How do you get an extra room on Animal Crossing?

YOu know how tom Nook tells u to pay of ur mortage. Well in the town hall at the post office (in DS) or an ATM (in Wii) you can make payments to your mortage. Once you pay it off if u go to Nook's store he will ask u if you want to expand and if you say yes u will have a new room in the morning. Kepp paying off your mortage and you will eventually get a mansion.