Drainage & The 2026 SuDS Revolution

How The National Standards are Shaking Up Deep Drainage, Surface Water, and Every New Development

 

Since January 2026, every new development in England has had to jump through a new hoop before work can even start on the first trench. The DEFRA National Standards for Sustainable Drainage, which were published on June 19th 2025, have fundamentally changed the rules on how surface water and deep drainage are supposed to be designed, approved and maintained. This article breaks down just what these new standards require, where traditional drainage still has a place, and what every developer, engineer and planner is having to get to grips with right now.

Quick Facts: What’s Changing in 2026 for UK Drainage?

Getting surface water management right is crucial for keeping our cities and towns resilient – and the new rules reflect just how urgent the situation is. A staggering 4.6 million properties in England are sitting in areas where surface water flooding is a big risk, with 1.07 million of those at real high risk of being flooded. The old ‘pipe it all away as fast as we can’ approach just wasn’t working, so the new standards are a reaction to that.

Three big changes are having have a big impact:

• Mandatory SuDS approval becomes the rule. Local authorities are now setting up SuDS Approving Bodies (SABs) to review and sign off on drainage strategies separately from planning permission. So even if you’ve got full planning consent, you won’t be able to start any serious groundworks until the SABs have given you the green light.
• The ‘first 5mm’ retention rule kicks in. New surface water drainage designs have to be able to cope with the first 5mm of rainfall on site, which catches about 80% of summer rainfall and 50% of winter events – that’s roughly 80% of the first bit of rain in the summer and 50% of the first bit of rain in the winter across most of the country. That means you can prevent rain from running off into sewers on a normal day.
• Deep-bored soakaways are no longer the default option. The Environment Agency and the Lead Local Flood Authorities are actively advising against deep-bored soakaways as the go-to surface water solution. They say there are risks of groundwater pollution, structural problems and the bypassing of natural filtration systems – so that’s no longer the default approach.

From now on, every new development – from a single plot to a massive new estate with 500 homes – has to integrate SuDS compliant surface water drainage and deep drainage all the way from the early planning stages. If you don’t get it right, you could end up with a planning refusal or some serious delays. If you deliver surface drainage services, deep drainage services or MAC Group drainage services then the scope of the job has just changed overnight.

How we got to this point: a little history

The groundwork was laid back in 2010 when the Flood and Water Management Act was passed with a bit of legislation called Schedule 3 which set up the framework for SuDS Approving Bodies. Wales got on the case back in January 2019 and now SAB approval is mandatory for most new developments there, but England has been lagging behind.

Here are the key dates in the story so far:

• 2010: The Flood and Water Management Act got the Royal Assent, which was when the framework for SABs was laid out.
• 2019: Wales actually started using Schedule 3, which meant that SABs became a real thing there.
• 2020: DEFRA reported that the drainage infrastructure in England is just getting older and older – and we’re not investing enough in it.
• July 2021: London got hit with nearly 100mm of rain in just a couple of hours and the city got badly flooded. Over 2,000 properties were underwater and 30 Underground stations had to be closed. The Fire Brigade got something like 1,755 calls in just four hours.
• 2023: The Government came out with the Sustainable Drainage Systems Review, which recommended implementing Schedule 3 but ministers thought they’d do something different instead – they decided to change planning policy instead.
• 2024: The National Planning Policy Framework was updated to make developers include more SuDS in their plans for new developments.
• June 2025: DEFRA finally published the National Standards for Sustainable Drainage.

By the time mid-2025 came around, English planning applications were more than twice as likely to be put on hold because of SuDS design issues compared with planning applications in Wales where SABs had been in place for six years. It didn’t take a genius to see that the writing was on the wall – things had to change, and fast.

What the new standards are all about

At its heart, drainage is just about getting rid of water – whether it’s from the surface or from underground. The National Standards build on that by saying that every drainage system has to mimic how water behaves in the wild: it needs to soak into the earth, get picked up by plants and flow slowly across the surface rather than rushing through pipes.

