The Shed, Southbank Centre, London

London has struggled with the legacy of the modernism of good intentions. Commercial modernism has thrived but municipal modernism, not so much. The Royal Festival Hall might have become popular but the other buildings of the South Bank have faced threats of demolition, while their forlorn residential contemporaries, social housing estates in London’s Elephant and Castle and Robin Hood Gardens, await the wrecking ball. The concrete mass of the National Theatre provoked public opprobrium when it was built and the futures of the Hayward Gallery and Queen Elizabeth Hall have been under a cloud of uncertainty for decades. The Southbank Centre is billed as the largest urban cultural complex in the world but it is not a place London is unequivocally in love with.

Perhaps that is a good thing. This concrete panorama is a provocative public place. Its architecture is complex and intriguing – if not always beautiful. It was an attempt to create a three-dimensional landscape of terraces and walkways, to lift pedestrians above the city. The idea of wrapping the public realm under, up and around the buildings makes this a thrillingly, radically unsettling cityscape.

Now, if you wander amongst the concrete levels, a curious, charismatic object catches your eye. A chunky, timber-clad red box with four stacks at its corners – like an upturned table – lurks beside the National Theatre. This is The Shed, a £1.2m temporary theatre designed by Haworth Tompkins and built for one year only to house productions while the Dorfman (formerly Cottesloe) is being rebuilt as part of the same architects’ £70m works to the National Theatre.

The Shed is indicative of a trend for big theatres to build small temporary venues to allow them greater flexibility and scope for experimentation. But beyond that, I think it also offers a clue as to how this extraordinary arts infrastructure might be augmented. The kind of major works under way at the National Theatre are expensive and inevitable: the replacement of technical equipment; the reorientation of the building to take account of a river walk that just wasn’t there when it was opened in 1976; the updating of interiors and cafés. The results might be difficult to notice even though they will change the way the building works and how it addresses the river.

The 250-seat Shed, however, is deliberately noticeable. Conceived and built more like a theatrical project than an architectural one, it was constructed from improvised materials, plywood boards, cheap timber slats, scaffolding, netting. An intervention like this allows a shift in understanding what can be done with that piece of the public realm. It is an architecture of possibilities – much as was the formal experimentation of Denys Lasdun, the architect of the National Theatre, and the Greater London Council Department of Architecture and Civic Design, which designed the neighbouring South Bank.

When the GLC architects built the Hayward Gallery (a space loved by artists) and the Queen Elizabeth Hall (less loved) in the late 1960s, theirs was the largest architecture office in the world. It was a time when architects were envisioning insectoid walking cities and inflatable house-pods, when the members of pop-provocateurs Archigram also worked at the GLC. There was a fearlessness and a desire to create a new type of open, non-commercial public realm.

Such ambitions have subsided. There is no real desire now to create visionary new worlds, and no money to fund them anyway; there is a sense that only consumption is capable of animating public space. We are stuck with tinkering with the existing infrastructure. That’s why I’m slightly uneasy about the Southbank Centre’s proposal to balance a couple of glass boxes atop the buildings to provide a huge rehearsal space and foyer.

The architects are Feilden Clegg Bradley – sensible and thoughtful designers who have also made good proposals for the site in terms of better using the existing multi-level landscape. But my inclination would have been to continue experimenting with exactly the kind of surprising, challenging and temporary (I’m straining not to use the expression “pop-up”) architecture which allows ideas to be tested and which facilitates real, momentary change without needing to be afraid of permanently changing the setting. Indeed, there’s another fine example on top of Queen Elizabeth Hall at present, David Kohn and Fiona Banner’s eccentric temporary nautical structure, “A Room for London”. The trouble with the proposed glass boxes (if that is what they are to be) is that they will fix the buildings in a moment and will fall out of fashion as fast as the chunky concrete Brutalism did when the rain first stained its coarse surfaces.

Not everything needs to be beautiful, unified or easily negotiable. Part of the charm of this unique cultural complex is its incoherence, its profile as a mantelpiece of modernist moments, not all of them successful, and the frisson of knowing that at any moment you might get lost in an unfamiliar concrete corner framing a new view of the city. Like a real city, it is unpredictable.

The resolutely public and determinedly concrete expanses of the South Bank and the National Theatre are as tough as they look – as massive and substantial in their way as a medieval castle. And just as such castles were once supported by a cluster of timber structures – adaptable and readily removable – that housed the activities of everyday life, the Southbank Centre too should embrace a little impermanence. It’s not as if culture is standing still, after all.

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