And on this particular issue, there are those who seem to read what isn't there.
"Must be no worse than existing," is a phrase quoted a lot, but nowhere in the regulations does it say any such thing.
They may not contain that exact phrase.
But what they do say is this:
An alteration is material for the purposes of these Regulations if the work, or any part of it, would at any stage result -
(a)in a building or controlled service or fitting not complying with a relevant requirement where previously it did; or
(b)in a building or controlled service or fitting which before the work commenced did not comply with a relevant requirement, being more unsatisfactory in relation to such a requirement.
and this
Building work shall be carried out so that, after it has been completed -
(a)any building which is extended or to which a material alteration is made; or
(b)any building in, or in connection with, which a controlled service or fitting is provided, extended or materially altered; or
(c)any controlled service or fitting,
complies with the applicable requirements of Schedule 1 or, where it did not comply with any such requirement, is no more unsatisfactory in relation to that requirement than before the work was carried out.
So I think that "Must be no worse than existing" is quite reasonable.