cturner wrote: .... She wants to gift some part of one property but realises it will be a gift with reservation of benefit
You have previously described in much more detail the circumstances at:
http://www.taxationweb.co.uk/forum/cgt-on-house-my-mother-originally-inherited-t36668.html
Re house B, your mother has a base cost for capital gains tax (CGT) from March 1982/1996 for 50% and for the other 50% a base cost at time of your father’s death (market value of entirety less 10% for having only a 50% holding).
It’s not clear how house A was owned on your father’s death but presumably it was always their PPR.
As mullet indicates, PPR relief on house B is not available or of benefit in these circumstances and there would probably be CGT to pay if your mother gifted the entirety of house B and such gift would be ineffective for IHT mitigation if she continued to live there.
If your mother does plan in future to reside at house B for the vast majority of her time, as she has since your father died, might it make sense for her to gift house A to you and your sister as a PET (with no CGT or SDLT payable); and if you to retain it (empty) for her to pay market rent to you for the little time she will spend there so that she retains no benefit from house A and the PET becomes successful after 7 years.