London architects Tate Harmer have pulled off an ingenious feat: fitting a 12-tonne staircase through a 1.4m-wide doorway. The freestanding staircase takes visitors down to the Brunel Museum in East London's Rotherhithe and into a 50ft-deep shaft or chamber, which opens this week as an unconventional performance space.

The Grade II* listed shaft was built by engineering father-son partnership Marc and Isambard Kingdom Brunel. It opened in 1843 and leads to the world's first underwater tunnel, making it 'the birthplace of the mass urban transport', says museum director Robert Hulse. Now, just 5m below the chamber's floor, London Overground trains regularly rumble by.

The staircase is supported by three columns and has a viewing platform at ground level, as well as two mini landings where it changes direction. Its 1,000 components, including the bright red handrail and the oak treads, were transported by crane through a new doorway at ground level, and assembled in the shaft.

The architects – who created TREExOFFICE, a pop-up workspace wrapped around a tree trunk for last year's London Design Festival – powder-coated the steel structure in black. This was in reference to the steam trains that once used the tunnel, and to help it blend in with the chamber's 1.5m-thick solid brick walls.

After some debate, Harmer and his team decided to leave the walls as they were, rather than smarten them up or try to return them to their earlier condition. 'The power of the space comes from the patina on the walls,' says Tate Harmer partner Jerry Tate. This patina includes the outline of a former staircase snaking up the brick, the soot, the peeling paint and the myriad dangling cobwebs. Industrial chic is an understatement.

The events programme kicks off tomorrow (16 April) with a concert performed on a grand piano with backing from a string quartet.