The definition of what constitutes a slum is at any time arbitrary and shifting, depending more upon the vagaries of the English social conscience than upon any precise and identifiable condition. In times of social crisis, when opinion is deeply disturbed the number of slums is generally thought to be very high indeed; at other times the number is thought to be few, and the slum is judged a relic, archaic and outmoded, of a way of life that is fast disappearing. Looked at in this way, the slum can be described as a condition of life which the English public pronounces intolerable, every 20 years or so, and then quietly forgets.

The slum is best understood in relation to the policy of the government rather than as an objective physical or social fact. At any given time, the number of officially recognised slums tends to correspond, more or less exactly, to the number of unfit houses (usually about 3 per cent of the nation’s total housing stock) which the government thinks it can pull down within a given period (usually five years) plus a slightly larger number which it hopes to clear later (usually about 3.5 per cent).

An ‘estimate’ of the number of slums, is an enumeration, sponsored by the government and conducted by local authorities, which includes some unfit houses in its count, but excludes others. Estimates vary a great deal from place to place, irrespective of housing conditions. The estimates of unfit houses are no more precise or dependable than the notion of the slum itself.

‘Obsolescence’ and ‘sub-standard’ are two further categories which, like the slum, are better understood as administrative conventions rather than as objective facts. They are legends fixed to unfit housing that the government does not want to clear or is not able to clear for many years to come.

A slum clearance programme is a target which the government fails to achieve by about 50 per cent. Thus, if the government declares it will destroy all the slums—those, that is, whose existence it recognises— within a five year period, it will in fact destroy about a half; if it intends to destroy only a half, then it will destroy a quarter. The number of officially recognised slums never diminishes very much, either because, if the programme really gets under way, additional slums are listed (as happened during the 1933–1939 campaign), or because progress is so slow that the original estimate is not seriously diminished (as happened in the 1950’s).