Effective drainage systems stop flooding and erosion – and they also help protect the soil and the slopes from the damage that uncontrolled water can do. The seven new standards are an attempt to get all this sorted:

1. The runoff hierarchy. Surface water has to be sent in the right order: first to non-potable reuse, then to infiltration, then to the water bodies above ground and then to the surface water sewers. Only as a last resort should you have to use combined sewers.
2. Everyday rainfall. You need to be able to cope with the first 5mm of rain on site.
3. Extreme rainfall. You have to design for the big events – and make sure the overflow goes somewhere safe.
4. Water quality. Pollutants have to be removed through a treatment process before you can let the water go.
5. Amenity. The drainage features themselves need to be usable and attractive.
6. Biodiversity. Supporting wildlife habitat creation or enhancement is essential.
7. Design for strength, operation, maintenance, and decommissioning. Everything has to be buildable, easy to keep in good nick, and easy to get rid of when its life ends.

Just one sentence sums up the philosophy behind all seven standards: “Sustainable drainage systems mimic natural drainage processes”.

The “First 5mm Rule”: how surface water drainage is being redefined

The first 5mm rule is probably the most talked-about requirement in the 2025 standards and here’s why: new surface water drainage designs must either keep the first 5mm of every rainfall event on site through features like soakaways, or treat it before any of it ends up running off the boundary.

Good drainage stops water pooling around buildings and paved areas, while surface water drainage keeps water from the heavy rainfall from flooding places. The first 5mm rule ensures that the majority of rain events don’t result in any runoff at all.

So what does this all look like in practice?

• A supermarket car park built back in 2010 likely just had impermeable asphalt, gullies and a single connection to a combined sewer. Under the new rules, a redesign would see permeable paving across at least part of the car park, rain gardens in the planters between the parking bays, and storage beneath the surfacing to hold that first 5mm.
• A new housing estate must include features like permeable driveways, soakaways in gardens, green roofs on the garages and tree pits along the access roads.
• A school could use the margins of the playing field as shallow detention areas, and the field itself as an infiltration surface.

Engineers now have to do some specific design checks to prove the design meets up with the new rules:

• Hydrological modelling of pre- and post-development runoff and things like infiltration and evapotraspiration for the first 5mm event
• Infiltration testing to BRE 365 standard, monitoring the level of the water table
• Proving that at least 80% of summer and 50% of winter events get intercepted
• Making sure that “retain” means infiltration, evapotraspiration or temporary storage and slow release – piping water straight to a combined sewer just won’t cut it

From pipes to landscapes: what a good SuDS design looks like nowadays

The contrast between old and new is really stark. Traditional hard-engineered surface water drainage relied on gullies, pipes and big underground tanks to get water off site as fast as possible. Modern SuDS design, on the other hand, integrates water management into the visible landscape.

Sustainable drainage systems help keep a lid on stormwater flooding, while also delivering some real amenity and habitat. Modern drainage approaches combine a few different methods rather than relying on a single system. The 2025 standards actually favour “treatment trains”, which are just sequences of multiple SuDS components that provide gradual filtration, storage and release.

Common SuDS tools – by development type:

Development type	Typical SuDS features
Housing estates	Permeable block paving on driveways and patios, rain gardens, swales along access roads, detention basins at site boundaries
Schools and hospitals	Green roofs, bioretention planters, shallow detention on playing fields, tree pits
Distribution centres	Permeable paving in yard areas, filter strips along building perimeters, attenuation ponds or wetlands near outfall
Retail parks	Permeable paving in car parks, swales between units, constructed wetlands before discharge

Swales are shallow, vegetation-lined channels that redirect surface water runoff. Dry wells just hold water temporarily and release it back into the subsoil. Both of these features are appearing in compliant SuDS designs because they are easy to build, simple to maintain and deliver some real water quality benefits.

Site constraints always shape the palette. Where the ground conditions are poor clay soils, high groundwater or contaminated land, infiltration features just won’t work. In those cases, above-ground attenuation (basins, ponds) or controlled discharge to a surface water sewer becomes the sensible solution.

Deep drainage in the SuDS era: what still needs a traditional system

SuDS are all about surface water. Deep drainage systems, on the other hand, are used to get rid of water from built up areas underground – they are still essential for foul sewage and combined systems, and for strategic trunk sewers serving big catchments. They help keep on top of stormwater and sewage when surface level features can’t do the job on their own.

