Nestled in a bowl in the rolling landscape of Yorkshire Sculpture Park stands a rugged brown wall, its gnarled sedimentary layers of gravel-flecked matter giving it the look of a great slice of earth hoisted freshly out of the ground. Emerging from a grassy slope, the monolithic slab runs for 50 metres with only a single break, where the presence of a glazed doorframe is the one telltale sign that this is not another piece of land art, but a habitable building. And an exquisitely made one at that.

“The question was whether it was going to be part of the landscape, or an object seen in the round,” says architect Fergus Feilden, one half of Feilden Fowles, the young practice responsible for the sculpture park’s new £3.6m Weston visitor centre. Their answer is that it’s both at once. From the car park, the building reads as a long enigmatic mass, a fissure in the landscape. But enter through the slot in the wall, and you find an expansive light-washed room, a warm timber-framed world where a long, slightly inward-curving glass wall frames a stunning view of the landscape, with tree-lined hills and sculpture-dotted meadows sloping down to a lake in the distance.

Facebook Twitter Pinterest Translucent wrapping marks the presence of the new gallery within. Photograph: Mike Dinsdale/Midi Photography

Housing a new gallery, restaurant and shop, the Weston is the latest addition to the Yorkshire Sculpture Park’s 500-acre grounds of fields, formal gardens, woodland and lakes – an extraordinary place where sheep roam freely among Henry Moore bronzes and highland cattle nose up against a James Turrell Skyspace in a former deer shelter. Founded in 1977 by former college lecturer Peter Murray, the park has evolved over the years, gradually taking over the grounds of Bretton Hall, an 18th-century Palladian mansion turned teacher training college near Wakefield, which closed in 2007, soon to be turned into a luxury hotel.

The landscape has been sculpted over the centuries by successive architects and gardeners, carefully moulded to look natural in the picturesque tradition, and dotted with assorted mansions, lodges, glasshouses and follies. More recently, the park has been an exemplar patron of contemporary architecture, commissioning Feilden Clegg Bradley to design the first visitor centre and an expansive semi-underground gallery in the early 2000s, followed by Tony Fretton’s conversion of a barn into another gallery at the other side of the park, and Adam Kahn’s concrete temple for Roger Hiorns’s copper sulphate-encrusted Seizure installation in 2013. But the Weston is one of their bravest projects yet.

“Feilden Fowles were by far the youngest and most risky choice,” says Clare Lilley, director of programme at Yorkshire Sculpture Park for the last 25 years. The architects hadn’t much to their name when they won the invited competition in 2014, but they have since become one of the country’s most sought-after young firms, with several school buildings and a huge pie factory under their belt, and a pair of Oxbridge college buildings underway. From their hidden oasis of a (self-built) studio at Waterloo city farm, they conjure sensitive projects that bridge urban and rural sensibilities, with a close attention to how things are made. And, just as animals roam the sculpture park, they are perhaps the only architects who can claim to have pigs poking their snouts against the office windows.

In the same way that their own studio presents an enigmatic blank wall to the street, and an entirely open glass and timber front to the garden within, the Weston building has a completely different character when approached from the park. It turns its back to the noisy M1 to the east, with the long earthy wall created by a sequence of meticulously mixed concrete recipes, poured to the architects’ exacting guidelines, after the nature of the local soil meant it didn’t make sense to use rammed earth. To the west, it opens up as a diaphanous freestanding pavilion, the long glazed front curving to embrace the landscape, while a fluted fibreglass lantern pokes out of the roof.

Facebook Twitter Pinterest A long enigmatic mass, a fissure in the landscape. Photograph: Peter Cook/YSP

Making the building very much of its time (and recalling the fibreglass walls of Carmody Groarke’s temporary Filling Station restaurant perhaps a little too closely), this undulating translucent wrapping marks the presence of the new gallery within, where rows of sculptural concrete roof lights plunge from the ceiling in powerful horizontal waves. Board-marked from the rough-grained timber shuttering, the concrete planes each swoop down before curving up in a broad bullnose, recalling a structure from the heroic age of concrete – like something a young Pier Luigi Nervi might have created, only on a more diminutive scale. A hidden labyrinth of 10,000 unfired bricks behind the wall provides a clever method of temperature and humidity control, allowing the gallery to opt out of the onerous national standards for conservation, yet still receive precious loans – a bold move, first pioneered by Manchester’s Whitworth Gallery. The opening show, by Delhi-based artists Thukral and Tagra, promises to raise the temperature, tackling issues faced by farmers in India and encouraging visitors to engage in traditional “kushti” wrestling.

After you’ve had a good tussle, there’s no better place to relax with a cuppa than the cafe-restaurant next door, sunken at a slightly lower level, where warm wooden cafe tables and chairs stand on a floor of ground concrete, while a Weetabixy ceiling of wood-wool panels floats overhead, above walls of faintly mottled lime render – a finely tuned space as carefully composed as the artwork it serves.