The entirety of the built environment (so not just new build, everything that exists now) contributes less than 5% of all river pollution.
The old nutrient neutrality rules were stopping all development unless the developer could pay into an offsetting scheme, working with a farmer or sewage treatment provider to reduce their pollution. The biggest polluter is agriculture. So the developers are paying about an extra £6-8k per home they build to provide a farmer with a solution to stop phosphate and nitrate run off from their fields - basically new housing had to foot the bill of sorting out pollution that comes from other industries, and that cost then gets passed on to whoever buys the new house. Completely blunt regulation that didn't target the main causes of the problem. All it has done is make it very hard to build certain areas, many of which have a high level of housing need - the Solent and Kent particularly, and make the new homes which were built more expensive.
What Government should have been doing, and are starting to do now with the new £280m fund, is more firmly regulating farming activity, and requiring water companies to upgrade waste water treatment infrastructure before giving massive annual payouts to shareholders.
You'll see that the RSPB have had to apologise for their original statement on this announcement.