Subsurface drainage systems keep water from reaching the soil table. French drains intercept groundwater to stop hydrostatic pressure against buildings and structures, while deep drainage prevents waterlogging and maintains the structural integrity of roads and buildings. All of these functions still need to be done, even under the new standards.

Where deep drainage is still needed for surface water:

• Dense city-centre plots where infiltration is just not possible due to clay geology, contaminated fill, or zero open space
• Sites with high water tables where shallow soakaways would just sit below the water table
• Connections to existing deep public sewers where there is no surface water sewer or watercourse nearby

The Environment Agency and loads of Local Flood and Water Management Authorities (LLFAs), including East Sussex County Council, are now very much discouraging the use of deep-bored soakaways for surface water drainage. Their reasons for doing this are because deep-bored soakaways can bypass the near-surface soil layers that naturally filter out pollutants, which risks contaminating the groundwater and often don’t perform all that well in infiltration testing. Somerset LLFA have taken it a step further by classifying deep-bored soakaways as “not a suitable infiltration SuDS technique” in their judgment.

Deep drainage services are unfortunately unavoidable in a lot of projects and that’s not likely to change anytime soon. The requirement is to marry them up with shallow SuDS features so that regular rainfall gets handled at the surface and only excess flows make it to the deep system at a controlled rate.

Surface water vs foul water: keeping the systems separate – it’s crucial

Drainage systems are typically split up into surface drainage and subsurface drainage, but for regulatory purposes the more important distinction is between surface water and foul water.

• Surface water: rainwater, snow melt, and runoff from roofs, roads, patios and any hardstanding areas
• Foul water: wastewater from toilets, sinks, showers, and industrial processes

Surface water drainage directs the runoff to sewer systems or soakaways, whereas foul water gets sent to treatment works via gravity sewers or pumping stations. The 2026 SuDS framework is reinforcing this separation and making sure it happens.

Misconnection problems are a common issue. In older housing across England from the 80s, roof downpipes were often tied into foul drains. During heavy rainfall these connections would flood the foul system, triggering combined sewer overflow events that discharge raw sewage into rivers and watercourses. The new standards make it very clear that new surface water connections to foul or combined sewers won’t be accepted where alternatives are available.

A compliant 2026 site layout looks something like this:

• Rainwater falls on roofs and surfaces, and is directed through gutters and downpipes into plot-level SuDS (rain gardens, permeable paving and soakaways)
• Overflow from SuDS feeds into site-level features (swales, basins) before discharging to a watercourse or surface water sewer at a controlled rate
• Foul flows from buildings connect to a separate deep gravity sewer network, running to the nearest foul or combined public sewer via manholes and connections sized for the development.

Surface water drainage systems: from plot level to strategic networks

Surface water drainage systems comprise gutters and stormwater drains at the micro scale. Curbs and gutters catch runoff from pavement and funnel it to storm drains. Catch basins are box-like structures that collect water, allowing sediment to settle before it carries on downstream. These components make up the first tier of any surface water drainage system.

The hierarchy of control runs through three levels:

1. At-source control (plot level). Gutters, yard drains, ACO channels, permeable surfacing on driveways and patios. In 2026, rainwater landing on a roof should pass through a downpipe and into a rain garden or soakaway within the plot boundary and only overflow to the site network in bigger storms.
2. Site control (public realm). Swales along access roads, filter strips beside car parks, detention basins at site boundaries. These features collect and clean water before it leaves the development.
3. Regional control (off-site). Strategic attenuation ponds, wetlands or connections to existing watercourses or rivers managed at catchment scale.

Designers are now expected to demonstrate this hierarchy in their drainage strategy reports, showing how each level connects up and how the complete drainage system performs across a range of storm events.

The new SuDS Approving Bodies (SABs): who are they and what they check

SABs sit within local authorities (typically unitary or county councils) and mirror the Welsh model that’s been in place since 2019. Their role is distinct from the planning committee: a development can hold full planning permission and still be blocked from starting groundworks until SAB approval is granted.

