Who changed the Design? Who is instructing the change? What is the nature of the change to the design? Sorry for the quiz, but I think these answers are required before we can usefully suggest an answer to your query.

Sorry Zoltan, I misunderstood your meaning. The answer will partly depend on whether the contractor has leeway and authority to make design changes, or has to react to design changes (often referred to as "AIs" or Architect Instructions) arising after the construction phase has commenced, or potentially both. Where the Contractor has to react to an AI, then the Client should be telling you what he requires of you in his pre-construction information. Even where you are given authority to make significant design changes (and there's limited scope for that in electrical work in Housing stock) I would still expect the Client to be providing you with the ground rules. Otherwise, its down to reacting to local circumstances, maybe having to re-route cables due to something "unforeseen". This usually comes down to accurate recording on a marked-up drawing and promptly advising the CDM-C (unless other comms. and authorisation arrangements have been stipulated by the Client). Short answer: the person asking you should be providing you with some clues here?!