Despite little vacant land in Alexandria, new construction climbed 15 percent during the last 12 months, ranking the city fourth in Northern Virginia's building boom after Fairfax, Arlington and Prince William counties.

New office buildings rather than houses dominated the $202 million in construction during the last year, city officials said, accounting for more than three-quarters of the total.

"The bottom line is we have very little vacant land residentially zoned. Most people want to develop it as offices," said George Colyer, chief of comprehensive planning for Alexandria's Department of Planning and Community Development.

Although only 400 of the city's 10,000 acres, or 4 percent, are vacant, Alexandria attracts developers because it has what other sprawling Northern Virginia jurisdictions lack: the historic and geographic focal point of Old Town, city officials say.

"A major advantage of the city is that there's a clear definition to it. Certainly the waterfront is part of that. The core is Old Town," Colyer said.

"It has amenities that other jurisdictions don't have. It has the history, the preservation of classical architecture," said Alexandria Mayor James P. Moran.

About 200 houses, apartments or condominiums were planned by developers during the 12 months, said Uwe K. Hinz, acting building official for the Department of Public Safety's Code Enforcement Division. The department compiled the statistics based on building permits issued for the last fiscal year, July 1, 1984, to June 30, 1985.

Two of the city's most extensive office and retail projects that began in the last year are being built by Savage/Fogarty Companies Inc., an Alexandria developer. The $125 million TransPotomac Canal Center is now being built on a 10-acre waterfront site at the foot of First and Montgomery streets. About 504,000 square feet of office space will be divided among four office buildings with heights varying from four to seven stories.

As part of a special arrangement with the city, Savage/Fogarty will restore an old canal lock, lay out a four-acre waterfront park and provide 3,000 square feet for a city-run museum at the site. The TransPotomac Center is expected to be finished by next summer, a Savage/Fogarty spokesman said.

As in other Northern Virginia jurisdictions, the Metro stations will be catalysts for developments, Colyer said.

Braddock Place, a four-building $60 million complex next to the Braddock Road Metro Station, will provide 330,000 square feet of office and retail space when it's completed by 1986. The Public Broadcasting Service plans to move its headquarters and 250 employes into the largest building of Braddock Place in January.

Moran said that the city plans to encourage mixed-use development by granting variances that lift restrictions imposed on developers, such as providing for a certain number of parking spaces.

Although residential construction has averaged 200 units annually for the last several years, next year as many as 500 condominiums, apartments and town houses could be built if several development projects begin, Colyer said CAPTION: Picture, Flags fly above construction of the $60 million Braddock Place in Alexandria at the corner of West and Madison streets. BY VANESSA HILLIAN -- The Washington Post