I can confirm with certainity that you do need to maintain buildings cover and notify your Insurer of any works. A Builders CAR policy only covers the new works and specifically excludes any existing structure. If there was a fire , it is only the new works ie the new extension which is covered, your home will not be. Also if you don't tell your insurer about the works being carried out , they will likely decline any claim for damage to your home, if they were not made aware of the planned works.