A room that works from home is not automatically an office. The word brings a piece of furniture with it, a large rectangular desk with a screen on it and a task chair pulled up, and once that arrives the room has been decided. If the working corner happens to be the first space you see from the front door, which it very often is, that decision spreads to the rest of the house.
So start with the work rather than the furniture. Some work genuinely needs a desk: two screens, drawings spread out, long stretches at a keyboard. Much work does not. Reading, thinking, talking to people, taking notes and answering things on a laptop for an hour at a time are all done perfectly well from a sofa with a firm seat and a small table at the right height beside it. Be honest about the split, because the honest answer usually shifts the balance away from office furniture.
A tuxedo sofa or any sofa with arms and back at a similar height works particularly well in this role, because it is symmetrical and reads as seating from every direction. Placed against the long wall it furnishes the room for visitors; turned toward the window it is where the work happens. A firm seat matters more than it sounds: a soft sofa is unusable for anything productive after twenty minutes.
If you do need a desk, consider a writing table rather than an office desk. The difference is storage. A writing table is a surface on legs and therefore reads as furniture; an office desk has a pedestal of drawers and reads as equipment. Most people who think they need the drawers need one drawer of paper and a box for cables, and would be better served by a table and a chest elsewhere in the room.
The walls are where a working room gets its character back. A gallery wall of things you already own is the cheapest and most effective move available: prints, photographs, a child's drawing framed properly, a page torn from something and kept. Lay it out on the floor first and work to a consistent gap of five to seven centimetres between frames, letting the outer edge of the group form a rough rectangle even when the frames inside it vary. A group hung to a shared edge looks considered; a group hung to no rule at all looks accidental.
Colour and texture do the rest, and this is where a working room most often goes wrong by staying neutral. A room that is used for concentration all day benefits from having something to look at that is not a screen. Pillows, a throw over the sofa arm, a rug with some pattern in it and a lamp with a warm shade are not decoration for its own sake here; they are the only things in the room that are not work.
Get the light right and much of the rest forgives itself. Daylight from the side is the aim. Daylight from behind the screen turns the screen into a silhouette; daylight straight onto the screen puts a reflection in it. In the evening, one warm lamp beside the seat and one bouncing off a wall is more comfortable than a single bright overhead fitting, and it also means the room can be turned back into an ordinary room at six o'clock by switching the working light off.
The test of the finished room is simple. When the laptop is closed and carried out, does anything remain that says this is an office? If the answer is no, the room will read as part of the house rather than as an intrusion into it, and it will be a pleasanter place to work as well.