There was a court case over a picture on a wall, this was about what was fixtures and fittings and what was belonging to the occupants, it was stated a screw is designed to be removed, a nail is not. Since there was a nail involved with removing the picture it had to remain in the house.
We have something similar with accessible, floor board nailed down then not accessible, screwed down it is accessible. Also anything placed on top of the floor boards, if there is a nail holding down the carpet then not accessible.
Where the problem lies is at the time of fitting it may be accessible but latter it becomes inaccessible, the same applies to things like bathroom fans, when the house is built it may have an opening window so the fan if fitted is not required by building regulations, then the window is replaced with non opening and then you need a independent way to switch on the fan. Not looking at date the fan was fitted but the date when the window was changed.
To me the problem is I can down load or buy current regulations, but I can't get hold of old regulations so if the window was fitted in 1990 I have no way to check up if an independent method of switching on is required or not.
Now I would assume if the electrician writes on the floor, "electric box under here" it means two things, one he expects it to be carpeted or he would not write on the floor, and if some one puts something harder to remove than carpet they are fully aware access is required. I have worked in many offices with carpet tiles which are removed so that the floor panels can be removed to pull in cables, the floor and floor covering clearly designed to be removed on a regular basis. in some cases enough room to crawl under the floor, although on has to be careful where one exits again, women in offices seem to complain if they find your below them looking up.