JUST AS the original rowhouse builders of Capitol Hill had their favorite design elements, so do the new remodelers.Back then, the designers were intent on varying the streetscape with fanciful roofs and window trimmings. Today's designers, rightly thinking that they could hardly improve on the old folks (except in the matter of doors, street numbers and entryway lamps), concentrate on the inside.

The original two-story Capitol Hill rowhouse had an entryway with a staircase, and, on the other side of the hall, three rooms in a row - the living room at the front so one could watch the fire engines go down the street, the dinning room stuck in the middle, and the kitchen in the back with windows so one could check up on the goat and the chickens and see if the laundry needed to be brought in out of the train, and a back door for deliveries at the rear. Upstairs were three rooms and a bath.

The house was cheap, practical and reasonably pleasant, because the front of the house often jutted out to make a bay and let in light on three sides both to the master bedroom upstairs and the living room on the main floor. The party walls made the house easy to heat and build.

But the houses had a big problem - the center of the house. The dining room and middle bedroom usually had only a single window onto a dark and dismal light shaft generally only suitable for hiding the garbage can.

The answer to this problem for remodelers has become ubiquitous - take out the floor and two or three walls of the middle bedroom and install a skylight above, thus making a central two-story high atrium to flood the house with light. It has become the standard usage because nothing else is so simple and effective. True you lose a bedroom, but you gain a greenhouse. For couples and small families the trade is worth it.

Three recently finished rowhouses (or almost finished - work was thick and heavy last week as the homeowners tried to do just one more thing before the annual Capitol Hill house tour next Sunday) all have the central sky-light-topped light shaft. But all have floor plans which are varied to fit the needs of the occupants. That's the great charm of remodeling - choosing the design and finish to suit you own tastes. All three houses are in the new area of Capitol Hill remodeling with its smaller, simpler, two-story houses in the neighborhood of 12th and 15th streets SE.

Patrick McCabe, a U.S. Postal Service photographer, lives at 12th and C streets SE. McCabe says he managed to redo his house with the help of a great many people, some of whom worked with him after hours and weekends after they'd finished their regular jobs, including Jon Irwin, an architect with the Justice Department.

"We were all learning - especially me," says McCabe.

Irwin's plan called for a seating area just inside the front door (furnished now with old-fashioned ceiling fan and an upright piano) and a staircase going straight up, as the old one once did. But where the middle bedroom would have been, he designed a large two-story atrium, topped with a 6-by-6 double insulated dome skylight with a bronze coating.

On the first floor, the dining area is just under the natural spotlight. Behind it is a kitchen, with the kitchen cabinet serving as divider. (Besides his kitchen equipment, the cabinets hold his building equipment.)

McCabe says Irwin's principal stroke of genius was to incorporate the old light shaft at the back into the building. The original outside L-shaped brickwalls now are inside and their original windows make doorways. The tops of the doorways were arched. The new space serves as a hall along side the kitchen, for access to the back sitting room, and a bicycle garage. Everybody on Capitol Hill, it seems, owns bicycle or two.

The sitting room is a completely new addition, about 18 feet deep. The city allowed the addition because the old house had not used the allowed 60 percent of the lot. Upstairs are two bedrooms and two baths, separated by the atrium. The stairwell is rimmed with a good-looking planter serving as a rail. Flowers flourish under the skylight.

McCabe had owned a rowhouse before, but it was one he'd bought already remodeled. His current house had sat empty for five years in an estate settlement. He and a friend, Fred Griffin, found it while scouring The Hill on bicycles. Finally the house was put on for auction in a judge's office.And there sat a big developer whom McCabe was sure was going to outbid him. But the procedure dragged on and on, McCabe said, and the developer had other deals to make, leaving McCabe to pick up the house for $29,000 in April of 1977. A shell in the neighborhood recently sold for $36,000.

After Irwin had worked out the design for him, McCabe set to work as his own general contractor, not knowing a soul in the business. Work was delayed by three weeks when he didn't know he had to have a ground plat to get the building permits. "If it hadn't been for Ernest Pifer, chief of the building permit office, I would have never made it through. But he walked me around and told me what I needed."

He found Benny Boykin, the brick-layer, on a Sunday when Boykin had brought along his son to work with him on a house near where McCabe was living. "I thought anybody who was that dedicated might be the man I wanted." Boykin did two arched brick fireplaces and the arches to the hall and kitchen and the complicated circular brick patio. He usually worked from 5 to 10 p.m. - usually after his regular day of work.

David Seagraves, a Mount Pleasant cabinetmaker, the son of a friend, did oak cabinets for the kitchen and the bathrooms. Ed Roach did the beveled windowsills, along with other carpentry. McCabe calls John Runyan the "electric carpenter" because he had an electric tool for everything, including one for hammering in nails. Runyan told him, when the floor joists turned out to have a distinct slope, "think of it as a war story. You'll enjoy telling it some day." Time has eased the thought of the $400 it cost to set it right.

