Before you lift the sofa: measuring is what counts
Most sofas will fit through; you just have to turn them the right way. How to get a sofa through a narrow door is decided before the first lift, with a tape measure rather than at the door frame. A few minutes of measuring will save you scuffed walls and torn upholstery.
Measure the sofa in three dimensions
You need the height, the depth and the so-called diagonal. The diagonal is the longest line across the side profile, i.e. the distance from the front bottom corner to the back top corner. It is this that determines whether the sofa will go through the opening upright or at an angle.
Measure the opening, not the door
What matters is the clear width of the frame once the door leaf is removed. The door leaf takes up three to five centimetres, which are often exactly what is missing. Measure the height of the opening too, and don't forget the handle, the architrave and any light switch right next to the frame.
Walk the whole route
The narrowest point is often beyond the door. Measure the landing, the width of the staircase, the banister and the inside of the lift, including the height of the car. On spiral stairs, note how much space is left at the turn. Clear the route in advance, including the coat rack and shoes in the hallway.
What can be removed from the sofa
Legs, armrests, backrest
Start with the legs; they unscrew in a minute. On many models you can also remove the armrests and fold down or detach the backrest, which is held on metal hooks. Put the screws in a bag and tape it to the underside of the frame so you can find them in one place when reassembling.
Secure the sofa bed mechanism
A sofa bed must be firmly closed. Strap or tie the mechanism shut. If it opens while you are carrying it, the centre of gravity shifts suddenly and a hand inside the frame can very easily get trapped.
Remove the covers and cushions
Carry the loose parts separately. Seat cushions and pillows take up less space in the van when stacked separately, and the sofa weighs noticeably less without them. Put a removable cover in a bag so it doesn't get dirty on the way.
Take photos as you dismantle it
Reassembly is always harder than disassembly. Take a few photos of each step, especially of the backrest fittings and the sofa bed mechanism. In two days' time, when you are standing over a pile of parts in your new flat, you will appreciate it more than you expect.
Take the door off its hinges
You can lift the door leaf off its hinges in a moment. Open the door fully, lift it and take it off upwards. With newer doors you first need to push out the hinge pin. This gives you a few extra centimetres and removes the risk of the door breaking against the sofa.