PORTLAND, Maine — Since 2014, the city has approved construction of more than 1,500 new homes, but the vast majority are too small to house families, which Portland needs in order to grow, city leaders say.

Over the past two decades, the cost of city services has risen while the number of taxpayers has remained relatively flat. The best way to span the resulting revenue gap is to add more people, according to city leaders, who want to boost Portland’s population from about 67,000 to 72,000 by 2027.

These new Portlanders will need homes and, spurred by high rents and low vacancies, developers are building lots of them. But they have too often been high-end, low-occupancy units that don’t attract the families who would drive sustained population growth and build vibrant neighborhoods, said Planning and Urban Development Director Jeff Levine.

“We need to focus more on providing family housing that people actually spend time in, as opposed to these pied-a-terres that people come to for the weekend,” said Levine. “Until a couple of years ago, a lot of the focus was on housing that attracted empty nesters.”

Developers have broken ground on more than 60 percent of the new housing approved over the last three years. This surge in supply may eventually offer some relief to the low- and middle-income Portlanders who have struggled to keep their heads above the tide of rising rents. And yet, because of their location and size, these new homes are unlikely to heat up the tepid population growth in Maine’s largest city and economic hub.

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Since 2014, 97 percent of the approved housing developments have had two or fewer bedrooms, according to city data, and 77 percent were built on the peninsula. Only 45 of the 694 units under construction since 2013 had three or more bedrooms — although the city data exclude free-standing houses.

This pattern fits with the national trend of young adults and aging baby boomers flocking back to cities, according to Patrick Venne, a Portland development consultant and former Berwick city manager.

“The people who want to be in the city only need two bedrooms,” said Venne.

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Portland has only gained 2,600 people over the last 15 years, according to the U.S. Census Bureau. And if it were not for thousands of immigrants who moved here, the city would likely have had a net population loss over that period.

Parents, who might be attracted by the culture and energy of cities, are often still looking for the good schools and space of the suburbs. They largely don’t want to live in the urban core, said Venne. This dynamic, coupled with the high rents, puts Portland at risk of being hollowed out and giving way to a sprawl of commuter suburbs, according to a recent report on the city’s growth authored by local experts.

Developing Portland neighborhoods outside the urban core, such as Deering Center and Morrill’s Corner, so that they blend the qualities of the city and suburbs, is the best way to attract families to the city, according to the report.

The city is tinkering with how to channel development money toward building the right size homes in the right areas.

The zoning code — which developers frequently grumble about and Levine described as an archaeological dig of regulations piled upon regulations going back to at least the 1970s — may get major changes over the next two years. And the planning department is considering a system that would reward developers who construct multi-bedroom units by allowing them to add extra units onto their buildings.

Venne said development markets in larger American cities have already shifted to provide families with neighborhoods, and that developers here will eventually respond to the trend.

“It’s just a scale thing,” he said.

BDN writer Darren Fishell contributed to this report.