What might be an idea is something in writing capturing the actual situation, that she is moving in to allow her to rent her place out, to make some extra money, in return she is giving you a share of the savings from that to help with bills and the money is not going toward owning a share of your house in any way, and should the two of you ever separate the understanding is that she will return to her own place (if that's possible the way tenants rights are going!). Getting her to sign something so you are both on the same page seems sensible. On the original question she might work out her old outgoings on her place (mortgage) minus the new outgoings net of rent, all the renting expenses, tax etc. and you work out some share of that saving that she is making, she would need to first set aside a fund to pay tax and for a maintenance fund for any repairs etc. Anyway best of luck with it, hope it all works out great for you both.