Just when you thought it might be safe to put your house on the market comes this little bombshell from the federal government:

The Federal Housing Administration has revised its guidelines for borrowers.

Responding to concerns about its capital-reserve ratio, which fell below the 2 percent threshold Congress requires, the FHA is raising mortgage-insurance fees and down payments for borrowers with lower credit scores.

It also is cutting the percentage of the sale amount that sellers can pay to help ease the transaction.

The FHA's action could have the effect of removing a big chunk of potential buyers from the real estate market, which has been depending heavily on the government for everything lately - from foreclosure bailouts to tax credits to push wary purchasers into home ownership.

The problem is that one-third of all mortgages approved in 2009 were FHA-insured. During real estate's mid-decade boom years, when breathing was a measure of creditworthiness, the FHA was lucky if it could get a loan in edgewise.

Here's what the new guidelines, which take effect at various times between now and July, mean:

Borrowers with credit scores of less than 580 will need to make 10 percent down payments on homes - up from the current 3.5 percent. The percentage of FHA loans in default is increasing, recent data show.

Seller concessions will be cut to 3 percent of a transaction's price from 6 percent.

The mortgage-insurance fee at closing will increase from 1.75 percent to 2.26 percent of the difference between the down payment and the standard 20 percent.

What effect will the changes have?

Although the guidelines seem to be more constricting, "I do not believe they will cause much pain to borrower," said Peter Buchsbaum, branch manager at Arlington Capital Mortgage in Horsham, since people with 580 scores were finding it increasingly difficult to get FHA loans anyway.

The higher up-front mortgage-insurance premium "sounds outrageous," he said, "but on a typical $250,000 loan, the increased monthly cost is less than $6."

"This seems to be a small price to pay in an effort to keep the only loan program out there that works in our new lending environment solvent," Buchsbaum said.

David Diamond, of Diamond Mortgage in North Wales, said the changes would "make it much more difficult for the marginal borrower to qualify, so either they purchase a more reasonable home or pull out of the market as the lower price range doesn't provide the amenities they're seeking."

"The real estate market needs every possible tool to absorb the housing inventory, so the FHA changes are a real negative," Diamond said.

Veteran Mayfair real estate broker Christopher J. Artur agreed that few borrowers with credit scores under 580 were having much success getting financing anyway (the average FHA score appears to be in the upper-600 range these days).

"The problem for my market, however, is the reduction in the seller assist," Artur said. "With a higher down payment and the end of grants for settlement assistance, cutting seller financing to 3 percent will hurt" first-time buyers - many of whom can afford the monthly mortgage payments but need help amassing the larger sums due at closing.

Artur said he believed people would find a way to circumvent this change "under the table."

Moody's Economy.com chief economist Mark Zandi also considers scaling back the ability to use seller financing the most significant of all the changes the FHA is making.

He said the action "does highlight more broadly that the government is slowly exiting its extraordinary support to the housing market."

"The market will struggle getting off of policy life support," Zandi said.

Inquirer real estate writer Alan J. Heavens is the author of "Remodeling on the Money" (Kaplan Publishing). His home improvement column appears Fridays in Home & Design. "On the House" appears Sundays in The Inquirer. Contact Alan J. Heavens at 215-854-2472 or aheavens@phillynews.com.