Flood Risk Assessment and Drainage Strategy: Do You Need Both?

Most applications that need one of these documents need the other too. A flood risk assessment and a drainage strategy answer different questions: the first asks whether the site floods and whether your development makes flooding worse somewhere else, the second explains where the water goes once the site is built. Councils routinely ask for both, and government guidance is explicit that they can arrive as two documents or as one.

That point causes more trouble on flood and drainage submissions than any other. An application goes in with a competent assessment and no drainage information. Or a drainage strategy arrives for a site the Environment Agency was always going to want an assessment for. Either way the application stops. It stops at validation or at consultee stage, which are the two most expensive places for it to stop.

GOV.UK's planning guidance, updated on 28 May 2026, now sets out both requirements on one page: an assessment where the site is at risk, and a sustainable drainage strategy where the development could affect drainage on or around the site.

What a flood risk assessment assesses

A flood risk assessment establishes the flood risk to your site and the flood risk your development creates elsewhere. It works through every source that could reach the land, sets a design flood level with a climate change allowance applied, and shows the scheme will be safe for its lifetime without pushing water onto anyone else.

The scope is wider than the flood map suggests. That is where a lot of assessments come unstuck.

• All the sources, mapped or not. Rivers and the sea are two sources among several. Surface water, groundwater, sewers, reservoirs and infrastructure all count, and several of them have no national map to check.
• The lifetime of the development. Housing is assessed over 100 years. That means a climate change allowance chosen for the location and the epoch, not today's flood level.
• Risk in both directions. Risk to the site, and risk from it. Lost floodplain storage, diverted overland flow, raised ground levels: all of it has to be accounted for.
• What is left after mitigation. Finished floor levels, safe access and escape, and the residual risk that survives once the defences and the mitigation are in place.

If you are still working out whether the site needs one at all, we have set the triggers out separately in when do I need a flood risk assessment.

What a drainage strategy explains

A drainage strategy explains how surface water and, where it applies, foul water will be managed once the site is built. It justifies where the water goes, how much leaves the site and how fast, how it is stored and treated on the way, and who is responsible for maintaining the system for the lifetime of the development.

Standard 1 of the National Standards for Sustainable Drainage Systems sets a five-tier order of preference for surface water destinations, and cost alone is never sufficient justification for dropping down a tier.

The surface water half is the part most people picture: greenfield and proposed runoff rates, attenuation storage sized with climate change and urban creep applied, water quality, the discharge hierarchy worked down tier by tier, and the routes water takes when a storm exceeds the design event. The 2025 National Standards are the reference point for most of it, and they are worth reading for what they are: non-statutory guidance, not law. Plenty of clients assume otherwise.

The foul half is usually simpler. On around nine schemes in ten the work is identifying where the foul water goes and confirming the receiving party will take it, rather than modelling a network.

The two documents answer different questions

They overlap at the edges, and both end up discussing surface water, but they are commissioned for different reasons and read by different people. The Environment Agency is the statutory consultee for flooding from rivers and the sea. The Lead Local Flood Authority has been the statutory consultee on surface water drainage for major development since 15 April 2015. Neither reviews the other's document, and neither is impressed by a report that answers the wrong question well.

So do you need both?

Usually yes, and often inside the same report. Government guidance settles the second half of that directly.

Binding them together is the normal approach rather than a shortcut. It is also the safer one on naming. A report titled "Flood Risk Assessment and Drainage Strategy" meets the naming expectation of essentially every authority in Unda's July 2026 review of drainage requirements across around 145 English councils. That review also turned up at least 35 distinct document titles in live use for substantially the same deliverable, and the single most common exact string appeared on only about 12% of the district validation lists we checked. Matching your council's precise wording matters much less than covering both questions properly.

There is a wrinkle. In more authorities than not the named validation requirement is a form rather than a report, and in outer London 13 of 17 boroughs name a proforma or assessment form. A proforma still needs a strategy behind it. The form asks for numbers, and the numbers have to come from somewhere.

A Flood Zone 1 site can still need drainage information

Flood zones describe the probability of flooding from rivers and the sea. They say nothing about what happens to rainwater landing on your roofs and hardstanding. A site can sit squarely in Flood Zone 1, with no fluvial or tidal risk worth discussing, and still need a full drainage strategy, because the trigger for drainage information is whether the development could affect drainage, not which zone it falls in.

