Should I leave in old pipe work and electric cabling?
In order to comply with current building regulations, if your electric wiring and/or plumbing works are not done to current building regulations, and they do change every couple of years, you are required to replace them in order to be able to get an electric certificate.
A common sense of safety should suggest not using old pipes. You could of course argue that they have worked for years, so why should there be a problem now; or that it would require additional work, particularly if the pipes are cast in concrete (which the builder will charge you for if this was not an expected cost at tender stage).
However, from a health and safety point of view, old pipes used to be manufactured from lesser-grade materials than today’s copper or plastic pipes, thus they can be prone to releasing parts of themselves into the drinking water and this could even lead to illness.
You may also find that your plumber advises you that some connections between various materials the pipes are made from are not very stable and therefore it’s not possible to reliably weld some pipes to others.
Old wiring is potentially very dangerous and considered unsafe by today’s standards, and any DIY additions can make it even worse. While a pipe connection can leak and cause you trouble or damage, a faulty or unprofessionally installed (or even uncertified electric wiring) can cause much more harm! Please do not attempt to add or modify an existing installation yourself, always hire a certified electrician to do this for you.