I suspect trades do not always stick rigorously to the regs. My house was an incredible mess. I think their had been a bodgers convention at my house. The porch frame was built from what looked like scrap wood, the ceiling was bits of hardboard. I replaced the frame and ceiling. At the sides of windows plaster had cracked after double glazing was put in, and wall papered over. When I had my bathroom done, a light switch fell off the ceiling, it was screwed into plaster board. The wall socket came off, of two screws only one was fixed, the other screwed into a small bracket that was not attacked to the mounting box. I repaired both. Some trades are brilliant, some are worse than bad DIY.