Residential mortgage interest rates in the Washington area appear to have stabilized after months of steady increases and even may be edging downward, according to Interest Data Reports.

Victor Peeke, whose publication tabulates mortgage rates offered by banks, savings and loans and mortgage companies in the District and its suburbs, reported the average for March 12 was 15.40 percent, down one one-hundredth of a percentage point from the preceding week. However, as many lenders raised their rates again last week as lowered them, indicating a firm trend has not been established.

In the current economic cycle, the prime has dropped from its December 1980 peak of 20 1/2 percent to 18 percent at most big banks. Economists predict the prime may fall further this week. Data Resources Inc. is talking about a rate of between 13 percent and 14 percent rate by midsummer. But mortgage rates have just begun to show signs of budging.

According to Interest Data Reports, Perpetual American Federal Savings and Loan was one of two District lenders lowering mortgage rates. Perpetual dropped its rate for 30-year, 95 percent financing (5 percent down payment) from 15.7 percent to 15.6 percent, with a point each payable by the buyer and seller.

Perpetual President William Sinclain said the move followed a drop in the weekly average auction yield at the Federal Home Loan Mortgage Co.

Perpetual's interest rate on construction loans is pegged to the prime rate, plus 1 or 1 1/2 percentage point, whatever investors in the secondary market will accept.

Better Homes and James T. Barnes are among the mortgage companies that have lowered their rates for conventional mortgages by half a percentage point in the past fortnight. Better Homes said it had found an investor willing to put up $1 million at 15 1/2 percent.

Jerry Hale of Unity Mortgage, which also has lowered its rate to 15 1/4 percent, said conventional loans were very slow but business was good in FHA- and VA-insured loans at 14 percent plus 3 1/2 points.