GARDENING IN the sky was new to Patty Cox and her husband John Butler.

But last year they managed to have dwarf cantaloupes, cucumbers, corn, even water melons on the deck of their penthouse, four floors above Lincoln Park. The plants were safe from most predators except, perhaps, low-flying airplanes.

This winter, they gardened in the two-story atrium of Their apartment-green foliage but not, so far, green house vegetables.

Their penthouse, across the hall from the one belonging to Nobu and Carol Akiyama, is very different. When you walk into the Cox/Butler's and look up, the effect is as starting as being hit over the head with a split-leaff philodendron (and you may very well be, if you're not careful).

Originally, a light shaft ran from the roof all the way down to what had been the drugstore on the first floor. Architect Robert A.Bell, when he remodeled the building, incorporated the light well's space into the apartments on each floor. Since Cox and Butler have the top floors, they got the skylight; the atrium thus created is two stories high. Bell covered the floor of the small area in marble tiles. The balcony above has arched openings looking down. Plaster columns (cast especially) hold up the first floor arches.

Cox and Butler have filled the atrium with a vast collection of ferns and vines, which sometimes seem ready to entwine the visitor and hold him captive-not that it wouldn't be a pleasant place to be held captive.

Against one wall, they have a handsome old wardrobe. "No, it's not a place to hang your coat up," said Cox.

Instead, it's gardening shed. "We keep our hose, hoe-all the equipment in it," said Cox.

Nothing is quite what it seems. A door at the end of the living room, which might also be a coat closet, is instead the utility closet-gas furnace (each apartment furnishes its own heat and air conditioning), Whirlpool stacking washer and dryer. ("I was surprised, but they are big enough," Cox said). Another discreet door is the bar, a help for the living room when you consider the kitchen is up a full flights of stairs.

When you look up in the attium, you want to go up, too. As with the Akiyamas across the way, the kitchen, dining room and roof deck are on the top of the building, where they only attic used to be.

From the roof, you get a great view not only of the city, but of Bell's hand-applied gold leaf finial or pinnacle.

Cox worked with Jack Chase to make the trellis where she hopes wisteria will flourish. So far this year the plants on the deck are only on loan from inside. But on the kitchen counter are all sorts of small beginnings which Cox hopes will rise sky-high this summer. With all the potting and wattering going on, they're very glad of the quarry tile floor and ceramic tops in the kitchen.

"First we put down big wooden boxes full of soil last year. Only to find that the fertilizer and everything burned holes in the tin roof underneath, leaving us with a leaky ceiling. So this year, we've made wood decking to fit to set the flats and pots on," Cox said.

Several home stores and lumber yards offer modular decking, but the Cox/Butler deck, like the house, doesn't fit into anyone's prefab materials.

There is a waist-high railing all around the deck, which Cox says "gives us a feeling of shelter and security. We don't mind standing up to see the view." Fortunately, the deck primarily has south and east exposures for maximum sun.

A few steps up in the dining room lead to the arched door of the deck. The steps currently serve as plant stands. The trestle dining table table and country-style chairs were handmade in Waltham,Mass.

"Handwork, animals and plants seem to be our style," said Cox. "My grandfather is a woodcarver. So for several years I have worked with the Christmas Store in Cambridge, Mass., Selling his reindeer and other carvings."

Cox and Butler have crafts of many periods and many countries-a Victorian Morris chair, a Japanese chest, a marquetry chair, a Haitian tin sculpture, to name a few. In the bedroom, a group of architectural drawings of roofs seems quite appropriate. Hannah, a dog who seems to have studied at Mr.Peacham's begging school in the Three Penny Opera, spends most of her day cuddled up to a stuffed sheep in a wing chair in the bedroom. There's an identical wing chair in the guest bedroom, but the cat had rather dispute Hannah for the one in the master bedroom. Cox and Butler don't stand a chance.

As does the other penthouse apartment, Cox and Butlerhs has a splendid 30 foot-high corner room. They use it as a study. Cox, the owner, and Chase, the cabinetmaker, built the custom-fitted cabinets in the room to house things such as both the big two-volume Oxford Dictionary and the Webster International unabridged.

Cox and Butler-she's 30, he's 36-moved to Washington from Boston just over a year or so ago. He is study director of child development at the national Academy of Science. In Boston they had a fine house on the south side of Beacon Hill with a garden. When they moved to Washington they found that "a rowhouse here half the size cost a third again as much," as Cox said.

She is research associate on an Office of Education project. They knew Bell's brother, who took them by to admire the project. "At that time both penthouses were sold, but not everybody can visualize a half-finished project. When the other owners backed out, we rushed in," Cox said. They paid $112,000 a year ago, but of course the apartment building has gone up considerably from there. CAPTION: Picture 1 And 2, High atop Robert Bell's remodeled building, which faces Lincoln Park, are the two decks of the pent-houses belonging to Patty Cox and John Butler (top left) and Nobuo and Carol Akiyama. Bell used the octagons and polygons of the old Victorian building to create alcoves and rooms with high ceilings and interesting shapes.; Illustration, no caption, Illustration by Architect Robert A.Bell; Picture 3, At left is Patty Cox in a skylight atrium; Picture 4, below is Carol Akiyama in her dining room with stairs leading to the dock; Picture 5, and at right is the Akiyama's bedroom with 30-foot-high ceiling. Photographs by Harry Naltehayan-The Washington Post