An arts centre shows that the best plan can be no plan at all, says Iain Borden, but a London office block betrays no sign of life

Underneath Queen Elizabeth Hall in the South Bank Centre is a classic example of SLOAP – Space Left Over After Planning. Concrete and paving slab banks rest at 45°, with no apparent purpose. This undercroft is dank, dark and disturbing, a place that any switched-on metropolitan resident might avoid for fear of being mugged. Yet this very space is also one of London’s finest buildings. Why? Partly because of the raw, brutalist concrete architecture, designed by the Archigram cell within the old London council. But mainly because of its occupation by skateboarders, who for 30 years have made this the home of British skateboarding. Here you see vibrant flashes of colour and moving bodies, you hear sharp, staccato sounds, and, above all, you see that public spaces don’t have to contain shopping malls. Together with the neighbouring walkways, galleries and cafes, this is true social space: a place where different people do things differently.

And the opposite? The maroon and grey office building that runs up Farringdon Road, just to the west of Farringdon Underground station. Devoid of any aesthetic character other than its sub-postmodern patterning, this building repels the urban citizen – it seems to say “work inside, or walk by”. There is no city life here.

Professor Iain Borden is director of the Bartlett School of Architecture at University College London