The share of new homes being built as rental apartments is at the highest level in at least four decades, as an improving jobs picture spurs younger Americans to form their own households but tighter lending standards make it more difficult to buy.

Residential construction—a pillar of the economy and employment—is starting to ramp up again overall, but in previous years the growth was driven by single-family homes. Last year, according to census data, construction was started on a little less than one million new residential units, and about one in three of those was a rental in a multifamily building, the highest share since data began in the mid-1970s. Single-family homes accounted for about two-thirds of housing starts last year, down from their peak of 87% in 1993 and about 80% in the years leading up to the recession, the census data showed.

The move toward apartment construction reflects the convergence of several trends. Mortgage credit is still tight. Also, Americans have seen muted wage gains, and others have high student-debt loads, forcing people who otherwise would have bought homes to rent instead.

At the same time, as the job market improves, larger numbers of young adults are leaving their parents' homes and forming their own households—adding more to the demand for rentals. And the supply of new apartments hasn't kept pace, especially in some key markets such as San Francisco.

"Builders are betting on more millennials leaving the nest this year," said Jed Kolko, chief economist at Trulia, a real-estate site. "But young people don't get a job one day and buy a home the next. The improving jobs picture for young adults will mean more renters this year, not a surge in first-time home buyers."