Timmy and Tommy's store
From Nookipedia, the Animal Crossing wiki
Timmy and Tommy's store is the main shop in every player's town in Animal Crossing: New Leaf. It is located near the center of Main Street. The store is owned and operated by Timmy and Tommy, who replace Tom Nook's store from previous games in the series. Tom Nook now runs Nook's Homes, a home improvement store. In addition to buying items, the player may sell furniture, wallpaper, and fruit to the store for eighty percent of the value they would fetch at Re-Tail. Many of the store upgrades are similar to Tom Nook's upgrades, but the signs generally show two leaves instead of one to signify the dual store ownership.
T&T Mart is the second upgrade of the store. It is quite similar to Nook 'n' Go, the second upgrade for the store in the past games. The building's exterior has many of the same features as Nook 'n' Go's exterior, but features an orange and white sign instead of a blue and white sign. This is the first store with the catalog machine.
Super T&T is the third upgrade of the store. It is similar to Nookway, the third upgrade of the store in the past games.
T.I.Y is the fourth upgrade of the store. It is very different compared to Nookington's, the last upgrade of the store in the past games, since it is a D.I.Y. store. The store is merged with the Garden Center, with two separate entrances.
The requirements for upgrading are summarized on the table below