—(1) Except where the competent authority imposes a condition to the contrary, every planning permission and every conservation permission shall lapse if the development or works authorised by it are not completed or effected within 2 years of —
the date of the grant of the planning permission or conservation permission, as the case may be;
the date of the final permission granted under section 17(4); or
if an appeal is made under section 22, the date the appeal is determined or withdrawn.
(2) The competent authority may, in his discretion, extend any planning permission or conservation permission on such terms and for such further period as he thinks fit.