GOING UP?: The State Department of Finance says Manteca's population was up to 66,451 by the first of the year. But a private consultant hired by the Manteca Redevelopment Agency said the population has actually dropped by 19 people.

South San Joaquin County continues to grow at a steady clip, according to recent population figures from the state, but a private consultant’s estimates and school enrollment numbers point to a stagnant population in 2007.

Manteca added 1,855 new residents from January 2007 to January 2008 — a 2.9 percent gain — for a total population of 66,451, according to estimates released May 1 by the California Department of Finance.

Lathrop saw a 6.5 percent population increase, the state estimates, adding 1,071 residents for a total of 17,429. Those figures indicate that Lathrop is the fastest-growing city in the county and the sixth fastest in the state.

Department of Finance estimates show a steady increase in the population of Manteca and Lathrop each year since 1990.

But the state’s estimates are based mainly on the number of new houses that have been built in the city since the 2000 U.S. Census, state demographer Mary Heim said, and with the overbuilding that has led to a huge inventory of unsold homes, that number could be inaccurate.

Enrollment numbers for the Manteca Unified School District — which also includes Lathrop and the Weston Ranch area in Stockton — have dropped 1.5 percent from March 2007 to March 2008, falling to 23,051, according to data collected from the district.

And a private consultant hired by the Manteca Redevelopment Agency also reported a slight dip in the city’s population.

Claritas, a New York-based demographic and marketing consultant, estimated that Manteca’s population was 64,688 in the beginning of 2007 and 64,669 in 2008, according to reports produced for the city.

Lathrop does not collect demographic data separate from the state estimates.

Two factors — rapidly rising home prices in the last several years and a proliferation of foreclosures in 2007 — may have led to a stop in growth, according to Jeff Michael, who heads the Business Forecasting Center at University of the Pacific

Net migration into the county has essentially dropped to zero in the past two years, Michael said, and recent foreclosures may have led people who moved to the county from the Bay Area to move back to the place where many of them still hold jobs.

However, the local impact of foreclosures remains a mystery to demographers, he said. If people are simply moving into rental homes down the street, populations should remain steady. But forced home sales could also send residents out of state.

It’s a question that state population estimates cannot answer, Michael said.

“The Department of Finance estimates aren’t adjusted for any foreclosure estimate,” Michael said. “If the school enrollment is flat but the Department of Finance estimate is going up … they may be underestimating the foreclosure impact.”

Similarly, Claritas’ Chief Demographer Ken Hodges said the effect of foreclosures was unlikely to show up in his company’s estimates, which are based mainly on births, deaths and tax return addresses collected by the U.S. Census Bureau.

“I expect it would be very difficult to anticipate with a high degree of certainty what’s going to happen at a neighborhood level,” Hodges said.

But even if foreclosures halt the area’s expansion for a year or two, Michael said, a drop in home prices is bound to bring back the fundamental causes of the county’s growth — cheaper homes and more open space.

“With prices coming down, it won’t last,” Michael said. “The incentive to move out here will reassert itself.”