Material change of use 

Means a change in the purpose for which, or the circumstances in which, premises are used, such that after that change the premises are used (where previously they were not so used)–

(i)     as a dwelling;

(ii)    as an institution;

(iii)   as a public building; or

(iv)   for the purposes of the storage or use of substances which if mixed with water result in a fluid which is classified as either fluid category 4 or 5;

Interpretation given in Part 1: Preliminary of the Water Supply (Water Fittings) Regulations and Scottish Water Byelaws