Why Surface Water Shouldn’t Become Sewage

The next time it rains, look at the gutter on your house. Follow the downpipe down the wall and ask yourself a relatively simple question: where does that water actually go?

I suspect a surprising number of homeowners would not know the answer. Until I became increasingly involved with drainage design through planning applications, extensions and other development projects, I probably did not give enough thought to it myself. We tend to regard a downpipe as the end of the conversation. Rain falls onto the roof, the gutter collects it, the downpipe takes it away and, provided that water does not appear again somewhere inconvenient, we assume the drainage system has done its job.

The problem is that the water has not disappeared. It has simply become somebody else’s problem.

Where that rainwater goes can connect something as ordinary as a house extension, driveway or patio to a much larger conversation about sewer capacity, wastewater treatment, flooding, storm overflows and the condition of our rivers and coastal waters. Once you understand the distinction between foul water and surface water, it becomes increasingly difficult to look at an ordinary rainwater pipe in quite the same way.

I think that distinction needs to become much better understood, not only by architects, designers and developers, but by homeowners as well.

Foul water and surface water are not the same thing

At its simplest, foul water is the wastewater produced by the activities taking place inside our homes and businesses. Water from toilets, sinks, baths, showers, washing machines and other appliances enters the wastewater network and ultimately needs to be treated. At a wastewater treatment works, that sewage passes through a series of processes designed to remove solids, organic material and other contaminants before the treated water is eventually returned to the environment.

In Southern Water’s region, for example, the company operates more than 40,000 kilometres of sewer network, 363 wastewater treatment works and more than 3,500 pumping stations. Its treatment process includes screening, primary settlement, biological treatment and, where required, further filtration, disinfection and nutrient removal before treated water is returned to rivers, lakes or the sea.

Surface water is different. It is predominantly rainfall that has landed on roofs, roads, driveways, patios and other surfaces. In principle, that water does not need to be transported to a sewage treatment works simply because it happened to land on a building.

The difficulty is that historically we have not always kept these two types of water separate.

Large parts of Britain’s older sewer infrastructure use what are known as combined sewers. In a combined system, foul sewage and rainwater runoff travel through the same network. Southern Water states that more than 100,000 kilometres of combined sewers remain in the UK. Since around the 1960s, newer sewer systems have generally been constructed with separate pipes for foul water and surface water, but the older combined network remains a significant part of the infrastructure beneath many of our towns and cities.

This historical system made sense in the context in which it was created, and Britain’s nineteenth-century sewer infrastructure represented an extraordinary achievement in public health and civil engineering. The difficulty is that the demands being placed upon that infrastructure have changed enormously. Population has increased, urban areas have expanded and we have created huge additional areas of roofs, roads, parking spaces, patios and other impermeable surfaces. Southern Water itself identifies population growth, more extreme weather and increasing impermeable area as pressures on its sewer network.

Every time we replace an area of ground that previously absorbed or slowed rainfall with something impermeable, we potentially alter the way water moves through the site. Rainfall that might previously have infiltrated into soil, been intercepted by vegetation or moved slowly across the landscape can become runoff which reaches a drainage system much more quickly.

That matters enormously where the receiving system is a combined sewer.

What happens when it rains heavily?

Under ordinary conditions, wastewater travels through the sewer network to a treatment works where it can be processed. During significant rainfall, however, a combined sewer may also receive enormous quantities of rainwater from roofs, roads and other impermeable surfaces.

The volume can increase rapidly.

There is only so much water that pipes, pumping stations and treatment works can convey, store and process at any one time. If the network becomes overwhelmed, there is also a risk that sewage will back up through the system and flood homes, businesses and streets.

This is where storm overflows come into the picture.

Storm overflows are effectively relief points within parts of the sewer network. During conditions in which the system becomes overloaded, they allow a mixture of wastewater and rainwater to be discharged into rivers or coastal waters rather than continuing to build up within the network and potentially causing sewage flooding elsewhere.

That explains why they exist. It does not mean that their environmental consequences are desirable.

