Type 57 is a slightly smaller version of Type 21, as Type 58 is of Type 20. Type 20s and 21s constitute most of the flats in the blocks which face north and south, with flats running from front to back, except garden-level flats and penthouses. Type 20s and 21s are designed to slot together round the stair and lift section of each stairwell of the block. The Type 21 takes the space at the rear behind the stairwell for a second bedroom. The space at the front goes to the companion Type 20 flat, giving it an L-shaped living room or a 'study'.
Every so often along a block there has to be a fire exit corridor right to the front and back balconies. Those take up part of what would be the Type 21 extra bedroom and the Type 20 study. The flats next to such corridors are Types 57 and 58. The Type 57 ends up with one larger bedroom instead of two bedrooms. So they have 1 bedroom and a living room (2 rooms). There are 12 in Defoe House and 6 in Andrewes House but apparently none in Speed House, although it has 6 Type 58s.
