Drainage Strategies for Planning

A drainage strategy is a site-specific technical report that sets out how a development will manage the water that falls on it and the foul flows it creates, to a standard the local planning authority and its drainage consultee will accept. In England it is the evidence councils expect wherever a proposal changes how a site drains, and it is normally submitted alongside, or within, a flood risk assessment.

Reciting policy back at the council is the easy version of this document, and it is the version that gets picked apart. What we produce is a drainage route worked out with the client, chosen because it gives the development the strongest possible case through planning while remaining 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 are reconciled.

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 (CIWEM), including Chartered Members (MCIWEM C.WEM), and every drainage strategy is checked and signed off by a senior consultant before it is issued.

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

What is a drainage strategy?

A drainage strategy is a technical report submitted with a planning application that sets out how surface water and foul drainage will be managed on a development site. It has to demonstrate three things: that runoff is controlled to an acceptable rate and volume, that it is discharged to an acceptable destination, and that water quality is managed.

The National Standards for sustainable drainage 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, so a strategy worth having says which of those aims bite here rather than treating all of them as universal tests.

Two boundaries define the document. It deals with runoff generated within the site, and with the site in its post-development condition. That is what separates it from a flood risk assessment for planning, which asks the opposite question: what risk is the site exposed to, from rivers, the sea, groundwater and surface water running onto it from elsewhere. Plenty of applications need both. Where they do, we normally write them as one report.

Strategy or detailed design?

A strategy answers "will this work?". A detailed design answers "how do we build it?". Our rule of thumb: if the outstanding question is how a solution will be designed, it can usually be dealt with by condition; if it is whether the thing will work at all, it has to be resolved before permission. Our article on what drainage information is needed for planning permission takes that distinction further.

A drainage strategy goes by many names

No two councils call this document the same thing. At least 35 distinct titles are in live use across English councils for what is substantially the same deliverable, and on the district validation lists we reviewed, the single most common exact wording appears on only about 12 per cent of them. Clients arrive unsure whether the thing their council asked for is the thing we sell. Almost always, it is. The titles fall into families rather than into a council-by-council list:

• Surface Water Drainage Strategy. The most-used technical term, and the closest thing this document has to a standard name.
• Drainage Strategy. Either bare, or bundled into a combined title such as "FRA and Drainage Strategy".
• Sustainable Drainage Strategy. Turns up most often in the wording of drainage conditions.
• SuDS Strategy or SuDS Report. Common across London boroughs.
• Drainage Assessment. The ambiguous one, because in a good number of councils that phrase names a form rather than a report.
• Surface Water Drainage Statement. Used interchangeably with the strategy titles above.

Behind the confusion sit two naming systems that frequently disagree: what the Lead Local Flood Authority calls the document in its technical guidance, and what the local planning authority calls it on its adopted validation list. In Surrey, to take one example, county drainage guidance and a borough validation list can ask for the same evidence under two different titles.

What matters is narrower than the naming suggests: the document has to answer the requirement on the validation list and satisfy the drainage authority's technical standard. A report titled "Flood Risk Assessment and Drainage Strategy" meets the naming expectation of essentially every authority we have dealt with.

The title on the cover is the least important part of the document.

Which drainage document do you need?

Drainage for planning is one discipline, but the document you need depends on the question it has to answer. Unda prepares all of the following, and this page is the right place to start if you are not yet sure which applies:

• "I need to show how surface water will be managed." The main event: the discharge hierarchy, runoff rates and volumes, attenuation storage, water quality and exceedance flow routing, worked through for the site: surface water drainage strategy.
• "The council wants its SuDS proforma completed." There always has to be a strategy behind the form, and our report normally contains everything the form asks for, so we complete the two together: SuDS proforma completion.
• "Where does the foul go?" Mains connection, off-mains treatment, pumping and water company pre-development enquiries, for sites on and off the public sewer: Foul Drainage Strategy.
• "My permission is blocked by a drainage condition." Post-permission work, scoped by the wording of the condition rather than by a standard template: discharge of drainage planning conditions.
• "Do I need a flood risk assessment as well?" Often yes, and where both are required we usually write them as a single combined report: flood risk assessment for planning.

If none of those quite fits, or you simply want a steer, call +44 (0) 1293 214444 or send us the site address and the exact wording the council used, and one of our experienced consultants will tell you which document the application needs before any fee is involved.

When is a drainage strategy required?

More often than most people expect, and the threshold usually quoted is incomplete. The statutory baseline is the definition of major development in the Town and Country Planning (Development Management Procedure) (England) Order 2015. That definition has five limbs, not three: minerals; waste; 10 or more dwellings, or a site of 0.5 hectares or more where the dwelling count is not yet known; 1,000 m² or more of floorspace; and a site of 1 hectare or more.

