The Seminary Plaza Professional Buidling, a 12-story, 10-year-old structure at Kenmore Drive and Seminary Road in Alexandria, has been sold for $4.75 million to SPB Limited Partnership by Seminary Plaza Professional Building Limited Partnership. Principals in the new ownership are Louis Donatelli, Roger Klein and Steven Goldberg. Darryl Wright and James Kibbe of Weaver Bros. handled the transaction.

O.F. Smith & Bros. Co., a building firm in Montgomery County for 30 years, is being disbanded. The firm has sold its Montgomery Building in Bethesda and adjacent land for $4 million to a group of local investors. The new owners plan to develop another building on part of the site of 4720 Montgomery Ave. The transaction was handled by broker Walter Propps. The Smith firm earlier built houses in Somerset, Fort Sumner Hills, Bedford Village and Willerburn Acres.

A class in how to sell houses with or without a broker will be conducted May 20 at the Key Bridge Mariott by the Consumer Information Institute. The class covers financing, contracts, listings, brokerage and other subjects. For information, call 649-6666.

Sales at the Colonies condominium at McLean, which now is being handled by Development Management Corp., earlier this week totaled 209 of a total of 287 offered units. A grand opening was held April 15 for the development, which is located inside the Beltway at Tysons Corner.

C. A. Paterson Realty Ltd. has opened for business at 4200 Wisconsin Ave. NW and plans to handle property in the Georgetown area. The firm said it is charging 4 percent commissions on sales, below the usual level in this area.

Broker Eugene Krebs, whose Riverdale real estate office is affiliated with the Realty World franchise, now has three daughters working with him. So Krebs recently changed the name of the firm to Krebs and Daughters. The young women are Barbara, Lorraine and Melissa Krebs.

The National Home Improvement Council has opened an office here in the National Press Building E. Bruce Harrison is Washington representative of the trade association, which represents the home improvement industry. The main office is in New York City.

The Carey Winston Co. has been named exclusive leasing and managing agent for the Foundry building at 1055 Thomas Jefferson St. NW in Georgetown. The multi-use, 211,500-square-foot building on the C&O Canal was opened in the fall of 1975. Only a relatively small amount of retail space is unleased. A subsidiary of Inland Steel developed the contemporary building.

Real Estate broker Sidney Spiegel has moved his office from the District to the Perpetual Building at 87800 Georgia Ave., Silver Spring.

Town houses priced from $45,300 are being offered by Lobo Corp. on Forest Drive in Annapolis. Designed and built by L. Edward Wolf, the Greenbriar town houses have spiltfoyer entrances and three bedrooms. Three hundred dwellings are planned for the 30-acre site.

Fairwinds of Annapolis, a complex of 388 town houses and apartments has been purchased by Walter J. Hodge and Stephen G. Yeonas, who are converting the project from rental occupancy to condominium ownership. Current residents are being givern the right of the first refusal to buy under a rent-purchase plan, "as is" or with renovations.