The Hayward was designed in the mid 1960s by tough-minded young architects of the then London County Council. Some were associated with the now-legendary Archigram group of hip theorists. The Hayward was part of an overall complex which also introduced the Queen Elizabeth concert hall and all the elevated concrete walkways. The South Bank's later design difficulties all stem from that Quixotic decision to raise everything, including the entrances, up in the air when the earlier (1951) Royal Festival Hall to the west - and the later (1976) National Theatre to the east - were planted firmly down on the ground. But for all that, what these manifesto-led architects produced is still a touchstone in the architectural debate. It is strong meat. You have to be pretty committed to the idea of mid 20th century modernism to like it, for this is uncompromising concrete at its most heavyweight, most muscular, most brutal. But I do like it, for it is a unique record of the period. It is the Mike Tyson of arts buildings and, like that gentleman boxer, it long presented an unreconstructed face to the world. Now then: imagine if Tyson had gone to an image consultant and succumbed to a little light cosmetic surgery. Rearranging Tyson's features? It takes bravery to do that.

The brave men in question here - Dan Graham and Graham Haworth - very wisely decided that the smart thing to do was put away the scalpel and instead give Tyson the equivalent of a pair of reading glasses. Voila! The famous features remain the same, but softened, given a slightly studious air. So it has gone: the Hayward's extensions take the form of two restrained glass additions that sit comfortably with the lightly-cleaned concrete rather than attempting to carve it up. The foyer is enlarged by bringing it forward in a double-height tall glass box which houses a staircase to a new little upstairs gallery, little more than a broad landing. Meanwhile a new elliptical education room, kitted out with video monitors permanently playing classic cartoons, is added off that gallery to one side, on what was previously a flat roof. It is on a level with Waterloo Bridge. This gives at least some gallery-goers a glimpse of the river that has so long paradoxically been denied them.