ARCHITECT Don Hawkins took another look at the house he'd designed for Leslie and Robert Riggs the other day. "I clapped my hand to my forehead and said, 'My Gosh, I copied it from the Executive Office Building.'"

Not everybody thinks the Riggs' house looks like the EOB. Other people think it looks like a waterfall.

Hardly anyone on Capitol Hill who has seen it lacks an opinion of the Riggs house. It's quite a novelty there, where most houses are new only on the inside. The Riggs house is new both in its contemporary design and in its ground-up construction. Yet the three-story (and English basement), 3,300 square-foot house cost about $100,000. That's $35 a square foot, less than some remodelings of the same size. Many new houses today cost $65 a square foot. But then the Riggs did their own contracting.

The waterfall effect of the house's facade comes from the bay windows which Hawkins said "spill over from the inside in a cascade." The roof has an unglazed slit at the top, to make a light well for the deeply recessed top floor windows. Each of the three successive bays steps further out. The basement bay (roofed with glass for a greenhouse effect) sticks out most of all. The first- and second-floor bay roofs are covered with a gray-blue tin to heighten the waterfall effect.

The stepped effect helps the windows bring in more light, especially since the front of the house faces due south. "Architecture is made of light, and something else." said Hawkins. "We want our house to be flooded with light," said the Riggs, who had lived for eight yeats in a dark Victorian house. That was easier on paper than on the 20-foot-wide Capitol Hill lot. Only the front, back and roof could have openings to the light. The side walls, of course, were party to the adjoining row houses.

The Riggs worked with Hawkins on four different designs - one a very simple, inexpensive plan, another quite complicated. Hawkins did schematics so they could figure the different costs on each plan. "We chose just the right one - not the most expensive, not the plainest," said Leslie Riggs.

"We took the plan that concentrated the highest ceiling in the rooms where we'd be the most time - the living room, the study and our bedroom. Don Hawkins' plan helped us have two-story ceilings in these spaces, without loosing the whole center of the house, as some of the Capitol Hill houses do."

Many remodelings on the Hill remove the middle bedroom on the second floor, to bring in light in the middle. Hawkins' plan was much more complicated - the living room is sunk into the ground to gain extra height. (An apartment in the basement is only under the front two-thirds of the house.) The study and master bedroom ceilings go up to the pitched roof line to gain height.

A staircase disguised as a space scuplture, topped with a skylight, acts as a lightwell for the whole house. "Bob Norris, our carpenter who did just about everything on the house, told us he expected the staircase to take him forever. He had left it as a gaping hole until the rest of the house was almost finished. Actually after he got started, he did it in just a few days.It looks complicated, but it isn't," explained Riggs.

Like most row houses, you enter on the piano nobile , the main floor, raised above the street. The rental unit with its greenhouse is tucked in what is called locally an English basement.

Through wooden front door, you come into the front hall. (Note the dark fiber mat set flush with the floor so you don't trip over it but have to walk across it.) The dining room is to the left, with good-looking vertical blinds which look like handwoven Colombian hangings. Ethel Litman, an interior designer, found them for the Riggs and helped with other decorating matters.

Beyond is the enormous kitchen. "We kept saying cut down on everything before the kitchen," said Riggs. The kitchen is actually a balcony, overlooking the lower-level living room. The half wall hides the clutter but allows the cook to talk to the company, "though that disembodied voice from above is rather eerie," she said. The floor is covered with handsome patterned Italian tiles from Dennis Tile Co., an accent against the all-white kitchen cabinets and appliances. The cabinets are from Miller's Building Supply.

"Because we both cook, I wanted extra burners," said Riggs. "But we would have bumped into each other if we'd take a standard six burner unit. So we bought one four-burner unit and one two, and put them at opposite ends of the kitchen." On one wall are their collection of Mexican plates from Tonala, where each potter has an animal representing his signature.

The two-story-high living room is a generous 19 1/2 by 15 feet, with a fireplace and a built-in wood box working together to make a rectangular black accent. French doors on either side lead to the terrace. Large square fixed-glass areas above bring in the steady north light.

On the next level is a comfortable library with its own fireplace. Two bedrooms and a bath are on the same floor. One up is a huge master bedroom which shares a skylight with the bath on the top floor, to the pleasure of the African violets.

"The house has wonderful storage, including this cabinet with the lock for medicines." said Riggs. "But nothing is as necessary as the laundry chute - it goes from our floor down to the laundry room on the children's floor."

The Riggs decide to build a totally new house after the prices for shells went up so high, leaving, for the moment, the price of vacant lots behind. "When the great Capitol Hill revival began, many of the buyers were amateurs," explained Riggs. "So they felt more comfortable remodeling rather than building from scratch. They were willing to pay more for a shell with at least rafters and some floors. Today, the cost of even four walls and some roof beams has gone so high, that now vacant lots are beginning to go way up."

The Riggs - she's an agent with BW Real Estate, he's an economist with the Office of Management and the Budget - bought their lot in 1975 for what seems now to be a bargain $30,000. Similar size lots now sell for about $75,000. It's not far from the Capitol. They think now they could sell house and lot for twice what they have in it.

They were veterans of three previous remodelings. The sales of which had, thank you, been profitable. "We loved our last house, a real gingerbready Victorian, but we never got as much light as I wanted," Riggs said.

The Riggs acted as their own general contractors, saving themselves from 10 to 20 percent, since they knew what they were doing. Leslie Riggs took care of purchasing and of evaluating the bids. Robert Riggs supervised the work.

"We were really pleased with most of our craftspeople," Leslie Riggs said. "Bob Norris, who did all sorts of jobs, was wonderful, Heinie Zimmet, the stucco man, did it the hard way - first a mesh and then the hand-applied stucco on the cinder block. Harold Willet, our tile man, was great."

The house is heated and cooled with a Carrier heat pump. The largest bill so far was $140 in the coldest month and $50 in the summer, thanks to heavy insulation and the house's north/south orientation. The house's many windows make cross ventilation work.

The Riggs moved into the house just 10 days before the baby was due. Obviously, the house had been carefully calculated - in the Riggs' precise way - to accomodate themselves, their 4 1/2-year-old Adam and the expected arrival. Only the new one was two. The twins, Jason and Sarah, came as the total surprise, not only to the new parents but to the doctor.

For a baby present, Hawkins sent them a sketch of the house, adding an extra floor on the top. On the bottom he wrote, "Congratulations. Now here's my plan. . . ."