“When the weather’s good,” says Rini van Beek, sitting on the rear terrace of her home in the woods, a 15-minute drive from Antwerp, Belgium, “I practically live outside. Putting this deck down here was actually the first thing I did when I moved in. The house was so awful—but the view was wonderful.”

The back of Van Beek’s Extension House overlooks a small lake, the Rommersven, which, in addition to being the oldest documented expanse of water in the surrounding woodlands, was the feature that induced her to take a gamble on her home the first time she saw it. The house itself was “dismal,” an abandoned, barn-style holiday cottage. “No one had lived in it for years,” she says. “And it was far too small. But it was cheap, and I loved the location.”

The A-frame hut had never been designed for permanent living, so Van Beek enlisted architecture office dmvA to help her rethink and remodel the house. “I travel through this area every day on my way to work, but the location was such a surprise,” says Tom Verschueren of dmvA, who, together with Toon Verboven and fellow principal David Driesen, made up the team that worked on the home. “What a fairy-tale setting! And those A-frame houses [Van Beek’s is one of several such cottages in the locality] dotted around the lake, so archetypal. Normally, I don’t like triangular forms, but here it’s the right form in the right place.”

The architects were charmed, and it was clear from the start that keeping the existing house, rather than demolishing it, was the ideal solution. But how to do that, while meeting Van Beek’s need for working and living space, a modern kitchen, a bathroom, and plenty of storage? Initially, a separate, portable unit was constructed. “The first attempt was the blob—a kind of egg-shaped pod containing my office,” says Van Beek, who, as the Dutch design agent for brands such as the British firm Established & Sons, was open to a quirky solution. “But the local council asked us to remove it because of zoning laws,” she says, “and an artists’ organization now has it.”

It was back to the drawing board, and strict local building regulations meant that the floor plan could not exceed 856 square feet—allowing only an additional 290 square feet of building space. “The design process was a search for the right form to fit in with the A-frame house that would still show a maximum respect for nature,” says Verschueren. “The more sketches we did, the simpler it became.” The outcome was a straightforward geometric solution, adding a rectangular box to the existing pyramid. “The new extension is a statement, an answer to the existing house,” he says.

The original building’s structural framework is a series of sloping studs; Verschueren left these intact on the side of the house to be enlarged, matching them to new vertical studs in the frame constructed for the extension. The old studs remain as angled beams slicing through the space, while the new ones form the basis of the floor-to-ceiling bookshelves at the far end of the addition.

“We could have gotten rid of the three beams that are now freestanding in the middle of the space, but we didn’t want to lose the character of the house,” says Van Beek. “The beams of the new extension repeat the same rhythm [as the facade] and the effect feels spacious and harmonious. Although the [new area] is just 32 feet by 96 feet, it’s had a dramatic effect, and the house looks much bigger.”

The beams also echo the tree trunks outside, which Van Beek now views, along with the little lake, from the huge glass window that runs unbroken from floor to ceiling in the extension. “Because you see no frame, it’s as if the glass isn’t there,” she says. The front of the house repeats the same expanse of glass, but here Van Beek can close a black sliding door at night, increasing the home’s sense of privacy and coziness.

The Extension House’s ground floor sticks to the 856-square-foot floor plan but achieves an exceptional spatial quality, considering that it contains an open kitchen, dining room, and living room; the bathroom occupies the only closed-off area, give or take a cupboard or two. Upstairs is the house’s only bedroom, a pyramidal tunnel with windows at both ends. For storage, there’s a separate structure outside clad in Cor-Ten steel—echoing the gateway to the house and the outdoor pizza oven by Weltevree.

“I do cook outside when the weather allows,” says Van Beek. “And gardening is easy. It’s basically a little wilderness out here, so it maintains itself. We had to cut a couple of trees down to extend the house, but we’ve really removed as little as possible. We have pines, oak, beech, rhododendrons, and chestnut trees. Every day, I move the table to a different place in the garden for lunch. I like to look at the lake, and sometimes I take a rowboat out. Even in the winter, I get outside a lot; when the family visits, we go skating on the lake—and we don’t have to go far to warm up again afterward!”

This Minimal Addition Looks Magical in the Belgian Forest