Surface Water Drainage Strategy (SuDS) for Planning

A surface water drainage strategy is the technical report that shows a local planning authority and its drainage consultee how rainfall landing on your development will be controlled, where it will go, and how it will be looked after once the site is built. It is the document most often asked for by name on an English validation list, and the one that decides whether a drainage condition is discharged quickly or drags on for months.

Unda has prepared more than 5,000 flood risk and drainage assessments across England and Wales since 2014. Our consultants are professionally qualified members of the Chartered Institution of Water and Environmental Management, including Chartered Members (MCIWEM C.WEM), and every strategy is signed off by a senior consultant before issue.

The work itself is a drainage route worked out with you: one that gives the development the strongest possible case through planning while staying buildable on the site as it actually is. Those two aims pull against each other more often than they should, and the strategy is where they get reconciled.

A strategy that satisfies a drainage officer but cannot be built is not a success. Both halves have to hold.

What a surface water drainage strategy has to prove

Four questions sit at the centre of every strategy, and a drainage officer looks for evidence against each of them before anything else.

• Rate. How fast water leaves the site, evidenced by greenfield runoff calculations for the impermeable catchment and the flow control that delivers the proposed rate.
• Volume. How much water leaves and how much is held back, evidenced by storage calculations with climate change and urban creep applied.
• Destination. Where the water goes, evidenced by a justified position in the discharge hierarchy and confirmation from whoever receives the flows.
• Quality. What the water carries with it, evidenced by the pollution risk the development creates and the treatment proposed against it.

The National Standards for Sustainable Drainage Systems also frame SuDS around amenity and biodiversity, and how far those apply depends on the scheme. A rural site with room for a basin is a different proposition to an infill plot behind a high street. A strategy worth having says which of the aims bite here; it does not treat all of them as universal tests. Where there is scope for a green solution we would rather use it: nature-based drainage usually costs less to build and lands better with a consultee than a buried tank.

Will this work, or how do we build it?

A strategy answers the first. It shows that the scheme can drain, at an acceptable rate, to an acceptable place, with the storage it needs fitting inside the developable area, and it is the document that supports the planning application.

The second belongs to the post-permission stage, where councils generally use the phrase detailed drainage strategy on their validation lists and the wording of the condition sets the scope. That is covered on our page on discharging drainage planning conditions. Knowing which you are buying matters: pay for construction-level detail that could have been conditioned and you waste money, submit outline-level work against a full application and you lose weeks.

The discharge hierarchy on your site

Standard 1 of the National Standards, published in June 2025 and updated on 30 July 2025, sets a five-tier order for where surface water should go. It is the first thing a drainage officer checks, and one of the very few parts of a strategy that does not change from council to council.

Working down the ladder is the easy part. Justifying where you stopped is the work, and it decides whether a strategy is accepted or comes back asking for more.

Ruling out a tier takes evidence, not assertion. Infiltration is discounted on test results and observed groundwater levels, or because the ground is contaminated or unstable, or because a soakaway cannot sit far enough from a structure. A watercourse discharge is discounted where there is nothing within reach, or where the riparian owner or the drainage board will not accept the flows. A surface water sewer is discounted where the undertaker confirms no asset nearby or no capacity in it. Each of those is a sentence in the report and a piece of correspondence in the appendix.

One rule is worth knowing before you start: cost alone is never sufficient justification for dropping down a tier.

That catches people out on brownfield sites, where an existing positive connection is sitting there and proving infiltration would mean a ground investigation. If infiltration is viable, the cost of demonstrating it is not a reason to skip the tier. Deliverability is a different argument on different evidence, and it does carry weight.

Seven-tier and eight-tier versions of the hierarchy are still in circulation, including in guidance that is otherwise sound. They mix two separate ideas: destination, meaning where the water ends up, and the management train, meaning how it is conveyed and stored on the way. A pond and a geocellular tank are how water is held, not where it goes. Approved Document H runs a shorter three-tier order, which is the building control position, not the planning test. Our article on the SuDS hierarchy unpicks the differences, and our note on the 2025 National Standards covers what they replaced and why they are non-statutory.

How we set the design criteria for your site

This is where the strategy is really written, and it is policy-led from the first hour. National policy sits above it all, in planning practice guidance on flood risk and coastal change, but the numbers come from closer to home. Before any calculation is run we read the council's drainage requirements alongside the Lead Local Flood Authority's technical standards, because between them they set the framework every later decision sits inside. They are the starting point, not something reached for when a preferred answer fails to come out.

Discharge rate

We calculate greenfield runoff for the impermeable catchment and work towards the 1 in 1 year rate where the site can achieve it, within the framework the council's policy sets. Some sites cannot get there: a restricted outfall, an existing connection of fixed capacity, ground that will not take a soakaway. Local policy is also the framework for agreeing and justifying a different rate, and doing that properly is an ordinary part of the process, not a failure. What it takes is the working: what was tested, why the target is out of reach here, and what the proposed rate achieves instead.

