HELEN AND Howard Koss's remodeling story is instructive if only because so many other people would like to do the same thing. The Kosses remained in the house they love but changed in the years when they didn't have the money to do anything about them.

In 1951 when the couple bought their house in Hammondwoods, a Silver Spring suburb, one of the things that attracted them was the way developers Paul Berman and Paul Hamond and architect Charles Goodman had sited the house on a hillside, sparing all the big old trees. Each house was not lined up with the street but angled for the best exposure.

The like Godman's design-clesn, low contemporary lines with a steep, pitched roof giving a feeling of both shelter and space-and the many tall windows. The Goodman design was almost a prototype '50s contemporary house, not modern enough to scare anybody, but totally shorn of the doodads many houses up to that time. Their house, alone with Goodman designs in Hollin Hills and houses by Keys, Lethbridge and Condon in Holmes Run Acres, were among the few contemporary ones being built then in middle-price developments.

The price was $17,000, up to $2,000 from the time they began negotiating, thanks to inflation caused by the Ko- rean conflict. It seemed like so much money then, even for a one-story, three-bedroom hillside house with a "daylight" basement, as it was called. The steep slope of the land made it possible to have a ground-level floor at the back.

What the Kosses didn't like was the kitchen-a tiny thing then thought efficient.But you can't have everything.

The Kosses are good-design buffs from way back. In 1946 they found in a Connecticut Avenue store their real prize, a dinning room breakfront designed by Early Modern Gilbert Rhode for Herman Miller furniture company in the 1930s. The blond wood and neatly desinged cabinet, a forerunner of today's storage wall, spent World War II in the store basement. They also have a free-form coffee table with glass top by Isamu Noguchi and a slatted bench by George Nelson, two real shockers in American avant-garde furniture design of the 1950s. (Nowadays people are just beginning to realize what a great time that was for American design.)

They moved it all to Hammondwoods and lived happily for better than a quarter of a century-except for that kitchen.

"The kitchen irritated me more and more," said Helen Koss. "It was inadequate. We'd tried to get it changed before we bought the house, but the developer wasn't willing.We also needed more dining space." Only eight friendly people fit into the dining area, actually one end of the kitchen with no division between.

"When we first moved in," she said, "the closets seemed luxurious, compared to the apartment. But over the years, they all filled up."

The Kosses didn't want to move. Hammondwoods is a close-knit neighborhood. The quiet cul-de-sacs have been a grand place to raise children. The children of the couples who moved in during the '50s are grown, as are the two Koss daughters. No one wants to move away. They like the peace and quiet, the long-time friends, their paid-up mortgages. And the well-designed houses still have a contemporary air.

So the Kosses called on their neighbor, Harold Esten, an architect who bought his house about the same time they bought theirs. "We thought he had done a tremendous job on his own house," said Hlen Koss. Esten wrapped a completely new house around his old Goodman house. He also remodeled several others in the neighborhood.

"We were thinking of something relatively inexpensive, something that would take about 90 days," said Koss. "We were rather surprised when he came up with a much more expensive and extensive remodeling that look six or seven months to do."

They had quite a time finding anyone willing to build it for them. "Several builders told us it would cost too much money: We could buy a bigger house for that," she said. And they never did get a contract price; they worked it out as they were building. The Kosses didn't have time to do any work on the house themselves-his firm rents ladders and scaffolding; she is a member of the Maryland House of Delegates and is up for reelectio this fall.

The new work is finally finished. The final price tag for the remodeling was about $60,000, according to Koss.

They got a lot for their money.

Esten gift-wrapped, or rather boxed, the old house and its new additions in redwood siding. This added to the insulation and makes the outide of the hoouse easier to maintain. "It also modified the general appearance to give the house a coherent form in keeping with its new proportions," said Esten. It made the house look s though the roof were flat-because the new exterior walls come up high enough to disguise steep roof that adds to a feeling of space. Two new decks-a triangular one off the living room and a smaller one off the kitchen-are more useful than the old side yards. A staircase from the kitchen deck goes down to the backyard.

The handsome original windows, which predated cheap sliding-glass doors, started at the ceiling and went to the floor. Esten filled in the lower half in the bedroom because the Kosses complained, "There were no solid walls furniture."

Under all the new design are a great many practical improvements. You enter through the new foyer and entry closet. The living room gained a 4-foot-by-18-foot strip, bringing the room to a total space of 16 feet by 18 feet. It looks like more because of the new wall of sliding glass doors overlooking the deck. The old kitchen/dining area became the new dining room, now seating 12. A totally new kitchen was built just beyond the old one with many wood cabinets and built-in ovens and refrigerator. A cantilevered window bay over the sink gains light from every side-a spot for plants to fourish.The deck will be a cherry place for breakfast, but the Kosses haven't had time yet to buy a table and chairs for it.

Both the master bedroom and a second bedroom have been enlarged. A new bathroom has been added for the master bedroom. On the lower floor, the old paneling was removed to insulate the walls and a new office and guest room laid out.

The Kosses added some less interesting but necessary things as well: a new furnace and water heater, a new roof and all-new insulating glass with more efficient attention from the beginning to the problems of making the house more energy efficient.

Esten's changes make the house as much as typical 1978 house as the original design was a '50s house. For '50s buffs, the changes are not all to be good. Those steep roofs with their overhangs had a pleasant, sheltering and open quality that the more introverted, self-contained '70s boxy wrappers don't have. Still, the house fits 1978 needs while still keeping the 1950s good siting, open layout, high ceiling and splendid sense of place. CAPTION: Picture 1, Helen and Howard Koss loved their Silver Spring house, but decided remodeling would cure a few ills. Architect Harold Esten, center, took the house in hand by designing a new kitchen, adding decks and wrapping the house in redwood siding. Photos by Harry Naltchayan-The Washington Post.

Picture 2, no caption, By Harry Naltchayan-The Washington Post