A big chunk of North Texas' building boom is apartments.

For the last few years, the Dallas-Fort Worth area has led the nation in new apartments. More than 100,000 new units have opened their doors in the last four years. And average rents in the area have jumped by almost 30 percent since 2012.

While there are still almost 48,000 rental units under construction in the D-FW area, a slowdown in rent growth and soaring building costs are likely to tamp down the hot apartment building market in the coming year.

But don't expect a crash in the rental market. With almost 100,000 new jobs being added to the D-FW market in the last year, there's still huge demand for apartments to house young workers relocating to the area.

"We've got the jobs that keep churning in this town, and we continue to be underserved in apartments," said Bradley Miller, president of builder Encore Multi-Family LLC. "Yes, there is a boatload of high-end apartment product coming through the development pipeline.

"It will be interesting to see what happens with that."

With apartment rent increases in the most recent quarter running about half what they've been the last few years, Miller says that soaring construction costs in some markets will slow new apartment starts.

"The construction costs have increased to such a level and rents are not able to keep up," Miller said. "With the hurricanes, it is only going to get worse. We are hitting the ceiling on rents and the numbers aren't working like they were a couple of years ago."

Miller also said that a lack of affordable, properly zoned development sites in the suburbs will be a headwind against starts. There's almost nothing in the suburbs that doesn't require rezoning," he said. "And some of our Dallas suburbs aren't that friendly to apartments."

Even so, Miller expects apartment developers and landlords to make it through a slight slowdown.

"For 25 years in this business, the average annual rent increase was 2.5 percent," he said. "Now we are mad about 3 percent."

Spencer Stuart, senior managing director with apartment builder Legacy Partners, said high construction materials costs are his biggest concern.

"With the hurricanes in Florida and Texas — and now Puerto Rico — building materials prices are going up," Stuart said. "I'm really worried about appliance prices and supply.

"I do think that the level of apartment production in the Dallas area is going down and will continue to decline a little bit. I don't think we are in for a hard landing because of the strong economic factors."

With only 5 percent of the D-FW apartment market vacant, it's hard to get too worked up about a slowdown.

"Vacancy rates are still low," said Jay Denton with RealPage. "Apartment demand is pulling back a little bit because job growth is not quite what it was."

For the 12 months ending with August, employment gains in North Texas added up to about 97,000 new jobs. That's still one of the strongest job growth numbers in the country. But it's down from the almost 120,000 annual job growth in recent periods.

"Apartment construction can't stop if we keep adding so many new people to this market that need someplace to live," Denton said. "Construction will remain high — but not as high as it has been."