McCabe says professionals such as Otto Seidel, the plumber, gave him good advice. Why not put new pipe all the way to the city hookup. Seidel suggested. Costs more now. Saves much grief later, when the old cast iron pipe might spring a leak.

Howard Nolan, the sheetrock man, pointed out that most noise spreads between rowhouses at the floor joist level. So McCabe spent most of one night stuffing insulation in the cracks so Nolan could finish the drywalling the next morning. William Tarant, the sheet-metal man, made trays for the planter box that rims round the light well, as well as the circular hood which makes such a feature of the kitchen.

McCabe, in spite of the horror stories he'd heard, even liked his city building inspector. Shirley Austin, who turned out to be a marvel at coming when he was needed, as well as helping with suggestions.

McCabe did some of the work himself, and Griffin, a college recruiter, did the painting and helped with bright ideas. At the end, when he moved in last October, McCabe figured he spent $55,000 for the labor and materials on the house.

Besides McCabe, James Enloe, an architect with the Air Force, cualifies as a remodeling professional. He has "done," as they on The Hill, three houses and he's looking around for another, not to count all those he's designed for others to do. Two of his houses are right beside each other on 15th Street SE. He and a partner (now bought out) paid $28,000 for each of the houses in December of 1975. The remodeling came to about $40,000, not counting his own work in the design and some of the demolition and painting.

Enloe lives in one. Sally Coler, a partner in the Tile Gallery (which seems to have sold tile to at least half the Capitol Hill remodelers), lives next door in the other.

The two houses are the standards size for the area: 16 feet wide, 46 feet long (with a four-foot bay at the back in Enloe's case). The front doors are set in a few feet, making a bay at the front of the houses. In the back, both have left the L-shaped light shaft to serve as entryways to the rear garden. The downstairs of both houses are essentially one open room, with cabinets used as dividers when necessary.

Both houses, like McCabe's have removed the middle bedroom floor and two walls to make a two-story high skylighted central atrium. Enloe's is slanted Plexiglas sheets, facing west. Coler's is a doomed skylight bringing in east and west light. Both highlight the dining area below and give light to the upstairs balconies.

Enloe used his street side bay for a tiny, sunny breakfast room. His kitchen is a long counter running parallel with the staircase. A divider, pantry on the kitchen side, coat closet on the other side, separates kitchen from passageway. At the back of the house, with a door to the garden, is his living room, up a step from the dining area. When he heard he was going to be on the Capitol Hill house tour, he was seized with a madness, pushed out the back wall by four feet, to make a glass-roofed bay for the fireplace.

In Coler's house, to suit her tastes, he put the living room at the front of the house, with the fireplace in the north party wall, and the bay used as a sitting alcove of the living room.

"I'm not that into kitchens that I wanted it to be the first thing you see," she said. That left a clear sweep, which makes the house seem larger. Her big square kitchen occupies all of the rear section of the house, except for the hunk out for the traditional light-shaft corridor, here serving as the passageway to the deck.

Upstairs is pretty much like Enloe's with Coler using her balcony overlooking the light well for a sitting room, and Enloe for a study. But after seeing hers, he's thinking about refurnishing it for a sitting room as well.

Both have two bedrooms, separated by the balconies and light wells, with baths off the front one. Enloe's back bedroom overlooking the garden, is currently equipped with several pieces of stained glass for the next house and a long ladder. Coler's has white ruffled counterpanes and an antique doll that belonged to her aunt.

Both have spacious master bedrooms - Enloe has a great brass bed and a sunny spot for the cats by the east windows. Coler uses her bedroom bay for plants. And she has a big-blade electrical fan, similar to McCabe's (installed by Enloe - he hopes it won't fall but he wouldn't sleep under it) over the bed.

Both Enloe and Coler have those splendid handmade Mexican files - $1.50 a quare foot, about $5 to $6 with materials installed. Matter of fact, that's how Enloe sold Coler the house - he bought the tiles at her Tile Gallery.

Both also have custom-made stained-glass windows in their bedrooms (looking out on the atrium). Enloe commissioned his with an arched top. Coler made her own, as well as a handsome art-nouveau stained-glass screen which hangs between dining area and kitchen. Enloe uses large tree-size plants to screen his kitchen from the dining section.

The Enloe/Coler gardens are separate but equally nice. Hers has a deck adjacent to the house, with a lattice screen at the back. His has an octagonal deck in the middle of the garden, with a sewer pipe as a flower holder set in the middle. Both have fine solid wood fences with the boards nailed on the diagonal. Both have a place to park at back secured with gates that slide on a track.

These three houses and seven more equally personal will be open from 1 to 6 p.m. next Sunday during the 21st annual Capitol Hill House and Garden Tour. The tour starts at Eastern Market, 7th Street at North Carolina Avenue SE. Tickets are $7 in advance from Capitol Hill shops or at the Capitol Hill Restoration Society, Box 9064, Washington, D.C. 20003 (Telephone 546-6520), $8 at the starting point the day of the tour. The tour benefits a defense fund aimed at keeping the single-family residential character of the area.