Paragraph 2 Small detached Buildings
A detached building designed and intended to shelter people from the effects of nuclear, chemical or conventional weapons, and not used for any other purpose, if–
(a)
its floor area does not exceed 30 m2; and
(b)
the excavation for the building is no closer to any exposed part of another bui1ding or structure than a distance equal to the depth of the excavation plus one metre.