While it's unknown where Steed lived during series 1, the address 5 Westminster Mews, S.W.1 is John Steed's  home during series 2, the first season with Cathy Gale.

He lived here from at least May 1962 until March 1963.

In Propellant 23 John Steed tells lieutenant Leclair, the gendarme at the French airport, his address. Later Cathy Gale gives the address of John Steed's flat to the porter at the auction in The Mauritius Penny after Steed bought her a stamp collection of 25 stamp albums.

The building where the flat is located can be seen in Warlock and the inside of the flat appears for the first time in the sixth episode of series 2, Mr Teddy Bear.

The interior decoration and furniture in this first flat are modern and typical 60s style.There are contrasting colors in the flat. A bold orange/light brown of the bedroom door fits a dark blue front door. The bookcases are also dark blue, like the curtain at Steed's "bathroom", so are the three seaters and a matching armchair, there is another leather armchair at Steed's desk. There is no wallpaper, but the walls of Steed's flat are painted in a dark gray, the brick walls in a light gray or white. Door frames and stairs are made of light oak. It's hard to say if you can see windows on top of the wall with the door leading to his bedroom. It looks like some light came through it however in The Mauritius Penny.

Compared to other flats, especially the Edwardian and Recency style of his third flat, his first flat is the most modern one, 60ths style with furniture and colours typical for that period and typical for younger Steed. Especially his bedroom with the plastic chair and kings size bed pays witness of it.
Compared to the second or third flat Steed's first apartment is not very cozy, it's a typical bachelor flat - functionally equipped and almost spartan. It consists of a living room, a small kitchen, a bedroom. A staircase leads to the "bathroom", it's possible that another room is located on this floor.
The furnishings are almost always the same. Sometimes Steed changes some pictures and objects, in some episodes he also has potted plants in the living room.

He let Mrs Gale have his flat in The Golden Eggs, while she is looking for a new one. During the time she is has to provide him with 'slap- up bumper meals' while he stays in an ice-cold hotel (The Golden Eggs, script)

Steed has a charlady for his flat, called Elsie. "I'm getting ready for another of your all-night parties, Mr. Steed", says Elsie in The Mauritius Penny.
Looks like John Steed has a lot of fun in this flat.
5 Westminster Mews, S.W.1
That's probably the building in which Steed's flat is located.

seen in Warlock
rare colour shots of Steed's first flat 5 Westminster Mews, SW 1

Trivia:


Steed brushes his teeth only once in The Avengers as this is the only flat with a "bathroom".

Mrs Gale is the only partner who is ever shown in Steed's bedroom (Intercrime).

No one died in this flat.

FlatsHomeSketch of 5 Westminster Mews, S.W.1
John Steed's Flat - No.1