Tom's Apartment is a location rarely featured in the show. Tom lived here before moving in with Edd and Matt into Edd's House.
The building itself is tall and grey. There is always a party in the room next to Tom's (complete with music and flashing lights). The inside of Tom's apartment is only shown once briefly in Zombeh Attack 3, it has grey walls and has a living room, kitchen, bedroom and (most likely, but never shown) a bathroom. In this episode, it's also seen that his apartment's number is 27.
Reason for Leaving
It was explained through a conversation in WTFuture that after Tom shot Tord's skeleton in Zombeh Attack 3, he never bothered to clean it up. So his landlord figured the skeleton was Tom's and assumed that he was dead, so he rented the room out to someone else.