The FBI, acting under the aegis of national security, has decided it cannot afford to liven up its fortress-like headquarters building with storefront boutiques.

It would simply be too dangerous, the bureau says.

The details are classified, but FBI Director William H. Webster said yesterday that there are very real hazzards involved, including "electronics penetration" and even the planting of explosives in the huge building's conduits.

It short, Webster told reporters, "It is not possible to have private operations conducted inside our walls . . . We'd like to be a good neighbor, but we've got a real serious security problem."

There are those, of course, who think that the mausoleum-like building Pennsylvania Avenue NW ought to be demolished, but more restrained arbiters of good taste, such as the Pennsylvania Avenue Development Corporation, have been pressing for months for a more moderate solution. They suggested that commercial shops could be installed along the block-long concrete wall facing the avenue.

The trouble is, Webster said, the stores would have to poke into the wall itself for electrical connections and all the other facilities that building codes require. They can't go straight down below the sidewalk either, he said, since "unfortunately our facilities go out under the sidewalk" as well.

But the bureau still has some ideas for gussying up the place that it presented to the development corporation. For instance, Webster said, the concrete wall might be touched up with decorative panels or glass-enclosed museum displays maintained by the Smithsonian Institution.

Webster had apparently meant to let the development corporation mull over the countersuggestions before making them public. He had architectural drawings and building models all carefully hidden under brown wrapping paper yesterday morning before starting a news conference to announce some top-level appointments.

But when reporters asked him what he was covering up, Webster said he had nothing to hide.

"I'm kind of excited by the idea," he said of the model displays. "This we could do right away."