Since December 2024 the NPPF has applied its sustainable drainage expectation to any application which could affect drainage, at all scales, rather than to major development alone.

Most of the market still describes SuDS as a major-development requirement. It has not been one since the December 2024 framework. What is proportionate now sits in how much detail an officer expects, not in whether anything is expected at all, and that is a harder conversation to have with a client who has read three consultancy websites saying otherwise.

A Flood Zone 1 site may need an assessment in its own right as well, if it covers a hectare or more, if the flood map shows it at risk from surface water, or if it falls within an area with critical drainage problems.

How much detail is expected, and when

A drainage strategy is not automatically a full detailed design, and at application stage it is not supposed to be. It demonstrates that a workable solution exists and satisfies policy. The detail follows after permission, and we have set that boundary out in full in what drainage information planning permission needs.

The stages run like this.

1. At application. The strategy establishes the discharge destination and rate, the storage volume, the water quality approach, the flow exceedance routes and the maintenance responsibility. Enough for an officer to be satisfied the scheme works, and enough for the layout to be sound.
2. In the decision notice. The remaining detail is conditioned. The condition wording sets the scope of what follows, and it varies a great deal between authorities. Levels and gradients are asked for on some schemes and not on others.
3. After permission. A detailed drainage strategy discharges the condition, typically covering the sizing of attenuation storage, the flow control specification, the calculation output, the exceedance flow routing plan and the maintenance plan.

Councils use their own vocabulary for that last stage. Salford's validation list distinguishes a "drainage strategy" at outline from a "detailed drainage strategy" at full and reserved matters. Borrowing the authority's own terms in your covering letter saves a round of correspondence.

Where submissions come unstuck

Two things are simultaneously the most consistently requested by authorities and the most consistently missing from submissions: exceedance flow routing and the maintenance plan. Both cost almost nothing to include when the strategy is written. Both are expensive once an LLFA objection has landed. We put flow exceedance routes on every drainage layout for that reason.

• A proforma with nothing behind it. The form gets completed from assumed figures because no strategy was ever prepared. The numbers do not reconcile, and the LLFA asks for the evidence.
• Infiltration assumed rather than tested. A soakaway-led scheme with no BRE Digest 365 results. Worse, groundwater monitoring is often wanted through the winter, and summer-only data gets rejected. That can cost a season.
• Foul left as "connect to mains". A connection is expected where the public sewer is within 30 metres, and that distance is multiplied by the number of dwellings. Ten dwellings, 300 metres. Beyond that you are into off-mains, and septic tanks, package treatment plants and cesspools are not interchangeable.
• The wrong test cited. BS 6297 percolation testing is for foul drainage fields. BRE Digest 365 is for surface water soakaways. Quoting one where the other belongs invites a request for further information, and the confusion is common enough that officers look for it.
• Capacity taken on trust. A water company pre-development enquiry is a procurement item with a lead time, not a desk check. Thames Water's pre-planning enquiry confirms capacity in around three weeks and issues a letter you can submit with the application.

Get any of these wrong at validation and the application does not start. We have written separately on what gets an application invalidated on flood risk and drainage grounds.

How we work with you

The question we start from on every job is the same: what gives this development the strongest route through planning while staying practical and deliverable? Those two pull against each other more often than anyone would like, and the useful work sits in the resolution rather than in reciting the requirement.

In practice that means working with the layout you already have. Clients almost always arrive with something drawn, and that is normal rather than a problem. It means tailoring the strategy to the specific authority's list instead of issuing a template, because what an LLFA asks for in its technical guidance and what the LPA asks for on its adopted validation list can differ inside the same county. And it means being straight with you early about where the evidence is thin, so a winter of groundwater monitoring or a three-week capacity enquiry lands in your programme rather than in your determination period.

Policy is the framework throughout, not something we reach for when a preferred approach fails. We calculate greenfield runoff rates for the impermeable catchment and work towards the 1 in 1 year rate where it is achievable. Where it is not, local policy gives us the framework to agree and justify something else. Worth saying plainly, because the two get conflated: QBar and the 1 in 1 year rate are not the same thing, and a number of councils do not accept QBar.

If you are working out what your application needs, ask us. Unda prepares flood risk assessments for planning and drainage strategies across England and Wales, separately or bound as one report, covering surface water and foul drainage, and we complete the council's proforma alongside the strategy where one is required.

Frequently asked questions