Second floor of the museum in New Leaf?
How can I get the second floor for my town's museum in ACNL?
Donate 20 items to the museum.
These must include at least one item from each category (Bugs, Fish, Paintings, Fossils).
Talk to Blathers on 14 different days.
Enter the museum when Blathers is thinking. Talk to him and he will propose the second floor as a public works project.