Consumer prices in the Washington area rose 1.8 percent during October and November, more than twice the increase of the previous two months, but slightly less then the rest of the nation, the Labor Department reported yesterday.

Higher costs of food, housing -- particularly mortgage interest rates -- and transportation in the form of new and used cars and tires accounted for most of the increase. Gasoline, entertainment and medical-care costs declined, bringing the area's annual inflation rate to 12 1/2 percent. The figures, however, do not take into account the seasonal adjustments that are used in determining the national consumer price index.

Food and beverage costs rose 1.9 percent, reflecting a 1.8 percent rise in grocery prices and 1.9 percent in costs of food eaten away from home. Prices for fresh vegetables, pork, eggs, beef, sugar, white bread and most dairy products rose, while fresh fruit, poultry, fish and seafood prices declined.

In addition, as many Christmas shoppers have discovered, apparel costs rose 1.3 percent, with increases concentrated in women's clothing and footwear. r

During the months of August and September, prices in the area increased only 0.8 percent, and housing costs declined 0.7 percent.

Consumer price increases in the Washington area equaled the average of U.S. cities for the past two-month period. For example, prices rose slower locally then in Chicago, where consumers paid 3.9 percent more, and in detroit, where prices rose 2.7 percent.

But consumer prices increased by only 1.2 percent in New York and by 0.8 percent in the Philadelphia area. In addition, prices rose 1.3 percent in Baltimore, the Labor Department said.

As the cost of new cars has risen with the introduction of 1981 models, so have the prices of used cars, the consumer price figures show. One explanation is that dealers are willing to pay more for cars they take as trade-ins to get customers to buy new models and lift laggin new-car sales.

But Bob Cross, a salesman at Seidel Chevrolet Inc. in Landover, said another reason for higher used-car prices is the increased demand from people who can't afford to buy new cars.

A new car with automatic transmission, power steering and brakes and air conditioning is likely to cost between $7,500 and $9,000, Cross said.

Many customers at Seidel are buying late-model and larger used cars, Cross said. For example, he said, the price of a new sedan is about $9,000 But a two-year-old car of the same type would cost about $6,500, Cross said.

The housing index rose 2 1/2 percent during October and November, reflecting higher costs for mortgage interest rates, home purchases, furnishings, water and sewage services and property insurance. But rates were lower for electricity, reflecting lower winter rates, the Labor Department said.

Mortgage interest rates in the October and November consumer price figures reflect rates at settlement. These basically were rates that were decided about two months earlier and do not necessarily reflect current rates.