The scale of their use has become a major public issue. Environment Agency figures for Southern Water in 2024 recorded 977 active storm overflows. Of those with usable spill data, there were on average 30.2 spills per overflow, and the average monitored spill event lasted 10.4 hours.

Nationally, conditions improved considerably in 2025. The Environment Agency recorded 291,492 monitored storm-overflow spill events across England, representing a 35% reduction compared with 2024, while total monitored spill duration fell by 48%. However, the Environment Agency also emphasised that 2025 was significantly drier than 2024 and that storm-overflow activity is strongly related to rainfall.

That relationship with rainfall is crucial to understanding this issue.

Southern Water currently states that surface water causes 64% of storm-overflow activation within its region. The company is consequently carrying out work specifically intended to prevent unnecessary surface water entering combined sewers, including the use of sustainable drainage features such as rain gardens and tree pits.

When the water company responsible for operating the wastewater infrastructure is itself trying to stop rainfall entering that infrastructure, it seems reasonable that architects, designers and homeowners should be asking the same question at individual properties.

One house is not the problem, but thousands of houses matter

It would obviously be unreasonable to suggest that somebody building a small rear extension is responsible for sewage discharges into the sea. They are not.

The problem is cumulative.

Imagine one extension adding another thirty square metres of impermeable roof. On its own, that is insignificant compared with an entire sewer catchment. Now multiply similar decisions across thousands of extensions, new roofs, paved gardens, driveways, car parks and other developments, all contributing rainfall to drainage infrastructure.

The individual contribution may be small, but the collective contribution is not.

This is something I have had to reconsider in my own work. In the past, where a project had limited space for a conventional soakaway, I have sometimes treated connection into the existing drainage arrangement as the obvious alternative. The practical question becomes: there is water coming off this roof, so where is the nearest place we can discharge it?

I now think that question needs to be asked differently.

Before deciding where to connect a downpipe, we should first understand where the existing drainage actually goes and then ask whether that rainfall needs to enter a pipe at all.

That is a subtle change in design thinking, but potentially a very important one.

The drainage hierarchy already points us in this direction

This is not simply an environmental preference. Current national guidance for Sustainable Drainage Systems in England establishes a clear hierarchy for managing surface-water runoff from development.

The highest priority is to collect rainfall for non-potable use. The next is infiltration into the ground. After that comes discharge to an above-ground surface-water body, followed by a surface-water sewer or another piped surface-water system. Discharge to a combined sewer sits at the bottom of the hierarchy.

Importantly, the current national SuDS standards state that surface-water runoff from development should not discharge into a foul drainage system.

The standards go further than simply telling designers where water should eventually go. They encourage surface water to be managed as close to where it falls as reasonably practicable, mimicking natural drainage rather than immediately putting water into underground pipes. They also establish an objective whereby at least the first 5mm of rainfall from the majority of rainfall events should not result in runoff leaving a site for surface waters or piped drainage systems.

That represents quite a different philosophy from the traditional approach of collecting rainwater and getting it away from a building as quickly as possible.

The question becomes not simply where can we discharge this water?, but what can we do with this water before discharge becomes necessary?

Start with infiltration where the site allows it

On many domestic projects, a properly designed soakaway can provide a relatively straightforward answer. Instead of directing roof water towards the sewer network, the water is temporarily stored below ground and allowed to infiltrate gradually into suitable surrounding soil.

That does not mean a soakaway can simply be installed anywhere. Ground conditions matter enormously. Infiltration rates need to be suitable, the position of buildings and boundaries needs consideration, groundwater and contamination can affect suitability, and the system needs enough capacity for the area being drained. Appropriate investigation and design are therefore important.

Nevertheless, where infiltration is feasible, the principle is extremely attractive. Rain falls onto the property and ultimately returns to the ground relatively close to where it landed rather than being transported through wastewater infrastructure.

Where a conventional soakaway is not possible, that should not automatically mean that the next answer is a combined sewer. There are other ways to slow, store, reuse and manage runoff.

Perhaps the garden should become part of the drainage system

This is where surface-water design can become much more interesting.

