Where any works for the erection or alteration of a building had been begun but not completed on the relevant land before the day on which the notice to treat was served or the contract made, as the case may be, subsection (3) of section seventy-eight of the principal Act (which provides that the development value of land containing unfinished buildings shall be calculated as if the buildings were completed) shall not apply for the purpose of determining the development value of the relevant interest.