Dab95

Hello, My grandparents gifted their house to my mum and uncle nearly 30 years ago with a clause that they can continue to reside in the property. It has always been my grandparents residence. We are looking at the tax implications of selling before and after they pass. My grandad doesn’t have long left and my nan is considering leaving the property and either renting or purchasing a flat instead. Will there be any capital gains tax to pay on the house, or would they be able to apply for Principal Private Residence relief on this? Thank you