Home mortgage interest rates are going back up again, threatening to hobble home sales just as the real estate business heads into the home stretch of the house-buying season.

In the Washington area, mortgage rates jumped an average of one-quarter of a percentage point last week to just over 15 1/2 percent, said Victor Peeke, publisher of Interest Data Reports, a weekly survey of lending rates.

A quarter-point increase is a big change in a single week, Peeke said, indicating a large number of lenders have raised their rates.

Nationally the downward trend in most interest rates "came to an abrupt end in early April," the Federal Home Loan Bank Board reported yesterday. The FHLBB said rates as of the first of the month were only 1.1 percentage point below the record level of 16.6 percent set in April 1980.

Some economists were predicting mortgage costs would be back down to 12 percent by this summer, but now it looks like 13 percent or 14 percent is the best that can be hoped for, said Dale Riordan, chief economist of the National Savings and Loan League, an organization of big lenders.

"Most people expect rates to decline very, very slowly," added Riordan, and many predictions are that rates will go back up again next fall.

The interest rates on government-insured mortgages went up effective Monday from 14 percent to 14 1/2 percent. That is a new record for Federal Housing Administration and Veterans Administration loan rates, which traditionally are a little lower then conventional mortgages.

Increases in interest charges now could cause new trouble for home sellers, who have seen the pace of sales increase in recent months. Generally home sales start picking up after the first of the year and continue to climb through the spring and summer.

The Northern Virginia Board of Realtors reported March sales of its members were up 32 percent from last year's seriously depressed level. During the month 1,935 homes were sold in the Virginia suburbs compared with 1,463 in March 1980, said Joe Haydon, spokesman for the board. That's still well short of the 2,400 sales recorded in March 1979, before interest rates took off.

The Virginia sales figures reflect houses that were taken off the active "for sale" listings during the month and trend to lag behind the signing of sales contracts. The Virginia Realtor's figures, however, are generally regarded as the most accurate indicator of local trends because other areas either do not make sales data public or report them in different ways.

Wayne Lancaster, chairman of the suburban Virginia real estate group, said increases in interest rates are having less impact on sales now than they did in the past. Consumers have grown accustomed to rates in the teens, he said, and home buyers have found more and more ways to avoid paying the high rates.

The average rate of 15 1/2 percent reported by Peeke is for new 30-year mortgages with a 20 percent down payment. Buyers often get lower rates by assuming the existing mortgages on the property -- which may carry rates as low as 5 percent to 8 percent -- then taking a second mortgage for the rest of the purchase price at current rates. The old and new mortgages can average out to 12 percent to 13 percent, resulting in monthly payments $100 to $300 less than with an all-new loan.

Lancaster said the Federal National Mortgage Association has made that process easier by setting up a new "resale purchase program." Peeke said his surveys show that in the last month and a half or two months, more and more local lenders have started participating in that program of the FNMA, which is known as Fannie Mae.

The FNMA resale program applies to any home with a government-guaranteed FHA or VA loan, or any home whose mortgage has been resold by the lender to the FNMA. The way it works is that the FNMA cancels the old low-rate loan and issues a new mortgage at something less than the market rate. The savings can be 1 to 3 percentage points, depending on the rate of the original loan and how much new money has to be borrowed.

The increase in such unconventional financing is reducing the demand for standard loans and removing some of upward pressure on rates.

Mortgage rates had been inching upward for sometime, the bank board's economists noted, but the rise appeared to break early last month. During the first two weeks of March, mortgage rates declined fractionally, but the improvement was short-lived, and by the end of the month rates were heading back up.

The increase "appears to have reflected weakness in deposit flows to thrift institutions as well as increases in the overall cost of funds to mortgage-lending institutions," the board reported.