A local planning authority may at any time prepare proposals—
If the Secretary of State directs them to do so, the authority shall prepare, within such time as he may direct, proposals for—
An authority shall not, without the consent of the Secretary of State, prepare proposals in respect of a structure plan if the plan or any part of it has been approved by the Secretary of State under section 35A.
Proposals for the alteration of a structure plan may relate to the whole or part of the area to which the plan relates.
Proposals prepared under this section shall be accompanied by an explanatory memorandum.
The explanatory memorandum shall state—
and may contain such illustrative material as the authority think appropriate.
Proposals for the alteration or replacement of a structure plan shall not become operative unless they are—