How do I transfer my parents house deed to me and escape inheritance/capital gains?

Myself and my son (8) has moved into my parents house to build an annex on the side (it's a corner plot bungalow) for myself and my son to live in, the reason for this is two fold, as there is no easy way for a single parent to get on the property ladder and also my father has had a couple of heart episodes and I need to be closer and on hand for my mums sake also, basically my parents are worried that to build the annex would mean putting the house deeds in my name because of the mortgage which would mean I could incur capital gains or inheritance taxes. Can anyone tell me the answer as i've looked around and seen so many different answers to this?!!! Help!