SABs review drainage strategies against all seven National Standards. A number of things they take a close look at:

• Greenfield runoff rate calculations and how the development limits post-development discharge to match
• First 5mm retention methods and the modelling evidence behind them
• Exceedance routing for storms beyond the 1-in-30-year event
• Treatment train design showing sequential pollutant removal
• Adoption and maintenance proposals, including funding and ownership
• Climate change allowances applied to storage and pipe sizing
• Biodiversity and amenity benefits delivered by the drainage plans

Getting verification that all seven checks have been successfully completed is the key to SAB approval. Incomplete submissions, missing hydraulic models, or vague maintenance plans are the most common causes of delay.

Navigating the approvals maze: planning, SABs, water companies, and the EA

A typical 2026 development has to coordinate with five different bodies. Here’s the sequence:

1. Pre-application discussion with the Local Planning Authority and LLFA to get a sense of local drainage constraints and policies
2. Outline drainage concept prepared at RIBA Stage 2, testing the runoff destination hierarchy and identifying which SuDS features are likely to suit the site
3. Detailed drainage design produced at RIBA Stage 3, getting down to the nitty-gritty of the drainage strategy, sizing up pipes, and selecting SuDS features.
4. SAB approval sought before the drainage strategy report is submitted.
5. Water company and EA approval sought after that, as the water company and Environment Agency will need to check that the drainage system meets their needs as well.3. SAB Pre-Consultation Check: to make sure the proposed strategy is likely to meet the 7 key standards before actually making a formal application
6. Full SAB Application: with all the details – hydraulic modelling, treatment train design, how to handle overflow, maintenance plans, and what happens when you need to take the project over
7. Section 104 / S106 Agreements with the water company – this is where surface water or sewers that connect to your engineering meet the standards
8. Environment Agency Consent – this is where discharges affect rivers or need special permits

Why SAB applications can be delayed

• You don’t have all the hydraulic modelling done (or have missed out on important climate change scenarios, the first 5mm analysis)
• You don’t have a proper maintenance plan in place or unclear funding arrangements
• You’ve left it too late to sort out the water company – they might object to capacity, cost you time and money
• You’ve ignored where flood risk zones are – makes for a lot of headaches

Get a drainage expert on board from the very start of the project. Plan integrated foul and surface water strategy from the start, so you don’t have to do costly redesigns down the line.

Designing for Climate Change: Rainfall Intensities, Storage, and Overflow

Rain is coming down harder and more often – heavy rain events are on the rise according to climate projections. The 2025 standards mean we need to allow for 20% to 40% more rain than before to design in some extra safety. This means:

• Storage tanks and pipes need to be big enough to handle the worst case for a 1-in-30 year storm (no water on the site) and a 1-in-100 year storm with some extra rain added to it (no water in buildings)
• We need to make sure water is flowing away from buildings during extreme events – that means designing roads, car parks, and open spaces to get that water away from buildings
• Gravity systems are always preferred over pumps – but sump pumps can be a useful last resort for surface water management

We use the Flood Estimation Handbook, the Revitalised Flood Hydrograph method, and the Wallingford Procedure to work out how a catchment will react to the new climate conditions.

Water Quality, Pollution Control, and SuDS Treatment Trains

By law, we need to make sure we’re keeping hydrocarbons, heavy metals, and silt from urban runoff under control – not just as an extra. SuDS help with all that – and also improve biodiversity. SuDS can help to clean the water before it gets into the rivers.

A SuDS treatment train might work like this:

1. Permeable pavement in the car park to catch all the fine sediment and light oils\
2. A swale along the edge of the site to slow down the water, filter out some particulates and let some of it soak in\
3. A detention basin at the lowest point of the site to store excess water, let the solids settle out\
4. A constructed wetland before it goes into the watercourse – that picks up the remaining nutrients and pollutants\
5. The outfall goes to the sewer or watercourse at a controlled rate – with clean water

Where there is pollution risk, we can still use proprietary oil separators and interceptors – like fuel depots, haulage yards, and busy lay-bys.

Combining Deep Drainage and SuDS on Constricted Urban Sites

When you are in a dense city centre, you’ve got all the hurdles – limited space, high costs, utilities, deep sewers, and existing buildings. You need to make sure you get it right – precise gradients for deep drainage, and all the right settings for effective flow. Accurate setting out is crucial – when you’re working with millimetres of leeway.

