J.C. Penney Co., the nation's third largest general merchandise retailer, is planning to join No. 2 K-mart with new stores in the Forestville area of Prince George's County - part of continued expansion by both firms in the Washington market.

The new 160,000-square-foot Penney unit and a smaller K-mart will be located in an enclosed mall to be constructed at Pennsylvania Avenue and Donnell Drive, across the street from the existing Penn-Mar shopping center.

Penney will close its store in the 20-year-old Penn-Mar center and move to the new $20 million mall. Construction is expected to begin this summer and be completed late in 1979.

To be named Forest Village Park, the new mall will have more than 40 smaller stores and 40,000 square feet of retail space - about half the size of the area's principal regional malls.

The mall will be located on a 40-acre plot and is to be developed by Penney's real estate division and Melvin Simon and Associates of Indianapolis, a shopping center builder that recently developed Golden Ring Mall east of Baltimore.

Sewer connections and zoning changes for the development have been approved in the recent months. The Park and Planning Commission still must approve commercial design plans for the mall, including a proposal for more landscaping near the retail complex and less landscaping in parking lots than normally required.

In another retail development locally, many Sears, Roebuck & Co. stores have been opened for business on Sunday afternoons. Although most large retailers here have been open on Sundays in recent years, Sears previously had opened area branches on Sundays only for the month before Christmas.