That last limb is routinely dropped from consultancy summaries, and it is the one that catches people out, because a single low-density commercial building sitting on a 1.2 hectare plot counts as major development even though it fails every one of the other tests. Worth checking your site area before you assume you are below the line.

Below the statutory line, many councils go further, and the triggers fall into recognisable categories: new or increased impermeable area, basements, sites within a critical drainage area, and sites near or containing a watercourse. Which applies, and at what threshold, varies by authority, so we check the local position for each site. Our local planning authority pages set out where individual councils stand.

Most of the market has also missed a change at national level. Since December 2024, paragraph 182 of the National Planning Policy Framework has applied to applications that could affect drainage, proportionately, at all scales. Only the three-limb list within it is specific to major development: take account of the drainage authority's advice, meet appropriate minimum operational standards, and set out maintenance arrangements for the lifetime of the development.

Most of the market still describes sustainable drainage as a major-development requirement. It has not been one since December 2024.

Outline, full and reserved matters

The application type sets the level of detail, and the gap between what an outline submission needs and what a full one needs is wider than most people expect.

1. Outline applications. The job is to prove the principle before a layout exists: that the site can drain, that there is a viable discharge point, and that the storage the scheme needs can physically fit within the developable area. You are not designing a system at this stage. You are demonstrating that one is achievable.
2. Full and reserved matters applications. The layout is fixed, so the level of detail rises: the drainage layout itself, storage sized and located, the flow control identified, flow exceedance routes drawn on the plan. Some councils give the document at this stage a different name on their validation list, calling it a detailed drainage strategy.
3. Discharge of conditions. Where permission is granted with a drainage condition attached, the work continues after the decision notice, and the condition wording sets its scope. That is covered on our discharge of drainage planning conditions page.

Scoping this wrong is expensive in both directions. Paying for design-level detail that could have been conditioned wastes money; submitting outline-level work against a full application costs weeks, and occasionally a validation refusal. We establish which stage the application is at before we quote, and we say so where a client is being asked for more than the application requires.

What goes in a drainage strategy

Every local planning authority publishes its own list of what it wants to see, and we build the strategy against that list rather than a house template with the site name changed. Two authorities either side of a boundary can ask for materially different evidence about the same kind of scheme. A common core turns up on most lists regardless:

• The site and its red line boundary, which governs the catchment areas everything else is calculated from.
• Existing drainage arrangements and outfalls. What is already there, where it goes, and what condition it is in.
• Existing and proposed impermeable areas, shown as a drawing rather than only as a figure in a table.
• Ground investigation and infiltration testing where infiltration is proposed as the discharge route.
• The discharge hierarchy justification, with reasons for ruling out each higher tier rather than a bare statement of the chosen option.
• The discharge point, with confirmation from whoever receives the flows: the sewer undertaker, the riparian or watercourse owner, or the highway authority.
• Greenfield and proposed runoff rates and volumes, with the method stated openly so a drainage officer can follow it.
• Attenuation storage, with climate change and urban creep allowances applied.
• The design storm calculations, presented as output a reviewer can check.
• Exceedance flow routing, showing where water goes when the system is overwhelmed.
• Water quality, addressed rather than assumed.
• A maintenance plan naming who is responsible for the lifetime of the development.

Two of those are worth singling out. Exceedance flow routing and the maintenance plan are among the most consistently asked for by authorities and the most consistently missing from submissions. We include flow exceedance routes on every drainage layout we produce. A missing maintenance plan is one of the commoner reasons an otherwise sound scheme attracts an objection from the drainage consultee, and gaps in the drainage information are a routine cause of applications being invalidated at the front door.

The standards we work to

Planning policy sits at the top of the chain. The version of the National Planning Policy Framework in force is December 2024, technically amended on 7 February 2025, and chapter 14 carries the flood risk and drainage policy. The Planning Practice Guidance on flood risk and coastal change was last updated on 17 September 2025, though that update dealt with the sequential test rather than the sustainable drainage paragraphs, despite being reported in places as a tightening of drainage requirements.

Below policy sit the National Standards for Sustainable Drainage Systems, published in June 2025 and last updated on 30 July 2025. They replace the 2015 non-statutory technical standards, set out seven standards, and establish a five-tier hierarchy for where runoff should go: collected for non-potable use, infiltrated to ground, discharged to an above-ground surface water body, discharged to a surface water sewer, and to a combined sewer only as a last resort. We have written separately on what the 2025 standards changed and how the hierarchy is applied.

The National Standards for SuDS are non-statutory guidance, not law. Councils give them force through planning policy, which is a different thing.