We can use hybrid solutions:

• A podium deck with planters over a car park – that’s where structural waterproofing meets bioremediation
• Blue-Green Roofs store rain in the drainage layer beneath a planted surface, letting it out slowly
• Under-street attenuation tanks connected to existing infrastructure – with flow controls
• Shallow Conveyance Channels at ground level that link to deeper pipes via drop manholes

Sump pumps can be useful when you can’t use gravity drainage to the sewer due to level constraints, and water starts building up in the basement.

A City Block Redevelopment on a Victorian Sewer: How to Succeed with SuDS

Take a close look at a 0.4-hectare city block redevelopment that’s been done right. The site sits on top of a 4 metre deep Victorian combined sewer system – not exactly the most glamorous start. And things only get tougher with clay geology and contaminated fill stopping any chance of infiltration. The design solution: green roofs on all the buildings to catch 50% of annual rainfall, permeable paving on the courtyard to soak up the first 5mm of rain, and a secret underground tank hidden beneath the courtyard. This tank is fed by a vortex flow control that stops the stormwater from down-pouring into the sewer – all at a respectable 5 litres per second. The designers knew they needed to work together – surface water system and deep drainage working in perfect harmony – because neither system on its own would meet the new standards.

Making SuDS & Deep Drainage Work for a Lifetime

The 2025 National Standards are no joke when it comes to drainage. Every single element of the drainage system has to have a named person responsible and a maintenance budget set aside – for the life of the development. That’s the only way to get the green light from SABs – no vague ownership or funding plans are allowed.

Ownership & Funding Breakdown

Element	Typical adopter
Public highway drains and swales	Local authority highways department
Open SuDS features (basins, wetlands)	SAB / local authority, or management company
Sewers serving multiple properties	Water and sewerage company (via Section 104)
Private plot features (soakaways, rain gardens)	Individual property owner
Watercourse banks and channels	Riparian landowner

Don’t even think about skipping maintenance – these systems need regular TLC to keep them working as designed. Some of the regular tasks include:

• Quarterly desilting of inlets & catch basins
• Twice annual mowing of detention basins and swale margins
• Annual CCTV inspections of manholes & pipes
• Every 1-2 years: clear out debris from permeable paving joints using specialist vacuum equipment\
• After every big storm: test penstocks, flow-control devices & orifice plates

For a small housing estate with 50 homes, annual maintenance costs for SuDS features come in at a few thousand pounds, usually covered by a management company service charge. For a big logistics park with 2 hectares of attenuation basins and a wetland, you’ll need dedicated grounds staff or a contractor to keep everything running smoothly – with an annual budget in the tens of thousands.

Don’t even get me started on what happens when you don’t keep up with maintenance. A blocked flow control can turn a detention basin into a permanent pond in no time. And a silted-up permeable paving surface will be impermeable in three years if joints aren’t kept clean.

SuDS, Deep Drainage & the Real Cost Story

The biggest myth out there is that SuDS always cost more than traditional drainage. Oh, but multiple case studies across the UK show the opposite: using landscape features to store and treat water cuts the volume of underground storage needed, reduces pipe diameters, and saves on excavation costs. Capital costs for SuDS-led schemes are often comparable to or lower than traditional systems.

And it’s when you start looking at lifecycle costs that the story really changes. SuDS features require regular landscape maintenance, while traditional underground systems need less frequent but more expensive interventions (CCTV surveys, root cutting, relining). Neither approach is maintenance-free.

Non-Financial Benefits

We also have to talk about non-financial benefits that affect project viability. Improved house saleability, higher BREEAM scores, biodiversity net gain contributions – all of these contribute to project viability and give you a better return on your investment.

A Cardiff Scheme To Watch – Canal Quarter

Cardiff’s Canal Quarter is a fantastic example of SuDS done right in the heart of a bustling city. A historic canal was reopened after being buried beneath hardstanding for years. The scheme combined traditional deep drainage with surface-level features: open canal channels, rain gardens & tree pits integrated into pedestrian streets and public squares.

