My son and I purchased a property in May 2013, each owning 50%. The whole property became my principal private residence for about a month while renovations were carried out. After that the whole property was let out. I now wish to transfer my 50% share to my son. Since I will be disposing of my share within six years of the property first being rented out, can you confirm that under the six-year rule the property continues to be totally exempt from capital gains tax (CGT)? 

I do not know of the ‘six-year rule’ that you refer to. To my understanding, you will be liable to regular CGT on the transfer of your half of the house to your son. I cannot see HMRC agreeing to any principal private residence relief being applied here just because you lived in the property for about a month while renovations were carried out.