The organisers praised the scheme, which beat off 21 other Greater London buildings to claim the prize, as an innovative solution to the problem of lack of space.
The tiny suite of modern bathrooms in west London's Shouldham Street is attached to the rear of a Georgian townhouse.
Peter Murray, chairman of Wordsearch and curator of New London Architecture, presented the award-winning team with their certificates.
Murray, speaking on behalf of the jury, said: 'The winner stands out as it is interesting on both an architectural and an intellectual level and it is spatially successful. This light and elegant project illustrates what happens when necessity becomes the mother of invention.'
The judges for the awards were Simon Foxell (Chairman, RIBA London), Silvia Ullmayer (Ullmayer Sylvester Architects), Paul Baker (Wilkinson Eyre Architects) and Stephen Marshall (Munkenbeck + Marshall).