I would see an accountant/tax specialist before planning on using room rental as a business use of your house. This doesn't seem like it would be possible. What seems more likely to me would be that you should report the money as income on your taxes without getting to deduct anything. I must admit I would probably just pocket the money and not worry about it if "renting" to family (hope that isn't too unethical for the board).

Speaking of renting - I'm in the processes of buying a new home after splitting up with my SO. We own a home jointly which is now up for sale. When applying for the new home, the mortgage company told me they would count the entire old mortagage "against" me even though my ex pays half. The only way they would consider counting only half would be if I could get him to sign a rental agreement stating that he would pay me rent each month equal to half the mortgage. Imagine the tax consequences of that! (assuming I could get him to agree to the ridiculous idea of paying rent to live in your own home).

Lael