When desrcibing guarding on stairs Approved Document K states:

1.39 In a building that may be used by children under five years of age, construct the guarding to a flight of stairs to:

a. prevent children being held fast by the guarding: ensure that a 100mm sphere cannot pass through any openings in the guarding, and

b. prevent children from readily being able to climb the guarding.

Does anyone know if this is retrospective ? Obviously it doesn't apply if the building isnt used by children under 5 , but it could apply in a block of flats where residents will receive visitors, some of whom may be young children. I suppose it's down to asessing the risk i.e. likelihood and frequency of under 5's in the building? 

Building Regulations are never retrospective in their own right BUT if change of use or refurbishment occurs to an existing building it must meet the regulations as currently written.