YOU REMEMBER that cold, cold weekend we had the first of the month? While most of us were huddled by the fire wrapped in lap rugs, the London family was basking in the sun, as toasty as if they'd gone to Florida.

Paul A. London, his wife Paula Stern and their son Gabriel, were cavorting in their new sunroom, their passive solar heat trap, which in effect moves their house several hundred miles south. As long as the sun shone, the house's two zoned gas furnaces didn't click on at all, despite the 26 degree weather outside. The new addition doesn't use artificial heat, and indeed the heat it traps hellps to warm the rest of the house.

The sun room works very simply to capture heat without hardware, just the way your car heats up when the sun shines through the glass or the way a greenhouse is hot even on a cold day. The sun comes through two stories of double paned insulated glass, and is trapped inside. There's nothing particularly new about the idea. In Europe, the "winter garden" is a standard feature of the old houses. In the United States, the "sun porch," heated only by the sun was a usual part of the '20s houses. Of course, grand houses have always had their conservatories, the same thing.

From the cul-de-sac street, we saw an unstartling grey shingle bungalow, with the typical east-facing veranda. On the side is a wing that looks new enough to have sliding doors, but there's nothing suprpising about it. It looks as though it might have been there for a while.

Inside the house, we went through the hall into a pleasant living room -- a bust of the husband, by the wife, was on the mantlepiece. At the end, you see sliding glass doors onto a small hallway, with a second set of sliding glass doors leading to the hillside garden beyond. This two-story hallway with its balcony hall on the second floor, links the old and new parts of the house.

Another door at this end of the room, leads to the great room. This studio/sun room is 2 1/2-stories high, with a large dormer and a curved window topping it all off. The arched window matches one in the old side of the house. A sink for Stern's ceramics is on one side, plants hanging from the ceiling, sit on the ceramic tile floor, and, above all, plenty of sunlilght streams in.

The sun room has 20 tons of rock below it to store passive solar heat captured during sunny days. The air warms the rocks in the daytime and then the heat is radiated back at night. Ducts in the rocks move the hot air caught by the windows in the sun room, and recirculated from the hot air return. A fan system can transfer excess heat from this room into the heating system. There's a gismo that isn't quite working yet, but everybody thinks it can be fixed in a day or so. The roof is angled so that solar collectors could be added. But for now, it's all passive solar heat, actually a glass envelope around that part of the house.

Through sliding glass doors of the sun studio is an inner sanctum, a study where Gabriel's toys mix with weighty papers. A stair to the side leads up to a bit of happy space, a mezzanine that came about because the roof slopes from high over the sunroom to a lower angle over this space. Underneath the cantilevered room is the entryway to the basement housekeeper's apartment. The curving room has a built-in window seat facing a niche, which now is a desk but could be a fireplace. Architect Michael Cohalan built the curve as a grace not to the back.

On the top level is a sybaritic master bedroom. The headboard is a built-in niche behind the Morrocan rug-covered bed. (Stern spent some years studying Hebrew in Israel and Arabic in Egypt as a journalist on an Alicia Patterson Foundation grant.)

Across from the bed, below the window that looks down on the sun room, is that current trendy luxury, a jacuzzi-tub. It's big enough for the whole family. If Gabriel were smaller and less agile, it could serve well enough as a playpen. The two washbasins are actually pottery bowls, made especially. aThe toilet is hidden discreetly behind a pocket door.

A door at one side of the bedroom leads to the balcony overlooking the glassed-in hall, a pleasant deck off Gabriel's bedroom and the library. There's another, good-sized bedroom, growing with flowered prints in the old section of the house, with an old-fashioned ceiling fan.

"The fan in winter helps recirculate hot air that's trapped at the ceiling," said London.

Paul London was more accustomed to worrying about solar heat on a grander scale. For a year he was with the Department of Energy, worrying about conservation and solar-energy policy, dealing with mega-barrels of lost oil. Then he moved to the directorship of the Coalition of Northeastern Governors. And started doing something about saving America's energy, starting at home in Cleveland Park.

When they first bought their house, six years back, solar heat wasn't their first concern. The house was, to put it mildly, not in superior condition. "There was ivy growing out of the upstairs closet. Now we like to say we have daffodils," Stern said. They did up the kitchen in fine style. The lighting system is especially cheap and effective -- two rows of hanging spherical bulbs. They also added a neat breakfast room to the kitchen, with doors to the pleasant back yard. We had coffee and coffee cake in the breakfast room and it seemed an agreeable place to eat.

When they decided to add on a section, London and Stern, still weren't really thinking about passive solar heat. They just knew that son Gabriel, at 2 1/2, made their bungalow seem smaller and smaller. Stern, a U.S. commissioner with the Inteernational Trade Commission, is a sculptor, as well as a writer on international affairs, and a dancer -- so she needed space. w

"We talked to five or six architects," said London. "We thought we'd add on the northwest of the house, to save the wide southern part of the lot. Michael Cohalan was the first person to say to us, 'That's wrong. You should build on the southeast corner, to let in the sun.'"

So Cohalan drew up the plans and theey hired John Fleming, a young builder, who'd just finished opening up his own Takoma Park house to the sun, to build it. London and Stern think they saved money by serving as their own contractors, and hiring Fleming on a salary as construction manager. Fleming in turn, hired the sub contractors, including Don Lawrence, a carpenter they were particularly pleased with. They kept up with the costs by adding it all up every few weeks. "I did everything that could be done quickly," said London. "Everything that took patience, Paula did."

Currently, everyone is slapping everyone on the back for bringing in the project at $52.43 per square feet. They added 1,706 square feet to the house -- 258 feet of that is two-story high space, almost doubling the size of the old house. The price includes the architect's fee, the new basement with a bath, new basement drainage, an sump pump, and several other such things that weren't much fun.

"Knowing what we do now," said Stern, "we had to put in a foundation anyway. But it cost a lot and it wasn't worth it." The new lower level, complete with a pleasant area way that gives it its own glass wall, is home to Maria Arcadia Lopez, the pleasant El Salvadoran housekeeper who kept everything straight while the walls came tumbling down -- and now keeps the house at a high polish, suitable for a place with no dark corners.

While they had the roof off to make the new addition, Cohalan had heavy insulation put into the entire roof. The new walls are extra thick to accommodate more inchess of insulation. All the new glass is double glazed. On the old north wall, London is adding an inch of rigid insulation plus drywall. Everywhere he has changed anything, such as te enlarged closet in the guest room, he has added insulation.

It wasn't all easy. "We started last January," said Stern, "when there was snow on the ground. One day, when all that snow was melting, a drainage pipe in the new excavation started to siphon wet cement. I couldn't reach anyone so I had to wade in, waist high in icy water to attack a pump. We finished the work under sunny circumstances, August."

London still is working on more plans to be energy efficient, including some new ways to insulate the windows at night.

But for now, the family is working on their suntans.