long-term interest in CC&Rs. States should consider requiring an affirmative vote to retain the restrictive covenants imposed by the developer — including the single-family lot provision — after the community is built out.

The decision in Minneapolis to address the legacy of segregation in its single-family districts deserves praise. But unless the city also resists the power of the restrictive covenant to create the same result through private agreement, Minneapolis — and other cities that follow its lead — may well find it has gone through enormous effort to see little result.

Stephen R. Miller is a law professor and associate dean at the University of Idaho College of Law.