The Department of Housing and Urban Development today raised the interest ceiling on federally insured, single-family home loans by one percentage point to 15 1/2 percent effective Friday.

The change is the second in less than a month. It affects houses insured by the Federal Housing Administration and the Veterans Administration.

Assistant HUD Secretary and FHA Commissioner Philip D. Winn cited the substantial premiums, or interest points, that sellers were forced to pay at the lower interest rate in order to sell their properties, a situation that discourages owners from selling to people with FHA or VA financing. Points are charged because FHA and VA rates have not been rising fast enough to keep pace with market rates.

Speaking at the National Mortgage Banking Conference here sponsored by the Mortage Bankers Association of America, Winn told a group of reporters that the Reagan administration might consider asking Congress to approve a floating rate for government insured loans as a part of its overall deregulation plan for the housing industry.

Federal regulators recently authorized adjustable-rate mortgages for conventional loans, Winn said he had just approved a demonstration program of 1,000 units in each HUD region whereby a mortgage company can have a negotiated rate if it will give a firm 30-day loan commitment.

Winn said he expects members of a presidential commission on housing to be appointed within 60 days. Its mandate is to determine how to produce more housing at a lower cost. As such, it will examine areas such as subsidized low- and moderate-income housing, block grants and deregulation. Recommendations are expected in about a year, he predicted.

While acknowledging that the administration's proposed cuts in subsidized housing units to 175,000 in fiscal 1982 from the 254,500 planned by former president Carter might make one group of people suffer temporarily, Winn said it was necessary for the good of the economy. Repeating the Reagan line that the truly needy will be taken care of, he added, nevertheless, "This country can't afford to subsidize everyone."

He emphasized the Reagan philosophy of turning as much as possible over to the private sector, but he roundly criticized private mortage insurance companies trying to do just that. He charged them with being "selfish" and "very destructive to the housing scene" in wanting to "get rid of FHA." Winn said he doubts that private mortgage companies would be able to take over the FHA's function.

On the issue of cutting housing subsidies to cities that retain rent controls, Winn repeated the administration's position that the federal government has no business telling cities whether to control rents.