CAROL TAYLOR HAS a passsive solar house built in Potomac in 1969. That's right, 1969.

It's a Techbuilt house, a component or, if you prefer, a prefabricated house. The Techbuilt system was designed in the early '50s by Carl Koch, a Cambridge, Mass., architect. When you see the house, you recognize it immediately as '50s contemporary. Not modern, you understand, but contemporary, a style that came more from Frank Lloyd Wright than from Mies Van Der Rohe.

Like Wright, Koch was influenced by traditional Japanese design. The house has a steeply pitched roof, long walls of glass with a wide overhang, and a pleasant deck. The tall glass is sited due south, to bring in solar heat all day long in winter. The overhang keeps the sun out in the summer.

It's a hillside house. These houses always look best nestled in the trees, set against a bank. Being on a steep slope, the lower floor is carved into the hill, making for important savings on heat because one whole floor is protected against the north wind.

The main part of the house, five bedrooms, three baths, living room, dining room, and lower floor family room, costs only $650 a year to heat with an oil furnace -- despite the fact that all the glass is single pane. It's the south orientation that makes the difference.

The house was designed by Washington architect Don Hawkins, for Mr. and Mrs. John Bredehoeft, the original builders in 1969. The house then cost $36,000 -- an astounding price to think of today. Hawkins used the standard Techbuilt components to design the house to fit the individual site and client. Working with Margaret Leaf, the local Techbuilt representative, Hawkins has custom designed 70 Techbuilt houses for this area.The process might be thought of as remodeling a standard house to suit the individual buyer -- before it's built.

Taylor bought the house eight years ago. Recently, she asked Hawkins to design a 2,000-square-foot addition: a lavish master bedroom and bath, with enclosed sunning garden, garage, workshop and pantry. The addition cost about three times the original cost of the house, showing the way prices have changed.

Hawkins sited the original house -- as Wright said -- to be "of the hill, not on it." So the balcony overlooks a ravine and the woods.

We glimpsed the south face of the house through the trees. But we had a completely different view when we pulled into the steep drive, and walked in through the gate to the entrance court. Here, Lucida Murphy, a landscape architect, has recently completed a two-level patio with a pleasant fence around a free-form swimming pool between the house and the road.

Paul Arnold, Taylor's partner in their remodeling and development business, came out of the workshop on the lower level to meet us at the fence gate. Taylor greeted us in the pool courtyard, with its rocks and chrysanthemums. The glass door entrance to the house opens into a pleasant foyer.

The entertaining area is, in high '50s style, designed as one open area. This idea seems to be revived again these days as the "great room." The foyer is marked by the tiled floor, which continues in from the entrance, and a bench facing the pool court. The living area has furniture grouped to face the balcony and the brick fireplace. The fireplace opens both to living room and dining room and provides some separation between the two.

The kitchen is divided from the dining room only by a counter, with a built-in Jenn Air which exhausts to the outside. A Hawkins-designed skylight has just been installed above the counter. The pantry is a few steps down from the kitchen, with shelves for can goods storage and the freezer. The workshop and garage are through the pantry.

To the right of the living room is the original master bedroom and bath, now used as a library. Behind is the new 18-by-16 bedroom, with its 12-foot-high ceiling and sybaritic bath.

"I told Don I wanted to be private, but be able to look outside," said Taylor. The addition was just finished this summer. It is planned more for summer climate control than winter. The double-glazed glass is oriented to the north and east. Its own private garden is designed to be out of sight of the neighboring house and shielded from view of the pool patio by a fence. Hawkins even built a model of the addition and checked the sightlines from a neighboring house to make it a private paradise. Actually, the neighboring house put in a window unexpectedly on that side, but it's still quite private.

"Arnold found the tub in a Playboy magazine," said Taylor. The bath (shower and tub) is an Italian prefabricated design. The round, red tub has plexiglass sides and built-in, heated towel racks. Of course, it's designed for two.

"First the bath was held up because there was an Italian dock strike," said Taylor. "Then when it arrived here, there was a New York dock strike. Then the truck delivering it got lost. When we finally got it here, the instructions were in Italian."

There's space allocated for a sauna, but it hasn't been installed yet.

The new addition is heated with a heat pump, but the bills aren't in yet for the winter.

Downstairs are more bedrooms for the Taylor children as well as a family sitting room, mostly for them. This floor has all its glass concentrated on the south, protected in winter by the first floor deck.

The Taylor house is certainly far from being a perfect energy efficent house. But the amount of heat brought in from the south-facing glass in winter, shielded in summer by the overhangs, has made the house move neatly into the high-priced energy decade.