If you own a house on your own and pay at least 10,000 interest PA then you are entitled to 10,000*20%=2,000PA (166.67pm) relief
IF 2 people own a house they are entitled to 20% up to a maximum of 20,000 interest paid ie 166.67*2=333.33.
For argument sake if your mortgage is 400,000 and your Interest rate is 5%, you pay annual interest of 20,000, if this mortgage is in your name you will receive Relief on 10,000 if the mortgage is in 2 names than you earn relief on 20,000