The Clean Air (Height of Chimneys) (Exemption) Regulations 1969
Under section 6(2) of the Clean Air Act 1968, a person having possession of a boiler or industrial plant attached to a building or for the time being fixed to or installed on any land (other than an exempted boiler or plant) is required to obtain the approval of the local authority to the height of the chimney if he proposes to construct a new chimney or to enlarge the furnace. By section 6(11), exempted boilers and plant mean those used or to be used wholly for any purpose prescribed by regulations ; and these regulations prescribe the purposes which carry exemption. They include temporary or local provision of heat or power during replacement or maintenance, building operations, engineering construction, investigation or research, and agricultural operations.