“The Supply in the course of construction of (a) a building designed … intended for use solely for a relevant residential purpose Notes (4) use for a relevant residential purpose means use as (b) home or institution providing residential accommodation (16) for the purpose of this group the construction of a building does not include (a) the conversion, reconstruction or alteration of an existing building or (b) any enlargement of, or extension of an existing building. (18) A building only ceases to be an existing building when demolished completely to ground level.”