“ … that the Company and those deriving title under it will not at any time erect on the said land hereby conveyed more buildings than one dwellinghouse on that part of the said land as is hatched pink on the said plan and one dwellinghouse on the remaining part of the said land and no such dwellinghouse shall be erected otherwise than in accordance with plans (including site plans indicating the proposed position of the said dwellinghouse on the land hereby conveyed) and elevations and of materials previously approved by the Vendors or their successors in title and no building shall be commenced upon the land hereby conveyed until the Vendors shall have given a certificate in writing of such approval.”