Finding a pristine plot is every architect's dream. When Casper Mork-Ulnes was approached by two San Francisco couples with a brief for a sleek retreat a hundred miles or so north up the 101, deep into Californian wine country, the site was the main attraction, the budget rather less so.

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'The clients had previously seen a prefab building that we had designed,' says Mork-Ulnes, a Norwegian-born architect who has lived in Italy and Scotland and currently divides his time between Oslo and San Francisco. 'They asked us if we could create a building that was based on the same solutions.' The design had to be low-cost but also prefabricated, as the location was hard to access with heavy machinery and trucks.

With design team members Grygoriy Ladigin and Andreas Tingulstad, Mork-Ulnes set about finding ways of touching the ground lightly - and inexpensively. The key generator of the form was the unspoilt views across the surrounding hills. Perversely, this also kept costs down. 'We reduced the number of windows to capture just the essential views,' says Mork-Ulnes. As a result, the house spreads across the site like a threefingered mitten laid upon the ground, each finger terminating in an expansive glazed wall and a carefully considered vista.

Local firm Double-D Engineering created a steel stilt framework to keep the structure above (and between) tree roots. The entrance is up six steps, mounted above a tiny concrete foundation, leading you straight into the living space. Here, viewing axes shoot off in the three carefully chosen directions, looking across to Eagle Rock, a local landmark, as well as a distant ridge and the vineyard-filled valley. To the right is the kitchen with the 'great room' beyond, with two bedrooms and bathrooms occupying the other fingers.

This is essentially a shelter for a very outdoor-focused lifestyle, and fixtures and fittings are kept to a bare minimum. The San Francisco artist Yvonne Mouser contributed a selection of simple furniture using burnt wood. 'We have some built-in furniture, such as bookcases, but the interior was really designed to be sparsely furnished,' says Mork-Ulnes, explaining that, at just 1,170 sq ft, the house needed to be uncluttered.

The sense of space is also preserved by the glazed upper sections of the twin washroom boxes, which maintain the flow of light right through the structure, as well as by the great expanses of uncut raw materials.

The house allowed Mork-Ulnes to translate earlier prefab techniques into a practical kit for low-cost house-building. By setting out a modular design based on off-the-shelf materials and framing, waste and off-cuts were minimised, as was the time needed to apply extra finishes to the OSB and plywood. Instead, these rough and ready materials are exposed, stained and sealed to make them look their best. The steel cladding will gradually weather, becoming more organic in appearance and acting as a foil to the shadows of the gnarled tree trunks.

The Moose Road House is shaped by the ingenious response to its site, respect for which has served up a design that turns limitations into bold, elegant and apparently uncompromising architecture.