Nook will charge you the following to expand your house:

1st Expansion (Larger 1st Floor) - 120,000 Bells
2rd Expansion (Largest 1st Floor) - 248,000 Bells
3rd Expansion (2nd Floor) - 368,000 Bells
Final Expansion (Basement) - 598,000 Bells
Normal House (You start off with this) - 19,800 Bells