Speak to a solicitor. The mortgage and the deeds are 2 different things and the lender can refuse. You dont say whether you are married to the other person, if not then its best to speak with a solicitor to ensure you are both aware of where you stand if one of you was to die. Sorry to sound morbid but having gone through a very expensive and drawn out battle (I hasten to add no-one died) its important that you are aware of the pros and cons.