QBar and the 1 in 1 year rate are not the same thing

They get used interchangeably, including in published guidance, and a number of councils will not accept QBar at all. QBar is the mean annual flood, at a return period of roughly 1 in 2.3 years, so it gives a more generous rate than the 1 in 1 year figure. Submitting one where the authority expects the other is a straightforward way to have a strategy sent back. We check which the council works to first.

Storage is a separate question

The design event for the safe storage of water on site is the 1 in 100 year plus climate change. That is a different question from the discharge rate, and the two get conflated often enough to be worth separating on the page. The rate governs what leaves the site; the storm governs what has to be held on it while that happens.

Climate change, urban creep and brownfield

Climate change and urban creep allowances are applied per current guidance, and the figure turns on where the site is and what period the development is assessed over, not on a single national number. Our free climate change and urban creep calculator gives a first look at the effect on your storage volume. Brownfield is its own question again: discharge rate policy for previously developed land varies a great deal between authorities, and it is one of the first things we check, because it changes the storage a scheme needs and occasionally the layout with it.

You will notice there is no table of figures on this page. That is deliberate: the criteria move from council to council and site to site, so a published national set would be wrong somewhere the day it went up. We review the local position on every job instead, and our local planning authority pages set out where individual councils stand. For a first pass at your own numbers, the drainage calculator suite covers vortex flow controls, orifice throttle sizing, swale capacity, weir overflows and channel drains.

Infiltration testing and ground conditions

Ground conditions decide more strategies than policy does. If the site takes water, the hierarchy is satisfied at tier two and the whole scheme gets simpler; if it does not, everything downstream of that finding changes, because the storage grows, the outfall turns into a negotiation with a third party and the layout may have to give ground to accommodate both. It is the single most useful thing to know early.

BRE Digest 365 is the soakaway test for surface water, and the point most often missed is that the design infiltration rate is the lowest of the tests carried out, not the average. One poor pit governs the design. Our articles on what BRE 365 testing involves and whether your site needs it walk through the method and the trial pit procedure.

Timing is the part of this that saves clients a season. Many councils want groundwater monitoring through the winter, and summer-only data gets rejected.

The acceptable window varies by authority, and a programme built on the assumption that testing can happen whenever the contractor is free can lose six months to a rejected data set. So we check the local position before testing is commissioned, not after the pits are dug. Early contact buys more on drainage than on almost anything else in a planning pack.

Infiltration is not always viable. Low permeability, a high water table, contamination that soaking water into the ground would mobilise, or ground stability concerns will each rule it out, and each of them has to be evidenced, not asserted. Where infiltration is out the storage question grows: attenuation with a controlled discharge, permeable surfacing where it earns its place, or a combination worked into the landscape. For an early steer, try our BRE 365 infiltration rate calculator and our ground conditions and soakaway checker, which reports the underlying geology for any English postcode.

On sloping or awkward sites a topographical survey pays for itself early, and permeable paving carries its own expectations in planning terms.

Water quality

Every strategy has to address water quality, and it is the one requirement we routinely see missing from the drainage sections of otherwise competent submissions. Most competitor pages skip it entirely. A drainage officer will not.

The method comes from CIRIA C753, the SuDS Manual, and is called the Simple Index Approach. It scores the pollution hazard a development creates, using land use as the guide, then scores the mitigation the proposed drainage provides against it. A residential roof carries a low hazard index; a lorry park or a fuelling area carries a high one. Mitigation comes from the treatment train, so a filter strip into a swale into a basin scores very differently from a pipe running straight to an outfall. The scheme passes where mitigation meets or exceeds hazard for each pollutant assessed.

On a residential scheme this is usually one of the easier requirements to satisfy. On a commercial yard it is one of the harder ones, and you want to know which you are before the layout is drawn. Our SuDS water quality calculator runs the indices for you, and we have written separately on what C753 changed and on designing SuDS early.

Exceedance flow routing

Exceedance flow routing answers a question the design calculations do not: what happens when the rain is bigger than the system was built for. Every design event is exceeded eventually. The council wants to see that when it happens, water leaves the site along a route somebody chose, and not one it finds for itself, and that the route does not run through a doorway or off the red line into a neighbour's garden.

On a drawing this appears as flow exceedance routes: arrows and overland paths showing the direction water takes above ground, the depressions and swales that hold it on the way, and the low points where it collects. Getting them right is mostly a matter of levels and kerb lines, not pipework, which is why it is cheap at the right moment and expensive to retrofit.

We include flow exceedance routes on every drainage layout we produce. It is one of the two things authorities most consistently ask for and most consistently do not receive.

The other is the maintenance plan, and between them they account for a large share of the requests for further information that land on drainage submissions. Both are avoidable.

The maintenance plan

A maintenance plan sets out how the drainage system is kept working for the lifetime of the development. Councils ask for one on almost every scheme, and because they are asked for so routinely they are also the item most often produced from a template with the site name changed, which is precisely the version that comes back.

