Consumer prices in the Washington area rose slightly during August and September, although the rise was tempered by an alleged dropoff in housing costs. Federal economists and housing officials said the housing cost figures were based on inadequate information.

Overall prices rose by 1.1 percent during the two-month period compared with a 2.1 percent national increase for the same period and a 2 percent increase here for June and July.

According to the Bureau of Labor Statistics, the relatively moderate area consumer price rise was aided by a 3.4 percent decline in home ownership costs.

However, the drop in home purchase costs reflects only Federal Housing Administration transanctions on which mortgages are limited to $65,000, a bureau economist said. Excluded are most area home transactions, which average over $100,000 for new single houses, and around $75,000 for resale houses.

"Periodically, the data gets the best of us," said Daniel Ginsburg, supervisory economist with the bureau. "The decrease appears to be due to the abnormality of the statistics and is not necessarily what has happened."

Nevertheless, costs of clothing, gasoline, and some food items rose markedly from July to September, the government reported.

Grocery store prices rose by 2.1 percent in September, led by a 2.9 percent rise in the cost of meat, poultry, fish, and eggs. Food prices dropped by one-half percent in August.

More dramatic ws the 4.9 percent increase here in the cost of fuels during the two-month period . The costs of fuel, oil, coal, and bottled gas increased by 11.6 percent during that time.

Gasoline prices increased by 7.6 percent and airline fares rose by about 10 percent during the two months.

The costs of clothing rose even more sharply, increasing by 7.1 percent, with a 17.5 percent increase in the cost of women's and girl's apparel.

In other consumer spending areas the cost of medical care here rose by 3.1 percent over the two months and the cost of transporation rose by 2.2 percent.

Yet despite the substantial increases in most areas of consumer spending, the housing data and the resulting reported decrease in the costs of home ownership kept the rise at a relatively moderate level.

Housing costs overall, declined by 0.3 percent, the government reported, although rental costs rose by 3.6 percent and "other" rental costs declined by 4.7 percent.

But housing costs here, according to the government, rose by 10.1 percent in the last year.

The bureau's figures indicate that actual home purchase prices declined by 6 percent, but increased financing costs lowered the decline to the 3.4 percent figure.

The figure's released yesterday, prepared for the bureau by the FHA, take into account the size and age of the home. But they do not consider neighborhood location.

Further, the housing data is based on an FHA loan program that offers a mortgage rate fixed by law.

In fact, the FHA-Veterans Administration mortgage rate rose to a record 11-1/2 percent this week. These are conventional mortgages, not the unique program used by the Commerce Department to evaluate consumer prices.

Mortgage rates here have risen by more than 2 percentage points in the past year, in most cases, and increasingly homebuyers are having to cope with mortgage rates that rise between the time of purchase and settlement.