Technical Booklet D Structure·Page 91·4E.5
Minimum depth of strip foundations
Strip foundations must be set at a depth equal to the greater of either the depth to selected bearing stratum or 450 mm minimum to avoid frost damage, though deeper placement may be needed in areas subject to prolonged frost or to reach satisfactory ground.
The minimum depth to the underside of a strip foundation should be the greater of –
(a) the depth to selected bearing stratum; or
(b) a depth of 450 mm to the underside of foundations. This should avoid damage from frost action in normal soil conditions although this depth may have to be increased in areas which are subject to long periods of frost or in order to transfer the loading onto satisfactory ground.
The susceptibility of ground to movement, action of frost and changes in water table varies widely and the advice of a suitably qualified person should be sought if the conditions are outside the parameters set out above.
More detailed guidance is provided in BRE Digests 240 and 241.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet D - Structure.pdf