How to build a home office that doubles as a guest room without compromising either

Most Australian homes do not have a room to spare. The home office almost always lands in the room that used to be something else, and in a lot of cases it is the room that still needs to be something else a few times a year when family visits.
The usual outcome is a compromise that fails twice. A cramped office with a bed shoved against one wall, or a guest room with a laptop balanced on a chest of drawers. Both are avoidable. The trick is to build the tech side properly first, then solve the f