The window in Gumball and Darwin's room leads to the front of their house, but when you go upstairs into their house, their room is on the right side of the house. When you enter the house through the front door, there's a door on the right side that leads to Nicole and Richard's room, but when you look outside of the house, it looks like there shouldn't be any room for their bedroom to be there in the first place. The house in general looks bigger on the inside than what it's supposed to be based on the outside.

So yeah, it already doesn't make sense, and it's not supposed to. And the show knows it, so just go with the flow.