Yes I agree on very large developments. But on developments such as less than 200 house/ flats they do not, but in a town where it is easy to have at least 2-5 of these developments a year it would soon mount up.Actually they do, on the bigger schemes at least.
Several very large developments are being proposed around here (2000+) and the promoters have had to show whether Thames Water have the capacity to accept their waste. In one case (Harlow North) the cost of upgrade is so high that the developer proposes to build a new sewage works on site serving the development. The other (Birchall Garden Suburb) has to contribute to an upgrade at Rye Meads Sewage Works before they can begin.
Drinking water, on the other hand... I raised the issue at the local plan examination that Affinity Water would struggle to supply the extra 16000 houses proposed in our district (plus the 24000 in neighbouring East Herts plus the many 1000's in their supply area in West London) and the Inspector said it was "outside the scope" of the planning system although he agreed it was mad.