Instead of thinking about drainage solely as underground pipes and chambers, we can begin using the landscape itself. Permeable paving can allow rainfall to infiltrate or be temporarily stored rather than immediately becoming runoff. Rain gardens can receive water from roofs and hard surfaces and temporarily hold it amongst appropriately selected planting. Storage tanks can retain rainfall for later use. Green roofs can slow the rate at which rainfall leaves a roof. Depending on the site, appropriately designed ponds and other water features can potentially provide storage, amenity and ecological value while incorporating a safe overflow for larger rainfall events.

I particularly like the idea of water being visible within a landscape rather than always hidden underground. A pond does not necessarily need to be enormous or elaborate. On an appropriate site, a modest and carefully designed water feature could receive suitable surface runoff, temporarily store some of it and provide habitat at the same time. There are obviously considerations around safety, water quality, maintenance, overflow arrangements and suitability for the particular property, so this should not become a universal prescription that every garden must contain a pond. The broader principle is that drainage infrastructure does not always have to look like drainage infrastructure.

This also connects with the biodiversity-conscious approach to design. A rain garden can manage runoff while supporting planting and insects. A green roof can slow rainfall while creating habitat. A pond can store water while supporting aquatic and terrestrial wildlife. Permeable landscaping can reduce runoff while changing the character of an external space.

The national SuDS standards explicitly encourage this kind of multifunctionality. Surface-water systems are expected not only to manage flooding, but where possible to contribute to water quality, amenity and biodiversity.

That is a far more interesting approach than simply drawing a pipe from the downpipe to the nearest chamber.

Existing houses deserve attention too

New development is increasingly expected to consider sustainable surface-water management as part of the design process. The more difficult question is what happens with the enormous number of existing properties connected to historic drainage systems.

If an existing roof, extension or paved area is already discharging into a combined sewer, there may be circumstances where that surface water could be disconnected and managed differently. Southern Water is itself carrying out projects based around precisely this principle: keeping additional surface water out of combined sewers to reduce pressure on the network and reduce storm-overflow activation.

That does not mean homeowners should start lifting manhole covers and altering drainage connections without understanding what they are doing. Existing drainage arrangements can be complicated, and what appears to be a foul sewer may actually be combined, while apparently separate systems can contain historic misconnections. Where the arrangement is unclear, drainage routes may need to be properly traced, surveyed or investigated before alterations are made.

The important first step is awareness.

When designing an extension to an older property, we should not automatically assume that because a rainwater pipe already enters a particular drain, the new roof should do exactly the same thing. The existing arrangement may reflect decisions made many decades ago under completely different circumstances.

A new extension presents an opportunity to do something better.

What about greywater?

There is a related conversation around greywater, although I think it is important not to confuse it with the main surface-water issue.

Water from baths, showers and washbasins can potentially be collected and reused through properly designed greywater systems for suitable non-potable purposes, including certain forms of irrigation or toilet flushing. More sophisticated systems can treat and store this water before reuse.

That is an interesting proposition because it raises a wider question about how resourceful our buildings are with water. We pay to abstract, treat and deliver drinking-quality water to our homes, use some of it once for washing ourselves, and then immediately send it away for wastewater treatment.

However, not every form of household wastewater should simply be redirected into a garden. Kitchen and dishwasher wastewater can contain food residues, fats, detergents and higher levels of contamination, and any greywater-reuse system needs appropriate design and management.

For the purposes of surface-water drainage, the simpler and more immediate opportunity remains the water falling freely onto our roofs and gardens every time it rains. Before developing complicated systems for recovering wastewater from inside a building, it seems sensible to become much better at managing the relatively clean rainfall arriving outside it.

The environmental and public-health consequences are real

Public concern around sewage discharges has increased enormously in recent years, particularly in coastal communities.

