You mean ACWW, as in Animal Crossing Wild World, I imagine.. If you rejected the extra room, I don't think there's a way to get the opportunity back.. Start saving up for the next mortgage, and when that pays off, maybe you can get the chance back. Good luck.
Do you have multiple characters living the house? (assuming this is Wild World) If so, try having one of them talk to Tom Nook. Or, create another character. You can always delete it later.