In a major development in plans to revitalize Washington's old downtown area, one of the city's largest and most prestigious law firms has concluded plans to move to a new building east of 15th Street.

Covington & Burling said Friday that they have concluded a lease to rent the top six floors of a building under construction at 12th Street and Pennsylvania Avenue NW for an undisclosed amount.

The move puts them in the middle of the area being upgraded by Pennsylvania Avenue Development Corp., on the edge of a shabby part of the city where porno shops and dirty bookstores have clustered. The "three A's" -- attorneys, accountants and associations -- generally have located in the area west of 15th Street.

"It's the vindication of the long and expensive investment that's been going on in downtown," said James Gibson, city planning director, discussing the move when it was rumored but before it had been nailed down. "It's key, because they're in the real world. They want their location to reflect the prestige their clients are attracted to."

Gibson said he expected the move would prompt other of the "so-called prestige functions that have avoided the traditional downtown to reevaluate that as a relocation option."

Besides redevelopment along Pennsylvania Avenue, city planners hope that the construction of a convention center will also help revitalize the area.

Covington & Burling partner Alfred Moses said the law firm has contracted for 187,000 square feet in the building with an option for three additional floors. The law firm will also have exclusive use of a large roof garden on top the 13-story building at 1201 Pennsylvania Ave. NW.

The firm will move into the building when it is completed in summer 1981, consolidating offices that are now in two buildings west of 15th Street -- 888 16th St. NW and 818 Connecticut Ave. NW.

Other tenants will include law firms, architectural firms and a public relations firm, he said. The building will include a large atrium, a garage for self-parking, a restaurant and restricted retail use.

The new location is in an area "where we think the future development of the city will occur," said Moses. "It's a part of the city that is being rebuilt. We think it will be the prime office location in the District."