View towards John Steed's living room with the stairs leading to the first floor of his flat, on his left the door leading to the bedroom. It looks like there are four windows or skylights, but what are those things outside the windows? Sticks or branches?
If the pitch of the roof causes the form of the ceiling, Steed's flat would be on the upmost floor of the building.

Steed's charlady, Elsie, cleaning up on the first floor in the episode The Mauritius Penny.
There is a door visible behind her, apparently leading to another room, but Steed never went inside the room nor does he enter it during series 2

.In episode The Golden Eggs a sort of a "bathroom" appeared on this first floor. It's  a small "room" surrounded by a dark curtain but it not existent in all other episodes.
Sketch of the flatHomeLiving Room 2
The Mauritius Penny
The Golden Eggs
John Steed's Flat - No.1
5 Westminster Mews, S.W.1