It is an elite list with some of the most significant and beautiful buildings and structures in England, including Battersea power station, Middlesbrough’s Transporter Bridge and the London Coliseum.

Now the Grade II* landmarks are being joined by a mysterious, limestone rubble “barn” on a grassy knoll in the Lake District, which was most recently used as a shelter for sheep and cows.

The Department for Culture, Media and Sport said it was awarding listed status to a building known as Henry’s Castle on the advice of Historic England. Only 5.8% of listed buildings are at grade II* level, meaning they offer “more than special interest”.

Research has revealed that Henry’s Castle has the hallmarks of a high-status building and could date back as far as the 14th century. It has been used as a field barn but what its original purpose was is not known.

“Henry’s Castle is one of those rare buildings that raises more questions than it answers,” said Sarah Charlesworth, Historic England’s listing team leader. “This mystery is part of what makes it so compelling.”

Rose Lord, built environment adviser at the Lake District national park authority, visited the building in 2022 and, despite its layers of animal manure, knew it was something “very special – I could tell it was definitely more than just a field barn”.

A corbelled (stepped) chimneystack and corbelled garderobe “even in their deteriorating state” suggested there was more to the barn than met the eye, she said. “The inside was plastered and limewashed and you could tell there had been a fire inside … the whole arrangement of fireplaces, windows, doors was something way different to anything I had ever seen.”

Archaeologists, architects and conservation engineers have undertaken four years of work to rescue and repair the building. Its most remarkable feature is the oak roof structure – “a precisely hand-finished, chamfered and pegged central truss of a type associated with high-quality carpentry of the 14th or 15th centuries”.

Theories on what role it had include being some sort of lookout dwelling with a defensive purpose. “Was it a hunting lodge for a deer park?” said Lord. “It could have been a very fancy, glamorous summer house that someone used sporadically.”

Historic England said that it shared characteristics with bastles – fortified farmhouses common to the Anglo-Scottish borders and built to protect farming families and their livestock from raiders.

But, nestled in a valley near Kendal, a half-hour walk from the village of Underbarrow, Henry’s Castle is too far south of the borders to be classified as a bastle.

In the 16th or 17th century it was converted to domestic use and by the 19th century it had become a field barn. It gets its name from a farmer, Henry Willison, who once owned it.

Lord said getting such a high listing was the “cherry on the cake” for what had been a “very rewarding” restoration project. “Most buildings which are that significant have already been picked up in previous surveys,” she said.

Interpretation panels are to be installed in the coming weeks and a box has been installed in the hope of encouraging owls to set up home