What is the lifespan of a building, or a building restoration? Despite the fact that much of the population lives in houses that are a century or more old, the answer nearly always appears to be 30 years.

The 1863 Temperate House in Kew Gardens now needs a £28m restoration. Go there today and you’ll see the proudly-carved stone inscription marking its re-opening by the Queen - in 1982. That marked the completion of the last restoration.

There is controversy over Make Architects’ (now approved) proposal to demolish part of Arup Associates’ Broadgate complex in the city of London and replace it with something very much bigger. The original Broadgate is only 25 years old.  No wonder there are moves afoot to get Richard Rogers’ Lloyd’s of London building listed. It too is 25 years old. 

Some buildings are of course designed for much longer lifespans - the British Library by Sandy Wilson and Portcullis House by Michael Hopkins both have something like a 250 year design life, and you can’t help noticing that medieval cathedrals and timber-frame houses last pretty well, too. But in the commercial sector, 25 to 30 years is the norm.

And not just the commercial sector. Consider the demountable new building for Garsington Opera by Robin Snell, in June’s RIBAJ. Demountable it may be, but not ‘temporary’. It will be put up and taken down again every year. And its planned design life is - 30 years.

What does this tell us? To stop treating every new building as ‘permanent’.  Frankly, in the City, buildings might as well be given temporary licences, annually renewed.