A fountain built in town Donate 500,000 Bells
A windmill or a lighthouse built in town Donate 1,000,000 Bells
First expansion 120,000 Bells; results in larger first floor.
Second expansion 248,000 Bells; results in largest first floor.
Third expansion 368,000 Bells; results in second floor.
Final expansion 598,000 Bells; results in basement.