This week's decline in the interest rate on FHA- and VA-insured mortgages from 15 1/2 to 15 percent will not help home buyers substantially -- but could mean substantially higher costs for hard-pressed homebuilders and other sellers, members of the Washington-area housing industry say.

In making the announcement of the change in the FHA rate -- which is set by the government -- Housing and Urban Development Undersecretary Donald I. Hovde declared, "This drop is additional evidence that the president's economic recovery program is beginning to work. . . . It opens the way to homeownership to approximately half a million more families who are now effectively cut out of the home-buying market."

"It's disastrous," said Kenneth F. Murphy, vice president of Long & Foster's new home and condominium division in Fairfax. A decline to 15 percent will not open the market to many buyers, and those FHA/VA rates that already have been bought down to perhaps 13 1/2 percent will not go lower, he said. But at the same time, some builders will have to pay thousands of dollars more in upfront points to lenders because of the change, Murphy said.

This is because lenders require a certain yield on the mortgages, and if the rate to the buyer goes down, the seller must make up the difference between the government-set FHA rate and the yield from a market rate. Upfront points on FHA loans had declined to about 2 to 3 points in recent weeks, but the rate drop means points now are between 4 1/2 and 6, according to lenders and members of the housing industry. Buyers by law pay only one point on FHA loans. A point equals one percent of the loan amount.

Hugh Graham, vice president of forecasting and policy analysis of the National Association of Realtors, was more upbeat about the change, but he calculated the dip would mean an extra 100,000 to 150,000 sales at most. He called the half-point decrease a "small boost to housing, a forerunner of a modest decline" in mortgage rates generally. He predicted that conventional mortgage rates, now just over 16 percent, also will dip to 15 percent by the end of this year before rising again somewhat next year.

Michael Sumichrast, chief economist at the National Association of Home Builders, also said the rate drop was not enough to make a large dent in the market, but said that the amount of points are likely to decline as mortgage rates generally come down. Sumichrast said HUD usually tries to lead the market when rates are declining and lags when market rates rise. This tends to penalize sellers during the transition, but it tends to bring other rates down, Sumichrast said.

But several local builders called the rate decrease a political move that was not supported by economic reality.

Peter E. Kaplan, senior staff vice president of the Mortgage Bankers Association, agreed with that view.

"It has the appearance of a political move, because we don't see any economic rationale to it," Kaplan said. Kaplan said that during the Carter administration, both MBA and candidate Ronald Reagan criticized HUD for "playing politics" with the FHA rates -- trying to make it look like rates were dropping more than they really were -- and now the Reagan administration is doing the same thing.

"It's nice to say, 'I will you, I command you to come down.' That won't change the market demand," Kaplan said. "The administration is returning to the wishy-washy, wishful thinking of previous years."

Joah Etchells, sales manager of Hylton Realty in Dale City, said he thinks the market would have to wait several weeks to see if the rate held. Meanwhile, his company plans no change in interest rates of 13 1/2 percent, bought down from the prevailing FHA rate.

This kind of buydown is typical, and area builders said that is why most home buyers will not see a half-point decline in rates they pay for FHA or VA mortgages, despite the half-point drop in the government-established rate. Builders will tend to simply buy down from the lower level.

Jim Tierney of the Winding Brook development in Chantilly and Don Foster of Foster Brothers, agents for Spring Woods in Springfield, for example, both said it is unlikely that the 13 1/2 percent they are offering on FHA/VA mortgages will change. A representative of Porten-Sullivan, which offers a 14 1/2 percent rate on FHA/VA mortgages at various developments, said he would think about making a reduction in the subsidized rate.

The mortgage limits in the Washington area on FHA-insured loans is $89,500, and the limit on VA-insured loans is $135,000.

Long & Foster's Murphy explained the economics behind what he termed an "artificial" decision to lower the FHA/VA rate. For the buyer, a reduction from 15 1/2 to 15 percent on a $100,000 loan means a $40 decrease in monthly payments from $1,305 to $1,265. At settlement the buyer will still have to pay one point (one percent of the loan), the amount set by law.

The lender--a bank, mortgage company or savings institution--still insists on getting a return of 15.75 percent on its money. So when the buyer pays less and the lender gets the same, it is the seller--a builder or homeowner--who pays more up front to make up for the reduced yield from the lower rate over the life of the mortgage.

Murphy said with the FHA/VA rate at 15 1/2 percent, sellers had to pay perhaps $2,000 in points on a $100,000 mortgage; at the 15 percent level, they now will have to pay perhaps $5,000 or $6,000. In such a case, the buyer is getting a break of $40 a month on the mortgage, but the seller has to put down $3,500 more in cash.

On buydowns, the total cost to builders will remain about the same despite the change in the rates, lenders said. This is because, regardless of what the official FHA rate is, the builder is paying the difference between a particular below-market and the market rate with upfront points. Of course, it is all the same to the buyer, as well, if the bought down rate remains the same.

Staff writer Sandra Evans Teeley contributed to this article.