Selling a house generally costs the government about $10,000. The outsides are weeded and the insides are scrubbed. Stolen appliances are replaced, brackish pools are refilled. And until the properties are sold, they must be maintained. Fannie asks contractors to mow lawns twice a month during the summer, and pays them $80 each time. That’s a monthly grass bill of more than $10 million.

All told, the companies spent more than $1 billion on upkeep last year.

“We may be behind many loans on the same street, so we believe that it’s in everyone’s best interest to aggressively do property maintenance,” said Chris Bowden, the Freddie Mac executive in charge of foreclosure sales.

Prices have plunged. So by the time a home is resold, Fannie and Freddie on average recoup less than 60 percent of the money the borrower failed to repay, according to the companies’ financial filings. In Phoenix and other areas where prices have fallen sharply, the losses often are larger.

Foreclosures punch holes in neighborhoods, so residents, community groups and public officials are eager to see properties reoccupied. But there also is concern that investors are buying many foreclosures as rental properties, making it harder for neighborhoods to recover.

Real estate agents tend to favor investors because the sales close surely and quickly and there is the prospect of repeat business. But community advocates say that Fannie and Freddie have an obligation to sell houses to homeowners.

David Adame worked for Fannie Mae’s local office during the boom, on programs to make ownership more affordable. Now with prices down sharply, Mr. Adame sees a second chance to put people into homes they can afford.

“Yes, move inventory,” said Mr. Adame, now an executive focused on housing issues at Chicanos por la Causa, a Phoenix nonprofit group, “but if we just move inventory to investors, then what are we doing?”

Executives at both Fannie and Freddie say they have an overriding obligation to limit losses, but that they are taking steps to sell more homes to families.

Fannie Mae last summer announced that it would give people seeking homes a “first look” by not accepting offers from investors in the first 15 days that a property is on the market. It also offers to help buyers with closing costs, and prohibits buyers from reselling properties at a profit for 90 days, to discourage speculation. Fannie Mae said that 68.4 percent of buyers this year had certified that they would use the house as a primary residence.

Freddie Mac has adopted fewer programs, but it said it had sold about the same share of foreclosures to owner-occupants.

The companies also have agreed to sell foreclosed homes to nonprofits using grants from the federal Neighborhood Stabilization Program. Chicanos por la Causa, which won $137 million under the program in partnership with nonprofits in eight other states, plans to buy more than 200 homes in Phoenix in the next two years. It plans to renovate them to sell to local families.

The scale of such efforts is small. The home ownership rate in Phoenix continues to fall as foreclosures pile up and renters replace owners.

But John R. Smith, chief of Housing Our Communities, another Phoenix-area group using federal money to buy foreclosures, says he tries to focus on salvaging one property at a time.

“I tell them, ‘O.K., you want to unload 10 houses to that guy, fine,’ ” he said. “ ‘Now give me this one. And this one. And one over here.’ ”

The Senate has approved a plan to give home buyers an extra three months to finish qualifying for federal tax incentives that boosted home sales this spring.

The move by Senate Majority Leader Harry Reid (D-Nev.) would give buyers until Sept. 30 to complete their purchases and qualify for tax credits of up to $8,000. Under the current terms, buyers had until April 30 to get a signed sales contract and until June 30 to close the sale. The proposal would only allow people who have signed contracts to finish later. About 180,000 homebuyers who already signed purchase agreements would otherwise miss the deadline.

Reid added the proposal to a bill extending jobless benefits through November.