The Department of Housing and Urban Development is sponsoring a series of workshops across the country to promote the use of its new rehabilitation guidelines to encourge preservation of existing buildings. The Washington-area session will be held next Thursday and Friday at the Sheraton Inn in Silver Spring: The cost is $40. For information, call 737-1460.

Earl A. Snyder, a Prince George's County appraiser who writes a syndicated question and answer column on realty investing, has compiled a new book based on his columns. The book, "Before You Invest -- Questions and Answers on Real Estate" (Reston Publishing Co. Inc., 211 pages, $12.95), describes the buying and selling process.

Rufus S. Lusk & Son has published an historic preservation map of Washington showing the historic districts of the city and a listing of desginated landmarks. The 22-by-34-inch map cost $4.95, plus $2 for postage and handling. Lusk's address is 1824 Jefferson Place NW, Washington 20036.

A seminar on Maryland's new condominium law will be held the morning of May 27 at the Bethesda Holiday Inn by the Washington Metropolitan Chapter of the Community Associations Institute. For information about fees, call 569-2147.

Fairfax County is sponsoring a seminar for prospective home buyers in the county from 7 to 9:30 p.m. Tuesday in the aauditorium of the William Cullen Bryant Intermediate School, 2709 Popkins Lane, Mount Vernon.The seminar will cover site and construction considerations and techniques, with special emphasis on contracts, financing and the settlement process. Walk-through inspections and warranties also will be explored.

Tenants of a 46-unit apartment building at 1365 Kennedy St. NW have purchased the structure for $385,000 with help from the city's department of housing and community development. It is being converted to a low-yield co-op.

A seminar on energy efficiency for real estate agents will be held at 9 a.m. Thursday at the Tyson's Corner Hoiday Inn under the sponsorship of the Northern Virginia Board of Realtors.