Irvine and Lake Forest are among the fastest-growing cities in California, spurred largely by upswings in new housing that should continue as the economy improves, the state’s Department of Finance said Monday.

Every Orange County city saw its population grow in 2015 – as did California as a whole, which nudged up nearly 1 percent to surpass 39 million residents.

Finance Department officials and demographic experts said the growth is a normal part of an improving economy, with Irvine and Lake Forest doing their part.

Lake Forest increased 3.7 percent from Jan. 1, 2015, to a year later, ranking third among California cities with populations of more than 30,000 people. Irvine ranked ninth with a 2.6 percent boost.

“I’ve never seen it grow this fast,” said Cindi Powalski, a 34-year Lake Forest resident and real estate agent for Berkshire Hathaway HomeServices. “It’s so crazy. Every house has multiple offers.”

Last year, Lake Forest added about 990 housing units and Irvine roughly 2,000, which explains the population increases, said Deborah Stickley Diep, the director of the Center for Demographic Research at Cal State Fullerton.

“When you build housing, the population will increase,” she said. “Especially in Orange County, the occupancy will happen fast.”

Orange County, which saw its population rise 1 percent in a year, adding more than 30,000 residents, has increased steadily every year since the end of the Great Recession in 2009.

It will continue to see its population increase, as will California, as more housing is added and the birth rate continues to outpace the death rate, said Doug Kuczynski, a demographer for the state Finance Department.

Irvine, Diep said, will likely continue to rank near the top of California cities for population increases as its housing boom continues.

Since 2011, the population of Irvine has grown 18 percent.

“A lot of permits have been pulled in Irvine,” Diep said. “Irvine for sure will continue to build.”

Lake Forest, too, is booming. In 2015, Lake Forest’s population grew nearly as much as it had in the previous four years combined, according to Finance Department data.

The city has seen several housing projects come online recently, including the 2,379-home Shea/Baker Ranch community. In January, the City Council approved a 52-house gated community near the Irvine border.

Powalski, the real estate agent, said three-quarters of her clients come from Irvine, especially as people search for lower-priced houses and become aware of the city’s new parks and strong school system. Lake Forest is also near Irvine and has close freeway access.

“There is something here for everyone,“ she said. “We are the gateway to south Orange County.”

Contact the writer: 714-796-6979 or chaire@ocregister.com