Improvements to your house will cost the indicated amount of Bells.

Unlockable	How to Unlock
Final expansion	598,000 Bells; results in basement.
First expansion	120,000 Bells; results in larger first floor.
Normal house	19,800 Bells
Second expansion	248,000 Bells; results in largest first floor.
Third expansion	368,000 Bells; results in second floor.