Chances are you've never noticed this building.
It's hidden away among the courts and squares of Lincoln's Inn — the legal enclave beside Chancery Lane.
This pretty lodge looks like a retirement home for pixies or a mausoleum for an eccentric antiquarian.
It's known as the ostler's hut, and is commonly cited as the smallest listed building in London. According to various accounts online, it was built in 1860, as a resting place for the ostler — the man charged with looking after the horses of those visiting Lincoln's Inn.
Historic England is less certain about its function. The Grade II listing notes it merely as 'late 19th century' and describes it as a porter's lodge.
But is it the smallest listed building? Another candidate might be the 'world's smallest police station' in Trafalgar Square. It is not, nor ever was, a police station — merely an occasional police observation spot. Even so, it is a habitable space and therefore, in our book, a proper building. Historic England includes the post within its Grade II* listing of features in the square. Is it smaller than the ostler's hut? Probably, though we neglected to carry our measuring tape when tracking down these buildings.
Beating both candidates, however, is this unique phone box at the entrance to Burlington House, Piccadilly. It is the original prototype K2 phone box designed by Giles Gilbert Scott. Unlike later models, it is made from wood and includes perforated crowns in the roof section. Historic England gives the kiosk a Grade II listing. Several other London K2s are also listed.
'Even when there's ten of them, they're hardly there at all'
But wait... we can get still slimmer. The later K6 phone box, also designed by Scott, is both shorter and narrower than the K2. Dozens of them can still be found across London, and many of them are listed structures (including those shown above on Duncannon Street). These, we reckon, are the smallest listed buildings in the capital, so long as you don't count stuff like bollards and lamp posts that cannot be entered.
Unless you know better...
See also: London's smallest house