A plan that survives review is built component by component, so that each element of the system carries its own entry setting out what it is, where it is, what has to be done to it, how often that has to happen, and what the signs would be that it is failing.

• Permeable surfacing. A brushing regime, and a stated interval at which the infiltration performance is checked.
• Filters, silt traps and forebays. An inspection frequency and a clear-out trigger, so whoever does it knows when full is full.
• Basins, ponds and swales. Inlets and outlets checked, vegetation managed to a stated regime, and silt removed once it reaches a stated depth.
• Flow control devices. Inspected and cleared on a schedule, and accessible enough that someone will actually do it.
• Responsibility. Who carries all of that out, for the lifetime of the development.

Generic plans get rejected because they describe a system, not the system. If the schedule does not match the components on the drainage layout, or it lists a feature the scheme does not have, a drainage officer notices within a minute of opening it. Maintenance is not controversial. The problem is that the document in front of them plainly belongs to another site.

When drainage affects your layout

Occasionally the drainage requirement pushes back on the layout. Some authorities carry expectations about where drainage features can sit, how much of the storage can go underground, or whether a single feature is allowed to serve a whole site, and any one of those can change how much developable area a scheme really has.

Where that applies to your site, we would rather you heard it from us in week one than from a consultee in month four, so we flag it as early as we see it and then work within the layout you have wherever that is possible. Most schemes come to us with the plots already set. That is normal and it is not a problem. There is usually more room in a fixed layout than people assume, once storage is treated as several small features tucked into the spaces a scheme already has instead of one large one.

Where the layout is still moving, an hour of conversation before the plots are fixed saves a great deal of redesign afterwards. If your site sits in a critical drainage area, have it sooner still: the local requirements there are usually tighter.

Completing your council's proforma

In more authorities than not, the named validation requirement is a form, not a report. Some councils want the form instead of a strategy and many want both, which catches out applicants who have priced one and been asked for the other.

We complete the proforma alongside the strategy as part of the same job. That works because the strategy already contains everything the form asks for, and because there always has to be a strategy behind the form even where a full report is not what gets submitted. A proforma with no calculations behind it falls over at the first technical query.

Which authority uses which form, and what each one asks for, is covered in our article on SuDS proformas and when they are required.

How we work with you

Every strategy we write has the same aim: to give the development the strongest possible route through planning while staying practical and deliverable on the ground. Two separate tests, and the document has to pass both. In practice that means four commitments.

1. Engage early enough to influence what can still be influenced. Ground investigation timing, the discharge point and the space storage needs get expensive once they are locked, and all three are usually still open when clients first call us.
2. Work within the layout you have. We start from the scheme you are promoting, not from the scheme that would be easiest to drain, and we look for the storage inside it before we ask you to move anything.
3. Raise constraints as they appear. Where ground conditions rule out infiltration, or the only viable outfall belongs to a third party who has not yet agreed to accept the flows, that is a week-one conversation, not a covering-letter footnote.
4. Be straight about what will get past an officer. Where an approach is likely to attract an objection we say so, and we set out what the alternative costs in space, money or programme, so the decision is made with the full picture in view.

The last of those is the one that matters most and gets said least. A strategy that sails past a drainage officer and then cannot be built has only moved the problem to a stage where it costs more to solve. Where the consultee raises a query or an objection we write the response ourselves instead of leaving you to translate it; our article on LLFA objections sets out how that usually runs. There is an example of finished work on our surface water drainage strategy project page.

Do you also need a flood risk assessment?

Often, yes, and where both are required we normally write them as a single combined report. A drainage strategy deals with the water generated inside your red line in its post-development state. A flood risk assessment for planning asks the opposite question: what risk is the site exposed to, from rivers, the sea, groundwater and surface water arriving from elsewhere. Different questions, but usually the same site visit and the same data, which is why one document is generally the quicker and cheaper route.

Where the two are combined, what sits inside the flood risk assessment is normally still a full surface water drainage strategy, not a reduced version of one. The binding is different; the content is not.

No validation list we have reviewed names a combined document as a requirement, so it is a choice, not a standard. Some authorities positively encourage it: Dorset's Lead Local Flood Authority expressly permits the surface water proposal to sit inside the flood risk assessment. Where a council's list itemises the two and its officers expect two files, we produce two. Our flood risk assessment hub covers the assessment side, and the drainage strategies hub sets out the thresholds, naming and application stages that apply to both. Foul drainage is a separate exercise with its own tests and consultees, on the foul drainage strategy page.

Frequently asked questions

Speak to our drainage team

Whether you are responding to a validation list, a request for further information or a query from the drainage consultee, the quickest way forward is a short conversation about the site. Our experienced consultants will tell you what the strategy involves for your scheme, what evidence the council is likely to want and what it will cost, before you commit to anything.

Call +44 (0) 1293 214444 or email enquiries@unda.co.uk and one of our experienced consultants will come back to you the same working day.