The Washington metropolitan area is one of the most expensive urban communities in the mainland United States for a retired couple, the Labor Department's Bureau of Labor Statistics reported yesterday.

In its annual survey of the budget required for food, clothing, shelter, medical care and other services, the bureau found Washington the second most costly area for a low-budget couple, behind Seattle-Everett, Wash., and in the top four for intermediate- and high-budget couples. A retired couple would have to budget $7,281 per year, not counting income tax, to live in this area and purchase the goods and services listed by the bureau for a low-budget standard of living.

In Seattle, the budget for the same goods and services would be $7,412. The nationwide urban-area average is $6,644.

The list reflects prices as of October 1980. Overall prices increased about 10 percent over the previous year, the biggest increase since 1974. Transportation costs and medical expenses led the increases.

The budget for hypothetical intermediate- and high-budget couples includes the basic services purchased by the low-budget couple, plus additional expenditures such as life insurance and travel.

For a couple in the intermediate category, the annual budget required here would be $10,269, fourth highest of the areas surveyed, behind Boston, New York and Seattle. The nationwide urban-area average is $9,434.

In the high-budget category, a Washington-area retired couple would need $15,109, third behind Boston and New York. The same goods and services would cost only $13,827 in Baltimore and $12,864 in Atlanta, the lowest-cost city in the survey in all three budget categories. The nationwide urban-area average in the high-budget category is $13,923.

The figures represent assumptions that do not necessarily reflect the actual cost of living of any real couple. They assume that the retired couple comprises a male age 65 or older and his wife. They are assumed to be self-supporting, in "reasonably good health" and living in an urban area.

According to the Bureau of Labor Statistics, the report may be the last of its kind because "a committee of experts has completed an in-depth study of the family budget methodology and has recommended a completely new approach."