When we had our attic space converted into a spare room, the electrician had to run power cables up the corner of the house (the fuse/distribution box is by the front door), which meant cutting channels in the brickwork and plaster (none of those hollow walls in this house) from the ground floor, up through what's now my study, to the top floor. Rather than have him attempt to wire up Ethernet, I just handed him a roll of CAT5e cable and asked him to do two runs: ground floor to corner study and ground floor to attic conversion, and to make sure he left 3 feet of cable sticking out at each end. I temporarily crimped connectors on the end to check continuity before they re-dressed the plastering, and then I did the wall boxes myself later.