Prince Charles might have done well to save his famous carbuncle speech for Preston. After years of battling to preserve the city’s majestic 1960s bus station, which finally got grade-II listed in 2013, the winning design for a £13m project to breathe life back into the building has been unveiled. The proposal is a cross between a “monstrous carbuncle on the face of a much loved and elegant friend,” as the Prince described the planned National Gallery extension in 1984, and a giant fridge-freezer, carelessly dumped in the station forecourt.

The winner of an international open competition, organised by Lancashire County Council and the Royal Institute of British Architects, the design is the work of New York-based architect John Puttick, who previously ran the Chinese offices of Make – a regular presence on the shortlist of the Carbuncle Cup for the ugliest building in Britain. Puttick may have left Make behind, but the Make way clearly hasn’t left him.

The striking beauty of the bus station, designed by the Preston-based Building Design Partnership in 1969, comes from the uninterrupted linear sweep of its curved concrete fins, which extend for 170m along the building like a sharp stack of razor shells. It was the biggest bus station in Europe when it opened, bringing the glamour of air travel to the road. The startling form of its bright white hangar cemented Preston’s role as a major hub for long-distance travel on the first motorway in the country. It is a temple to the age of coach travel.

‘Like a giant fridge-freezer dumped next to the station’ … the building is curved in response to the concrete fins of the bus station. Image: John Puttick/Lancashire County Council

Soon it will be a temple with a lumpen add-on, the Parthenon of buses with a bulky life-support machine plugged in. Crashing into the north-western end of the structure, the new building appears plucked from a completely different context, as though it’s found itself here by mistake. Half trying to merge with the existing building, half trying to exert itself as a freestanding object, it squats awkwardly alongside the station, managing to obscure a good chunk of it in the process.

Planned to house a “youth zone” of sports facilities and arts spaces, the new building has a curved prow at either end (a half-hearted reference to the curving profile of the bus station’s car park floors) while the surrounding masterplan, including a skate park and a seating area, is conceived as a “textured carpet”, “drawing on Preston’s proud heritage of textile manufacturing”.

After years of campaigning to save the bus station, which saw a gigantic retail scheme fought off and two attempts at listing turned down, Eddy Rhead of the Manchester Modernist Society is unimpressed with the conclusion to this long-running saga.

“This design is total rubbish,” he says. “What disappoints me is the lack of ambition. On a pragmatic level I appreciate they had to do something, but this is such a wasted opportunity. It looks like the compromised result of design-by-committee.”

The curving concrete fins extend for 170m along the building like a sharp stack of razor shells. Photograph: Alamy

He blames the way the competition was organised, both as an open call – with the five-strong shortlist subject to public vote, taken account of in the judging – and the limited scope of the brief. Rather than considering the entire structure, and addressing the wider problems the building has with its context, the RIBA-led competition focused only on strapping a new youth zone to the side and allowing minor interventions to the ground floor of the building – which will continue to serve as a bus station.

The five floors of car-parking, each the size of two football pitches, as well as the expansive (and unused) rooftop, and the central retail spine of the building were all off limits, as was the dysfunctional relationship to the surrounding streets. As a result, the bus station will stand as a zombified relic, neither fully fixed nor put out of its misery, but kept in compromised stasis by its bulky new appendage.

Still, Rhead is optimistic the great concrete cruise-liner will outlive whatever the council can throw at it. “The only saving grace is that this new extension will probably be closed down or turned into a bar, then eventually knocked down,” he says. “As long as its physical integrity isn’t compromised, the bus station will endure.”