Alongside those we work to CIRIA C753, BRE Digest 365 for infiltration testing, BS 8582 for surface water drainage on development sites, and Approved Document H, which sets the building control position rather than the planning test. Satisfying one does not automatically satisfy the other, and we have seen schemes come unstuck on that assumption. Then your council's own standard, which in practice is the one that decides the job. Authorities are adopting the 2025 National Standards on different timescales, so we check which version applies to your site before we start.

How we work out what your site needs

Discharge rate is where most drainage strategies are won or lost. 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 own policy sets. Policy is the starting point for that work, not something reached for when the preferred answer fails to come out.

Some sites cannot achieve the target rate: a restricted outfall, ground that will not take infiltration, an existing positive connection that constrains what is possible. Local policy is also the framework for agreeing and justifying a different rate, and doing so is an ordinary part of the process rather than a failure. What matters is that the case is properly made, with the evidence a drainage officer needs to accept it.

Brownfield sites are their own question. Discharge rate policy for previously developed land varies a great deal between councils, and it is one of the first things we check, because it changes the storage a scheme needs and occasionally the layout.

You will not find a table of figures on this page, and that is deliberate. The criteria change 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. For a first look at where your own site might land, our free drainage calculators will get you a long way, and the detail of how criteria are set is covered on the surface water drainage strategy page.

How we work with you

Every drainage 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. Those are two separate tests. The document has to pass both.

In practice that means engaging early enough to influence what can still be influenced. Most clients come to us with a layout already fixed, and there is usually more room in a fixed layout than people assume. Where the layout is still moving, an hour of conversation before the plots are set saves a great deal of redesign afterwards.

Most clients come to us with a layout already fixed. That is normal, and it is what we design around.

It also means raising constraints as they appear rather than at submission. 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 rather than a covering-letter footnote. And it means being straight about what will get past an officer: where an approach is likely to draw an objection from the drainage consultee 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. You can see how that reads on the page in our surface water drainage strategy project.

What it costs, and why we quote per site

There is no single price, because the work is scoped to the site. Two schemes with the same dwelling count can be very different exercises depending on what the ground does and what the council asks for. The main drivers are:

• Site area. Larger catchments mean more contributing areas to model and more discharge options to consider.
• The scale and type of scheme. A residential extension, a 200-unit site and a commercial yard are three different pieces of work.
• How many discharge options need testing. Where infiltration is plainly viable the answer comes quickly; where it has to be ruled out and a third-party outfall pursued, it does not.
• What the council's requirements list demands. Some authorities want considerably more evidence than others for the same scheme.

Every Unda quote is fixed-fee, free and tailored to the site, and we set out what is included before you commit. Tell us the site address and what has been asked for and the quote comes back within 60 minutes on a working day.

Why choose Unda for your drainage strategy

Most developers commission a drainage strategy once or twice; we produce them every working day. That experience shows in the documents and in how they land with the people who review them.

• 5,000+ assessments since 2014. Residential, commercial and strategic sites across England and Wales, for developers, agents, architects and self-builders.
• Chartered, CIWEM-qualified consultants. Unda is a CIWEM Business Partner and the team includes Chartered Members (MCIWEM C.WEM), with every strategy checked and signed off by a senior consultant before issue.
• Daily contact with drainage consultees. We work with Lead Local Flood Authority standards and validation requirements constantly, which is how we know what a given team will accept before anything is submitted.
• Flow exceedance routes on every layout. One of the two things authorities most often ask for and most often find missing; it is standard on the drainage layouts we produce.
• Tools and technical depth. Around twenty free calculators in our drainage calculator suite, alongside more than 200 technical articles.
• Follow-through after submission. Where the drainage consultee raises a query or an objection, we handle the response rather than leaving it to be translated.

Local requirements by authority

Drainage is local in a way national policy is not. The same rainfall behaves differently on chalk, on clay and on a made-ground brownfield plot, and two neighbouring councils can ask for very different evidence on the same catchment. We publish detailed local guides so you can start from your own patch: browse our local planning authority pages, where each council's Local Plan policy, SFRA and validation requirements are set out, or work through the picture town and county by town and county or river by river. Our article on how local planning authorities handle flood risk and SuDS explains the relationship between the planning authority and the drainage consultee, and where flood risk is also in play the flood risk assessment hub is the place to start.

Two neighbouring councils can ask for very different things on the same catchment, so the right strategy starts with the right local requirements.

Frequently asked questions

Speak to our drainage team

A validation list, a condition on a decision notice, a query from the drainage consultee: whatever has brought you here, the quickest way forward is a short conversation about the site. Our experienced consultants will tell you which document you need, what it involves 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.