Key Details

• The reopened canal channel carries surface water runoff from surrounding areas and provides a focal point for cafes, seating & art
• Rain gardens along the canal edges treat the first flush of water from adjacent roads, filtering silt and hydrocarbons before it enters the canal
• The scheme won a national award in 2024 for innovative urban drainage and placemaking
• Post-completion monitoring showed a measurable reduction in surface water entering the combined sewer system, easing pressure on downstream infrastructure and reducing combined sewer overflow frequency

The lessons from Cardiff are clear: early collaboration between drainage engineers, landscape architects & urban designers produces a scheme where regulatory compliance drives better placemaking.

• Putting water at the heart of public realm design has certainly generated some serious commercial value: with footfall and property interest in the Canal Quarter shooting up following completion you can see why
• The project has also had a lasting influence on the thinking behind the 2025 National Standards, proving – in no uncertain terms – that visible water management features can easily coexist with dense urban environments

How drainage experts support compliance: from feasibility to as-built – a full lifecycle

The full project lifecycle for getting drainage compliance sorted out now runs from feasibility all the way through to as-built verification:

1. Feasibility drainage review – getting to grips with it : A desk study to get a handle on geology, existing drainage infrastructure, receiving watercourse capacity and local SAB policies
2. Outline strategy for planning: A drainage concept that’s aligned with the seven National Standards – all submitted alongside that planning application
3. Detailed design and modelling: Using hydraulic models (such as MicroDrainage, InfoDrainage, or some other equivalent) to prove out first 5mm retention, attenuation volumes, exceedance routing, treatment train performance – all that sort of thing
4. SAB submission: Everything including drawings, calculations, maintenance plan, adoption strategy, and climate change analysis all wrapped up in one neat package
5. Construction support: Setting out, inspecting the work on site, dealing with site conditions that differ from the original desk study – that kind of thing
6. As-built verification – signing off the work: A survey to confirm that the installed work matches what was approved – so we check levels, areas, volumes, and flow controls all line up with the design

Experienced teams are now coordinating surface water, foul water, and highways drainage alongside the likes of structural, architectural, and landscape design. Drainage services, whether its surface drainage or deep drainage, are now no longer separate commissions – they need to be integrated from day one. Its MAC Group drainage services that make a great example of this – we handle both SuDS features and traditional networks under one coordination framework.

By getting in on the act early – maybe even at that first optioneering workshop where drainage engineers sit down with the architects and landscape designers at RIBA Stage 1 – costs can be kept down and the costly pattern of retrofitting SuDS into a fixed layout can be avoided. Phased strategies for multi-plot sites make more sense – each phase connecting in logically to site-wide attenuation and treatment features. And getting expert help to deal with SAB queries keeps things on track, and all the delays that come with late or incomplete submissions are avoided.

What developers should do now: adapting to the new SuDS requirements

Here’s a practical checklist of what developers should now have in place:

• Audit your live and pipeline projects. Check every scheme in your programme for SuDS implications – any site without a compliant surface water strategy is at risk of refusal or condition.
• Get your teams up-skilled. Make sure your project managers and site managers can get their heads round the seven standards and can explain them to their design teams and clients in clear and simple terms.
• Bring in the drainage experts early on. From RIBA Stage 1 or 2, get the first 5mm rule and SuDS treatment trains baked into the layouts before plot positions and road alignments get fixed in place. Retrofitting SuDS into a finished masterplan only costs money, causes delay, and produces sub-par results.
• Chat to your local SABs. Each SAB will have their own local preferences and capacity constraints – being proactive about this saves time. Have a chat with them about preferred SuDS features, local infiltration rates, and adoption arrangements.
• Get in touch with water companies and the Environment Agency. Understand what the sewer capacity is, what discharge consent requirements are, and what environmental permits your sites will need.
• Review your maintenance and adoption budgets. Make sure you’ve got adoption timelines, long term funding, and management company arrangements all built into your financial models from the off. SABs will check all this at submission.
• Treat water as a design element, not an afterthought. Its the developments that treat water as a design element – visible, clean, contributing to the environment and protecting every property on site from flooding – that will perform well under the 2026 regime.

The standards are out, the SABs are getting staffed up, and the likelihood of refusal for non-compliant drainage plans is getting higher with every planning cycle. Developers who get it and build it into their processes from day one will build faster, sell better and spend less on redesign.