MARK KELLY had his dream on Christmas night, 2 1/2 years ago. He saw a vast mansion with turrets and verandahs and gazebos and long rows of windows, half-hidden in the trees by the water, like some long-vanished Adirondack resort hotel. He stayed awake the rest of the night, in the glow of that wonderful image.

"Soon as it was daylight I went right out to Dart Drug and bought every toothpick they had in the place. And Elmer's Glue. And I started."

The mansion sits in the middle of his small, dark, cluttered living room over the Turf Club bar on the strip in Laurel. It has maybe 500 rooms, including a working greenhouse with air ferns behind glass, a powered windmill, a waterwheel that runs, and a lighthouse with a tiny winking light at the top.

The dream weighs 60 pounds and covers a coffee table. It has 6,572,000 toothpicks, held together with nearly two gallons of Elmer's Glue.

"I buy toothpicks by the case. And they've gone up from 20 cents to 50 cents a box. I've gone through five pairs of nail clippers. My mother says, 'Will you shut them nail clippers up?' I work maybe 14 hours a day on it. Except when I go to my job."

For the past nine years Kelly has worked at the nearby Tastee Diner, as waiter, busboy, whatever he's needed for.

"I don't eat the food there. When I come home around 10 I'll work till 6 a.m. on this."

It will be finished in 1990, he thinks. He takes no measurements, draws no plans. It is all there in his head.

Kelly began building with toothpicks when he was 13 and recovering from an accident that left his legs temporarily paralyzed. He has made dozens of smaller buildings and given them away. He has made other things, like a miniature stone fireplace and chimney that draws fire when you put a cigarette in it. His picture of a sailing yacht is done in shards of mirror on red cloth, a striking vision that evokes the glitter of the sea in streaks of glass.

But the mansion is his life now. He has been offered $11,000 for it. "I didn't make it to sell, it's something I had a dream about." While he works he listens to classical music. There are 14 speakers ranged about the small room.

"I studied ballet for seven years," he says. "People say I'm hyper: I walk a lot, I walk very fast. But I don't go partying. I have 12 brothers and sisters, and they want to go partying, and they couldn't understand what I did in the house all the time. Then I had the mansion out to show it and they said, 'Now we know what you do.' "

Once the apartment house caught fire, and when Kelly carried out his creation "the firemen put down their hoses and all crowded around to look."

He still visits the family place in Winchester, Va. His father had a machine shop there, and "made a lot of money with an invention for electric hospital beds."

Sometimes Mark Kelly drives to Florida or New England with friends. He collects coral and laser paintings of seashores and sunsets and forests. The miniature trees and rocks around his mansion come from many different states. He doesn't like to hang out at the shopping centers nearby. He doesn't like the plastic things for sale there, the fast-food places, the raucous bars, the grimy repair shops, the highway outside his door.

"I just come in and look at my building," he says, "and it cheers me up.