Roughly a third, or about 7,200, of the six-county region’s vacant homes have been empty longer than a year. About 3,500 have been empty longer than two years.

The vacancy count, revealing a vast excess of unused shelter in a region that overbuilt during the housing boom, stems from a U.S. Postal Service survey of houses and apartments where mail has not been picked up for 90 days.

Unoccupied houses put further stress on neighborhoods already hit hard by foreclosures.

“If I could afford to move out of here I’d do it,” said Angela Trejo of Sacramento’s Oak Park. Across the street is a house vacant for months, now for sale for less than $20,000.

In the northern sections of Oak Park, and parts of West Sacramento, more than one in 10 homes is vacant. More than one in 20 homes is vacant in parts of Oak Park, south Sacramento, North Sacramento, North Highlands and Citrus Heights.

Reasons vary for the surge of vacant dwellings.

Area real estate agents and others Monday cited recent foreclosure moratoriums and banks increasingly sitting on large numbers of repossessed homes. Apartment communities also report rising vacancies as 11.3 percent regional unemployment forces renters to double up or move back in with family members.

Novato apartment industry tracker RealFacts reported last month that 7.8 percent of 76,000 apartments in large complexes in El Dorado, Placer, Sacramento and Yolo counties were vacant in the first quarter of 2009. A year earlier 6.9 percent of the region’s apartments were vacant.

Meanwhile, 8,189 homes were for sale in El Dorado, Placer, Sacramento and Yolo counties as March ended, reported Sacramento researcher TrendGraphix. It was not known how many were empty.

“There’s several houses in my neighborhood that were supposed to go into foreclosure in November of last year,” said Coldwell Banker real estate agent Mike Toste of Roseville. “They’re still sitting vacant. Banks are trying to keep this excessive inventory from coming into the market like last year and the year before.”

Highlights of the data:

• The nearly 24,000 vacant homes and apartments in El Dorado, Placer, Sacramento, Sutter, Yolo and Yuba counties compare with 17,000 the same time last year.

• Regionwide, 26 of every 1,000 homes have not picked up mail for at least 90 days, well above the statewide average of 17 per 1,000.

• Among the 10 largest counties in the state, only Riverside had a higher vacancy rate – 4.1 percent – than Sacramento County’s 2.8 percent.

• Yuba County now has the highest percentage of vacancies in the state; 8.9 percent of homes in the county have not picked up mail in 90 days. And almost half have been empty for more than a year.

“It doesn’t really surprise me, frankly,” said David Burrow, a Keller Williams real estate agent in Yuba City. “With all these foreclosure moratoriums, properties are vacant, but nothing is happening.”

Yuba County has taken some of the downturn’s hardest blows after a five-year housing boom planted 4,200 new houses and attracted many Sacramento commuters who bought with loans that turned toxic when values fell. The rural county has seen nearly 1,200 foreclosures in the past 15 months, according to researcher MDA DataQuick.

Collectively, thousands of vacant properties and empty apartments across the region have steadily driven down sales prices and rents. In some neighborhoods vacant homes have become crime magnets and scenes of squatting. Many vacant properties also have neglected swimming pools, breeding grounds for mosquitoes that carry West Nile virus.

Monday, Jovella and Louis Christian moved into a rental home near Ninth Avenue in Oak Park. Homes on both sides of them are vacant. On one side the house was getting repairs. On the other, only a halfhearted attempt to nail boards over a few windows.