Channel 4’s 2026 factual drama Dirty Business brought some of those concerns into mainstream television, examining Britain’s sewage problems through the experiences of people and communities affected by polluted waterways. One of the cases referenced was that of eight-year-old Heather Preen, who died from an E. coli O157 infection following a family visit to Dawlish Warren in 1999. Her family has long believed sewage contamination was responsible. It is important to distinguish that belief from a proven causal link between a particular storm-overflow discharge and her death, but the case illustrates why contamination of recreational waters is not merely an aesthetic or environmental concern.

Sewage pollution can present genuine risks to human health, just as it can damage aquatic and coastal environments. The response cannot therefore simply be to accept ever-increasing volumes entering ageing infrastructure and assume treatment works and storm overflows will deal with the consequences indefinitely.

Water companies have a substantial responsibility here. Infrastructure needs investment, maintenance and upgrading, treatment capacity needs to respond to population growth and environmental regulation needs effective enforcement. The Environment Agency has itself said that sustained maintenance and investment in drainage networks is necessary to produce lasting improvements, and billions of pounds are now committed to storm-overflow and wider water-infrastructure investment.

Those responsibilities should not be shifted onto individual homeowners.

At the same time, the fact that large infrastructure improvements are required does not mean that the design of individual properties is irrelevant. Both things can be true. Water companies can be required to improve the network while architects, developers and homeowners simultaneously stop adding unnecessary surface water to it.

We need to stop treating rainwater as somebody else’s problem

This is ultimately where I think the architectural profession has a role.

When we design an extension, the drainage strategy should not be an afterthought added once the floor plan and elevations have been completed. We should understand the existing drainage, establish whether foul and surface water are separate or combined, consider the suitability of infiltration and then look at what opportunities exist to slow, store, reuse or incorporate rainfall within the site before connecting it to downstream infrastructure.

Sometimes the answer will be a soakaway. On another site it may involve permeable paving and a rain garden. Somewhere else there may be an opportunity for above- or below-ground storage. A larger garden might accommodate a pond or another landscape feature capable of receiving suitable runoff. A flat roof might contribute through a green or blue roof strategy. Often the best solution may involve several relatively modest interventions working together rather than one large piece of drainage infrastructure.

There will also be sites where infiltration is impossible and storage or landscape solutions cannot reasonably deal with all of the design rainfall. In those circumstances, a properly controlled discharge to an appropriate surface-water system, or ultimately a combined sewer where permitted and justified, may still be necessary. Sustainable drainage is not about pretending that every drop of rain can always remain indefinitely on every property. It is about avoiding unnecessary runoff and controlling the water that does eventually have to leave.

That distinction is important because I do not think homeowners need another environmental lecture or another expensive requirement imposed upon every small extension. What we need is greater awareness of the consequences of a very ordinary design decision.

A downpipe is not simply a line drawn down the side of an elevation.

It is the beginning of a drainage system, and the decision about where that water goes can ultimately connect an individual building to infrastructure serving thousands or millions of people.

Britain’s historic sewer system has provided an extraordinary public-health service for generations, but parts of that network are under considerable pressure from population growth, urbanisation, impermeable development, groundwater and rainfall. At the same time, we are seeing the consequences when wastewater infrastructure cannot contain everything entering it.

Against that background, deliberately sending additional clean rainwater into wastewater infrastructure without first considering the alternatives increasingly makes very little sense.

We already have the basic tools required to approach this differently. We can infiltrate water where ground conditions allow it. We can store it. We can slow it. We can use permeable surfaces. We can create rain gardens and appropriately designed water features. We can harvest rainfall for non-potable uses. We can design landscapes and roofs that temporarily hold water rather than immediately shedding it.

None of these measures alone will solve Britain’s sewage problem, and it would be misleading to pretend otherwise. The sewer network itself requires enormous investment and improvement.

But every development still has to decide what happens to the rain that lands on it.

Before drawing another rainwater pipe connected to an existing drain, perhaps the first question should therefore be the simplest one:

Where does this water actually need to go?

If the answer is that it can safely remain, infiltrate, be stored or be put to useful purpose on the property, then perhaps it never needed to become part of the sewage problem in the first place.

What are you planning to build?

Extension

Loft Conversion

New Build

Garage / Annex