Something’s weird, but the Doctor’s well used to that. A lot of things are weird, which is why he’s always around, travelling through time and space to fix things that shouldn’t be the way they are.
This particular weirdness is especially interesting, a perfectly ordinary building, two storeys tall, in a row of other perfectly ordinary buildings. It’s been divided into two flats, one up, one down, with a small entrance hallway and a flight of stairs leading to the flat on the upper floor. That’s where the sense of wrongness is coming from.
There shouldn’t be a second floor.