Office building developer Lowell E. Baier figures he has a way to beat the current slowdown in the Washington-area real estate market: Offer his tenants a touch of Europe in the 14th century.

Baier Properties Inc. of Rockville has completed the 220,000-square-foot, Gothic-inspired One Cambridge Court office building in Merrifield, a changing commercial area of Fairfax County near the intersection of the Capital Beltway and Route 50.

The area once was mostly known for its ugly collection of warehouses -- and there still are some left. But the Beltway-Route 50 area now also includes Mobil Oil Corp.'s headquarters, the building bought by the American Red Cross for its main office, the Fairview Park office and hotel complex, and other major office buildings.

"We can't wait for Merrifield to urbanize," Baier said this week as he toured the six-story, twin-towered Cambridge Court property, which he called a "cathedral of commerce."

The $50 million project includes architectural touches reminiscent of the early Gothic period of the 14th and 15th centuries, including a glass-spired atrium, a lobby information desk that looks like a cathedral pulpit, a parking garage resembling a cloister, a carillon and lobby murals copied from the "Book of Hours of the Duke of Berry" (1487) depicting the various seasons of the year at the duke's chateaux.

"It's a trophy project, a downtown office building in the suburbs," Baier said. "It's a tough time, but we don't compromise on quality. I've been through three downturns {in 1969, 1973 through 1975 and 1981}. We've been here before and we'll get through and life will go on."

But at the moment, only 10,000 square feet of the space at One Cambridge Court is leased. Baier, however, said he has two large prospective tenants and that if both lease space, the property will be 75 percent filled.

George Washington never had it so good. Certainly, Mount Vernon, his mansion on the Potomac River, is a beacon of American architectural splendor, but did it have a Jenn-air range? Could he play tennis in his back yard or relax in a whirlpool tub in the house?

Roblyn Development Corp., a New Jersey real estate company, is planning to blend the architectural "testimony to American heritage" with modern household amenities. The result, developer Merrick Wilson said, will be a collection of 29 mansions in Millstone, N.J., that will replicate the homes of 29 former presidents.

Wilson said no president's home has been ruled out for replication, but noted that some lived in such simple abodes that the latter-day version of their houses would not fit in the 115-acre development, located about 15 miles from Princeton.

"We've identified at least 29 homes that are stately and grand enough to fit into the neighborhood," Wilson said.

The developer said the three-acre plots of woods and blueberry bushes will provide room for private tennis courts and swimming pools, and will include "unique" replicas of such presidential mansions as Thomas Jefferson's Monticello, Franklin D. Roosevelt's Hyde Park, Washington's Mount Vernon and Andrew Jackson's Hermitage, but not Abraham Lincoln's log cabin. A replica of the Woodlawn mansion, once part of Mount Vernon, in Fairfax County has been completed and is on the market for $559,900.

While the exteriors will assume the architecture of the originals, the mansions' interiors will be custom designed in line with the tastes of the buyer. The houses will vary in size up from 4,800 square feet and will be priced from $529,900.

IN THE BUSINESS ... Luxury home sales were up in the Washington area from 1988 to 1989, but declined slightly in the first quarter of this year, according to a report by the Rufus S. Lusk & Son Inc. real estate information service. In the price range between $400,000 to $1 million, 3,342 units sold in 1989, up 16.7 percent from 2,864 in 1988. In the first quarter of 1990, however, 600 home sales in that price range were sold, down 6.7 percent from 643 during the first three months of 1989. Sales of houses costing more than $1 million increased 10.9 percent to 275 in 1989 from 248 in 1988. During the first quarter of 1990, 42 homes costing more than $1 million sold, compared with 51 during the same period of 1989 ... More than 50 real estate agents quit the Montgomery County offices of Prudential Preferred Properties and were hired by new branches of Long & Foster Real Estate in Potomac and Bethesda ... An exhibition of "New Chicago Architecture" will open Wednesday at Union Station. Organized by the Chicago Athenaeum: the Center for Architecture, Art and Urban Studies, the exhibition highlights the designs of more than 60 new or planned Chicago buildings, including the Harold Washington Public Library and the Miglin-Beitler Tower, which, when built, will be taller than the Sears Tower ... The Maryland Housing Research Corp. began construction of Rivera Heights Addition in Severn as an example of bringing the state, real estate and business communities together to provide affordable housing. Realty companies "adopt" the homes and donate materials, such as carpet, appliances and paint, to complete them. The 14 homes, in the Baltimore Washington International Airport noise zone, are worth about $115,000, but will sell for between $48,000 and $92,000 ... Donohoe Construction Co. completed Portsmouth Station, a 141,000-square-foot shopping center in Manassas ... The Council of American Homeowners, a nonprofit lobbying group in McLean, was formed to represent the nation's 70 million homeowners. Charles F. Haywood was named president of the new group ... Samaritan Inns, a nonprofit housing organization, raised close to $2 million for a $3.7 million project to build Lazarus House, which will house 80 homeless people at 14th and Euclid streets NW ... The overall mortgage delinquency rate for one- to four-unit residential properties was 4.41 percent in the first quarter of this year, down from 4.96 percent in the fourth quarter of 1989, the Mortgage Bankers Association of America reported ... F. Daniel Cathers & Associates Inc., an architectural, interior design and civil engineering firm in Columbia, Md., changed its name to Cathers & Associates Inc., and expanded its services to include space planning, civil engineering, planning, landscape architecture and graphic design ... WJD Associates, an Alexandria commercial real estate firm, plans to start a 24-hour hot line with information on listings, mortgage rates and general industry tips . . The Far Southeast Community Organization has scheduled a free Landlord Tenant Affairs workshop for Thursday at 2041 Martin L. King Ave. SE at 6:30 p.m. ... The Landscape Contractors Association of Rockville plans to hold a seminar on financing Wednesday from 8 a.m. to 4 p.m. at the Greenbelt Marriott. With lunch, the program costs $250 for nonmembers.

PERSONNEL FILE ... The Maryland Chapter of the American Society of Appraisers named Robert G. Poinsett president for the next year ... Equitable Real Estate Investment Management Inc. named Lawrence B. Dendtler head of its Washington office.