Below is the technical answer directly from the Building Regulations. We realise that this may perhaps be rather confusing. If you are still unsure, please contact Building Control.

There is a material change of use where there is a change in the purposes for which a building is
used, so that after that change-
(a)the building is used as a dwelling, where previously it was not;
(b)the building contains a flat, where previously it did not;
(c)the building is used as an hotel or boarding house, where previously it was
not;
(d)the building is used as an institution, where previously it was not;
(e)the building is used as a public building, where previously it was not;
(f)the building is not a building described in Classes I to VI in Schedule 2,where previously it was:
(g)the building, which contains at least one dwelling, contains a greater or lesser number of dwellings than it did previously.

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