A modern extension for a Grade II* Listed farmhouse

Having once been a building at risk, Slackwood Farm in Lancashire has, under its new owners and a team of architects, become a sight to behold, thanks in part to a strikingly modern curved glass extension

Hunkered down on a hillside in Lancashire, a contemporary glazed circular extension to a listed seventeenth-century farmhouse has landed as lightly as a rarely sighted bittern in the Leighton Moss nature reserve it overlooks. Slackwood Farmhouse was on the English Heritage (now Historic England) Heritage at Risk Register when birdwatchers Angela and Adrian Waddingham bought it five years ago. Living in London - where Adrian is a consultant actuary and was, until 2014, Sheriff of the City - the couple planned to use it as a holiday home before eventually retiring to Lancashire.

One of the first things they did was bring in the conservation architect Elaine Blackett-Ord, who made two of the barns habitable. Elaine recommended they work with the architect Paul Archer, whose portfolio is a hymn to the modern aesthetic. As it happened, Paul was familiar with the area through his excursions as a mountain walker. 'Although the farmhouse looks like a two-storey house, it is actually four storeys and felt top heavy with bedrooms,' says Adrian. The Waddinghams decided to build an extension to provide them with more ground-floor space and discussed the matter with English Heritage. 'They didn't want anything pastiche, but modern and cutting edge,' Paul recalls.

His design included a glazed rectangular structure to house the dining area, which links the house to the new curving glass and zinc 'garden room'. With its high walls and glass ceiling, this space solved the problem of not having a 'proper dining room'. Angela and Adrian are both keen twitchers, and the garden room, with only half-height walls, offers a good 360-degree view.

Appearing smaller than it really is has earned Slackwood Farmhouse a Grade II* listing. It is an early example in northern England of a double-pile house, so called because it is two rooms deep along the width. The discovery of a Bronze Age dagger in the plaster above the Jacobean staircase was exciting. So, too, was uncovering a small window in the loft, originally intended for owls to fly in and catch mice.

In his The Buildings of England guides, architectural historian Nikolaus Pevsner described Slackwood Farmhouse as having 'mean windows'. However, an infrared scan conducted by an English Heritage archaeologist revealed it actually had additional ones that had been bricked up by the Georgians. These were unblocked and the façade is now illuminated by a line-up of wooden-framed windows.

By using local craftsmen in wood, stone and metal, Angela and Adrian have upheld the vernacular architecture. Even the cattle grids were made in nearby Carnforth, from whence came the reclaimed flagstone flooring and the handcrafted kitchen. The slate floors, walls and roofs came from a Cumbrian quarry. An old yew tree cut into wooden shingles was used to create a sliding panel in the glass dining room.

The glass extension follows the curves of the landscape as smoothly as a dry-stone wall. 'Inside, it feels as if you're sitting in the landscape,' says Paul. Getting those curves in glass was a challenge. Every detail of the concrete foundations upholding the swooping glass walls was resolved with architectural-glazing company Cantifix. 'Just as a gliding swan looks so elegant and graceful, below water it is paddling like hell,' Paul says, describing the hidden engineering.

Four steel posts support the zinc roof, chosen because it weathers to a greenish-grey like Cumbrian slate. Flat-topped, the roof flips up a little on either side, and a water channel runs its length, to negate the need for guttering and direct rainwater into a bird bath hollowed into the grasslands. Hidden fins support the glazed beams in the dining room, while wiring hidden inside these light pendants above the table.