Hechinger Co. stepped up its expansion plans yesterday, announcing it will open seven new stores this year, including one more in the Washington area and another in Baltimore.

The seven stores planned this year and seven more scheduled for 1984 will expand the Landover-based home-improvement-center chain from its present 34 outlets to 41 by year's end and to 48 a year later.

Six of the stores to open this year will be in buildings previously occupied by other retailers taken over by Hechinger. Remodeling will be completed in time for openings in the first half of the year. The seventh store, which will be built from scratch, is to open in the fall.

No locations were announced, but the company said this year's plans include one each in the Washington, Baltimore and Philadelphia markets and four in North Carolina. The 1984 additions will be in Washington, Baltimore and Philadelphia. Hechinger now has 16 Washington-area stores.

If the new stores match the $7-million-a-year average of existing Hechinger units, they will add nearly $50 million a year to the sales of the chain, which posted sales of $226 million for the 52-week period ended last July 31.

Hechinger raised $22 million in cash through a stock offering in 1980 and will use some of that money to finance the additional stores, Treasurer Clark McClelland said.

Since the stock offering, "we've been waiting for some real estate opportunities to come along," he said. "We're very excited about the locations we've found."

The company will spend about $500,000 apiece remodeling the stores it is taking over and will invest about $1.6 million in inventory, fixtures and working capital for each new store, he added. Each of the new units will follow the Hechinger pattern, with 60,000-square-foot buildings plus a 20,000-square-foot outdoor garden shop.