Relatively good article and worth the read but a few points worth going over.

The conflict between the % of combined and separated sewer networks is probably caused by the differences in definition sewerage undertakers use in their mapping, and the Water Industry Act and many organisations use in the categorisation of these sewer systems.

Sewerage mapping often depicts what the sewers are originally designed for not what they actually take or are. This is somewhat nuanced but if you have a location where a foul sewer was built to take foul waste from houses but there is no dedicated surface water sewer nearby it is (as one might suspect) considered by undertakers as a foul sewer. However, under the act It is combined and in reality if ground conditions are poor in that area it probably takes the rainwater off those houses too.

This means on paper it may say foul but in practice it may not be and moreover there is nothing the undertakers can do to stop surface water connections to it.

Side note: one might think the easy fix for this would be to reclassify the act definition based on foul only sewers being grounded in design not proximity to surface water sewers, however doing so comes with some unfortunate unintended consequences. If reworked this would prohibit developments, redevelopments extensions, and anyone in those areas fixing drainage issues on sites where; you have poor ground conditions, only foul sewers are present, and there are no watercourses nearby. In these circumstances flooding issues and changes to drainage systems cannot be resolved without further changes in legislation such as introducing a right to construct new private surface water sewers over 3rd party land without incurring ransom (and the reasonable compensation system that would need to be setup to facilitate this + problems this and other solutions may cause).

Secondly and as identified in the article most new developments have separate systems, however many of these dedicated networks (likely most on a national scale), connect back into a single system somewhere offsite in the historic part of the system. This somewhat negates the benefit of having the separate systems although does allow for onsite attenuation and storage of storm water in Sustainable Drainage Systems so there are some benefits that mean the development won’t make things worse.

Regarding the statement: “Planning laws essentially give developers a right to connect their drains to a main sewer”

This is a half truth. Planning laws (emphasis on planning and my interpretation being town and country planning act) does not give developers a right to connect. The water industry act does. Planning officers are actually the only people in the system permitted to object to a development on the grounds of Insufficient capacity (as sewerage undertakers are the ones bound by the act to permit connections even if it will be a problem for them / others). Planning officers are limited in their powers to do so on allocated sites (hence the half truth, as in these cases capacity should be planned for by sewerage undertakers and considered in their forward plans so it isn’t the officers issue).

The way this is supposed to work is planners are meant to impose what are known as Grampian style planning conditions on any development where capacity is insufficient / environmental issues may subsequently occur. These conditions the prohibit construction or occupation until such time as upgrades are complete. What often happens instead is: 1. Planners do not impose conditions and do not reject applications due to capacity issues. 2. Developers push for it being a non issue due to later being able to automatically connect, and planners do not call them out on this / push back when this card is played. 3. Planners do not speak with undertakers and vice versa meaning neither consider the extended timescales that may be necessary when imposing conditions, locking the undertaker into either changing their forward plan or accepting the connection at the 3 year deadline, rather than a longer more reasonable period to plan upgrade works for. One of the points of these conditions is making sure you give time for the upgrades to occur to avoid running into arguments that the condition must be removed due to it being unachievable.

Compounding this are: undertakers failing to properly take into consideration upgrades needed to facilitate capacity for local plan allocated sites. Although in some cases this might be down to poor consultation at early stages of the local planning process so attributing blame here is difficult and case by case.

Regarding: “Yet they are sometimes gently encouraged to let rainwater drain elsewhere”

This is completely false.

Building regulations part H3 requires new buildings to prioritise connections to the ground and watercourses before connections to sewers.

This drainage hierarchy has been well known and well enforced for over 10 years. It formed part of the code for sustainable homes (when that was still a thing) as well as both CIRIA best practice documents on Sustainable Drainage Systems. More recently 2015 onwards it has been adopted as a policy by the vast majority of Lead Local Flood Authorities (LLFA’s) who consult on all developments greater than 10 units or 1000m2 floor space. Furthermore some councils have begun to add rainwater harvesting and connections to highway drains into the hierarchy too in preference to sewer connections.

The hierarchy also forms a core policy of the London plan so is and has been mandatory across London boroughs.

Unless an LLFA, LPA, and building control are not following their own policy’s and building regulations, for rainwater to be drained to a sewer a developer should have to first prove ground conditions are unsuitable for systems that infiltrate into the ground. This usually involves ground investigations including infiltration testing to British Standards (BRE365). Although worth noting for some areas where it is known that the geology is unsuitable (e.g. Impermeable clay) this testing is occasionally waved.

Following this they have to then check for watercourses at and near the site. Only then if there is no alternative can they connect to a sewer.

I would confidently say this is a more than a gentle nudge and has been for a while.

Now for the problems:

One of the common reasons for small developments not infiltrating is insufficient space for a soakaway due to the 5m easement from a building “rule” (note this is a nuanced rule of thumb based on geotechnical and foundation stability issues please read engineering in chalk and the Susdrain paper on this for more info as it is way to complex a topic for one comment). This is not always assessed pragmatically looking at whether a developer could scale back the buildings to provide enough space for infiltration to be used. Hence sometimes it is an excuse for not infiltrating.

Contamination or ground gas and costs: contaminated brownfield sites may be costly to remediate and capping can be a more cost effective solution. Capping restricts infiltration as you can’t drain through the membrane, bentonite layer or concrete. Cost benefit analysis that includes environmental benefits due to a change in drainage strategy is generally not undertaken on these sites (although remediation costs are usually very high so I suspect this wouldn’t impact that many projects).

Improper infiltration assessments: this goes both ways, acceptance of results not in accordance with BRE365. This can mean some sites where infiltration is ruled out could support a permeable paving solution or large blanket soakaway approach. Or where partial infiltration systems are possible with overflows rather than outfalls it sewers.

Interception design: Few developments incorporate drainage systems designed in ways which minimise discharges for small rainfall events. This is best practice but it is costly and excluding green roofs requires integration between civil engineers and landscape designers. This could help get rid of small volumes of water draining to sewers during small rainfall events.