Good news: More new apartments will come on the market this year in the U.S. than in decades. Keep reading for the bad news.

Builders are expected to complete some 371,000 new apartments in 2020, compared to 247,000 in 2019 and 119,000 in 2010, according to the real-estate analytics firm RealPage. The problem is that many of the new apartments will be too expensive for lower- and middle-class families. RealPage data show that in many metropolitan areas between 60% and 89% of the apartments under construction are in neighborhoods...