When you are first starting out in the game you are going to have a two floor house with a save room and a 4×4 area. It will cost you 19,800 bells from Tom Nook. By the time you are done you are going to have a three floor six room house. You’ll want to figure out floor plans and different ways to lay out your furniture to appease the HRA score gods. I suggest making a grid and doing “test runs” on where to put what furniture until you get it all designed out properly.
First House Expansion
The first expansion will widen your first floor area from the 4×4 area to a 6×6 area. You will pay 120,000 bells for this privilege from Tom Nook.
Second House Expansion
This expansion will increases the first floor layer area from the 6×6 area after the first expansion to the 8×8 it will be after this expansion. It will cost you 298,000 bells from Tom Nook.
Third House Expansion
This one is a bit tricky. You will have your save room increased to a third floor room that is also a 6×6 room and now you will be able to use the old save room for something else. You will have to pay 598,000 bells in mortgage costs to Tom Nook for this expansion.
This is the expansion where you will get another 6×6 room that is added to the side of the house. This expansion will cost you 728,000 bells, and again it is from Tom Nook for the mortgage.
Four expansions are all you got in the first Animal Crossing, but in Wild World you get another 6×6 room added to the other side of the house. This one costs 848,000 to get. Tom Nook is your landlord.
Sixth, and Final, Expansion
This is the last expansion to your house, and it’s another 6×6 room to the back part of your house. It is visibly larger to see from the outside as well as roomier inside. This one will cost you 948,000 bells to complete.
All in all, after all the expansions are said and done, you are going to be out 3,559,800 bells for the house. The rest is all about decorating after this work is done.