I don't understand the logic here. Both parties input 25% of the PP but only 1 party is paying the mortgage (75%). OP, I am assuming that the mortgage is in joint names of both parties, which if only 1 party is paying the mortgage could lead to problems up the line. In theory you could be classed as 50/50 owners of the property as presumably you are registered as joint owners. However, the martgage arrangement could cause problems, if/when the property is sold or your brother wishes to take it over. Best to formalise the position now, with the help of a solicitor, rather than risk problems arising later. i.e. What will happen if you want to take out a separate mortgage in the future. Any bank assessment will take into account your liability on this mortgage!! Also, as your brother pays down the debt, he should be legally entitled to a higher share in the property if/when it is sold, if no other agreement is in place!! Unless you come to an agreement now, there is a high liklihood of problems arising in the future.