Subject to any rules made in this behalf, any person generally or specially authorised by the Board in this behalf, may, at all reasonable times, enter upon any land or premises and do such things thereon as may be necessary for the purpose of lawfully carrying out any works or for making any survey, examination or investigation, preliminary or incidental to the exercise of any power or performance of any function by the Board under this Act:
     Provided that no such person shall enter any building or any enclosed courtyard or garden attached to a dwelling-house without previously giving the occupier thereof at least three days notice in writing of his intention to do so.