To HEAR BARBARA Matusow and husband Jack Nelson tell it, their terraced deck "has changed our lives." Quips Nelson, "I skip Class Reunion" -- a favorite pub -- "and come straight home."

More specifically the deck has "tripled the space" -- adding 700 square feet of livable space and a great deal of pizzazz to what was a rather small and ordinary Washington house.

The problem was one you see a great deal of around Washington: a split-level lot without a split-level plan. Because the house is built on a hillside with the high side at the front, the back of the house reared up 10 feet, a whole story off the ground. Underneath at back is a drivein garage (with a turn so awkward nobody ever drove the car in) and a stingy-windowed basement. The principal public rooms of the house, therefore, had no way to connect with the outside except through a tiny screened porch, too small for a table, and a high, rickety stair.

This was a particular hardship for the Nelsons, who like to eat outside. Also, coupled with the size of the house, it made mass entertaining very difficult indeed. Matusow is assistant news director for WJLA (Channel 7). Nelson is Washington bureau chief of the Los Angeles Times. They are gregarious types who like having people over.

They laid these problems upon Richard Ridley, an architect who especially likes to design rough-sawn effects with natural woods. Ridley, of course, could have produced the standard deck, making a sort of balcony with a steep staircase down. Such decks are often quite pleasant. But they have a distinctly "tree house in the sky" air, not at all what the Nelsons wanted. They wanted the house firmly brought down to earth.

So Ridley, with help from his associate, Dave Sternberg, designed a 700-square-foot deck which would be, in effect, a series of three wooden terraces, two of them triangular, stepping down as they go.

The top level has a built-in bench with a back. Each level is generous enough in size for tables and chairs. At the top is a fairly high fence of rough-sawn cedar with a pine latticework center (constructed on the premises) to give the upper deck a sense of enclosure and a measure of privacy. The fence or wall narrows as it goes down, but finishes up full-sized. There are "windows" or cutouts all along for breeze and to keep the deck from seeming too forbidding.

An existing fence, dividing the Nelson land from a public park, was removed to steal visual space. The deck has wiring along the way to provide both power for the "parking lot" type of industrial light fixtures and outlets.

The deck became, as Matusow said, very big in their lives. "People would ask 'How's the deck?' just as though it were a real person." Nelson loves it like a son. "My wife asked if I were going to send it to college." The Nelsons were so taken with it that they started to use it while construction was still under way. The usual kibitizers came by and were so awestruck they just stood and stared. The landscaping contractor didn't like it at all while it was still under construction, but came around enthusiastically when it was finished.

Ridley extended the old back porch, making it into a 140-square-foot sumroom, which easily accommodates a table, chairs and many plantS. The vertical siding is shiplap cedar. The new roof is 4-by-10-foot clear plexiglas. The fixed glass has Pella awning windows below, a good solution for lettering in air while keeping out rais. Since the deck is angular, the sunroom windows don't look at it at all but, instead, straight ahead at the park.

The large glass areas, including the roof, collect winter sun, but because there are many shady deciduous trees to the south, the deck stays cool in the summer. Such climate control considerations are worth more and more thought with today's expensive utilities.

Barry Schaye and his Rock Creek Construction Co. did deck and sunroom in five weeks. The construction costs, including labor and materials, came to $8,800. The architectural and engineering fee was $900. Meadow Farms landscaping was done in a day and cost another $1,800, bringing the complete job to $11,500.

The Nelsons thought it was all well worth it on Memorial Day when they gave a massive party with the terraced deck serving as dining room and lounge space plus bleachers for the furious ball game in the adjacent public park. Next on the agenda: converting that garage into an in-and-out transitional room for the ground floor.