Absolutely correct its not a change of use, however it is a material alteration under Part A and B there's also a change in energy status, other regs may apply dependant on the exact nature of the work,
The professional disagreeing is not a BCO and doesn't always fully understand the legislation, however there will always be situations where interpretation can vary due to ambiguous wording, it also doesn't help when some authorities interpret things in such a way as to make life easier, for example I worked somewhere where we treat conservatory extensions on pubs as a window display to a shop to get over Part L {thermal insulation) requirements, even though we knew this weas wrong!
Application is needed, installation of a controlled fitting, Part M could apply if you are moving the WC from a different position.
It did say "could", however there was no mention it was a conversion in which case it won't apply. It COULD apply to a part M compliant flat though as a material alteration.
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