The National Geographic Society, with 85,000 square feet of space to rent out in its new building on M Street NW, is offering prospective tenants a little something extra if they'll move in.

The society will have a special firm design the newly rented office space to suit the individual tenant.

"It is a way to compete with a lot of the office buildings in town, and in this town there are a lot of office buildings," said Ward S. Phelps, assistant vice president of The National Geographic Society. "You have to have an edge, and this seems to be a very appealing one."

The idea also has occurred to other landlords. Indeed, office interior designers say business is booming as tenants go bargain-hunting across the city and landlords cast about for sweeteners that can make the difference in swinging a lease deal.

"I would say this is a definite trend," said Arthur Auerbach, president of Design for Business Interiors. He said several of his clients are building owners who have given their new tenants a lump sum with which to design their new office space.

"We're seeing more and more of this," said Auerbach. "Tenants would rather have a design firm tailor space to their needs than be at the mercy of the building owner contractors."

Building owners and designers say the deal works two ways. In some cases, the building owner allows a certain amount of money for office design. The tenant then goes out and hires the design firm of his or her choice.

If the cost of designing the office is higher than the amount allotted, then the tenant picks up the extra tab.

Other building owners retain their own design firms which work with the tenant directly. Again, if the cost of designing the office exceeds a certain limit, then the tenant pays the difference.

Building owners are reluctant to say how much they allow for office design, for the obvious reason that, as one designer put it, "If prospective tenants know what deal you've made with Tenant A, then you're in trouble."

However, they say that, with a glut of office space on the market, the cost of an interior designer is less than the cost of empty office space, and what a tenant can get is what he or she can negotiate.

The interior designers boast that they are an excellent drawing card for a building because most tenants have a long list of specifications for their new offices.

"Designing office space has become extremely complex," said Maree Simmons-Forbes, whose Friday Design Group Associated is working on The National Geographic Society M Street building. "Traffic flow, computers, air conditioning, lighting--all have to be taken into account."

Simmons-Forbes said that her business is up 35 percent over this time last year and that several of her clients are building owners. "August is usually a slow month," she said. "Business has really picked up, and much of it is due to this new practice."

The designers say they usually enter the scene before the new office building is even finished--working with the architects and new tenants to decide where the walls go and how many electrical outlets are needed.

"Clients are a lot more sophisticated about what they expect from their office space," said Auerbach. "They have very specific needs and cannot afford to just move into any empty space."

Designers say their clients often need such special features as a security station or computer room. Clients may need extra electrical outlets or air conditioning units for their computers or special lighting features for their workers.

Not only do individually designed offices draw tenants to a new building, they keep them there longer, say building owners and designers.

"Tenants are willing to sign a longer lease because they have put something into the building," said Phelps. "That's the real importance" of individually designed offices.

Auerbach agreed, saying many of the tenants he is working with are planning to stay in their rented quarters for a substantial amount of time.

"They are willing to invest more money in someone else's building if they have a long-term lease," he said. "Many are spending some of their own money on these new offices and want to receive something long term from their investment."

The National Geographic Society's new building at 1600 M St. NW is almost completed, and most of the three top stories that are up for rent already have been spoken for by tenants who are working with the society's interior design firm.

"We like to have special features that make our building unique," said Phelps. "We have a nice location, beautiful terraces--many buildings in the city are beautiful. We also have a good design firm to suit the building to the tenant."