Uses of land and buildings are classified into "use classes" and any change from one use class to another use class is automatically a "material change of use" amounting to development. Some small scale changes between use classes are nevertheless "permitted development" and hence do not require planning permission, subject to any site specific restrictions. Certain types of use or activity do not fall into a specific use class and are termed "sui generis". Any change of use to or from "sui generis" use requires planning permission. In practice most uses are a composite of several uses so that, say, a factory might well have an ancillary office and perhaps storage uses, all within the same premises. In such a case however the primary use would be that of a factory (use class B1 or B2).