There are a lot of things to consider. You may be able to use the same wires as your extra low voltage lamps used but more likely you will need to replace all cables as most low voltage systems need earths.
Extra low voltage up to 50 volt and low voltage is 50 to 1000 volt high voltage over 1000 volt a.c. So I assume you are referring to extra low voltage when you say low voltage.
Unless your in Scotland you will most likely need the paper work submitting to building control under Part P.
Basically your question is far too vague to give an answer it's like asking how long is a piece of string.
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