Scott's Seaboard Corp. will acquire Frank's Hardware Inc., to become one of the Washington area's largest retailers of hardware, building materials and garden supplies.

Scott's Corp., a Scott's Seaboard subsidiary, under the agreement will acquire Frank's seven hardware stores in the Washington area. Combined with Scott's plans to open three new stores, the move will mean Scott's will have 18 retail outlets in the Baltimore-Washington area by the beginning of next year.

John W. Hechinger, president of Hechinger Co., said yesterday he doesn't fear the new, larger Scott's. "The whole is equal to the sum of the parts," said Hechinger, who heads the area's largest hardware chain with 20 stores in the Washington-Baltimore region. "They may change the name up on the sign, but they'll still be selling the same products."

"They aren't going to be any competition to Hechinger's" agreed Robert Chew, editor of the industry newsletter National Home Center News. Chew said that, based on Scott's published sales figures and estimates of Frank's sales for 1980, the two firms combined had $31.5 million in sales of hardware and home-improvement products.

Neither company would disclose the terms of the agreement between Scott's, whose shares are traded over the counter, and Frank's, a privately held company. A joint announcement said the deal is subject to several conditions outlined in the agreement. Completion of the transaction is expected on or before Sept. 11.

The companies also declined to discuss whether any changes in management or selling strategies were planned for the combined organization.

Frank's operates seven hardware stores in suburban Maryland and two in Northern Virginia. According to the estimates by Home Center News, the company had sales of $17 million last year, down from a peak of about $18 million in 1979.