Professional standards
It's that sort of thing that reminds you why legislation is brought in to prevent DIY electrical work.
When I bought a house a couple of years ago I had the electrics checked. It turned out that two double 13A sockets in the sitting room were being fed via a 0.5mm^2 flex taken as a spur off the cellar lighting circuit. That arrangement was the work of professionals and had been signed off by professionals when, before I bought it, the house was let out.