17.—(1) Any approval or agreement which is given by the relevant planning authority or relevant highway authority under these requirements must be given in writing.
(2) Where the words “unless otherwise approved by the relevant planning authority” or “unless otherwise agreed with the relevant planning authority” or “unless otherwise agreed with the relevant highway authority” are used in these requirements such approval or agreement may only be given in relation to immaterial changes where it has been demonstrated to the satisfaction of the relevant planning authority or relevant highway authority that the subject matter of the approval or agreement sought is unlikely to give rise to any materially new or materially different environmental effects from those assessed in the environmental statement.