Pardon my language for a moment, but Explanatory Booklet my a*se. The following words illustrate exactly what a bunch of t*ss has been written in the pursuit of being "explanatory" (sic.).
[code:1](r)(i) If I want to install fittings such as a WC, shower, washbasin, or
kitchen sink within my home, or any other type of building,
will the Building Regulations apply?
YES – if the installation of the fitting or fittings will involve
alterations to, or new connections to, a drainage stack or an
underground drain. You should also check with your Building
Control Service to find out how much information is required
about the fittings you intend to use.
NO – provided the work only involves installing an additional
fitting or fittings in the same or a new location which does not
involve extensions to the drainage pipework.
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The two paragraphs aren't even mutually consistent! Apparantly, if a change involves the soil stack but not extending the drainage network, then the BR's both apply and don't apply.