Flood risk and drainage in the planning process: where they fit at every stage

If you are applying for planning permission for the first time, the part most likely to catch you out is not the design or the neighbours. It is flood risk and drainage in the planning process. Flooding and surface water sit quietly underneath almost every stage, and an application that is otherwise sound can stall for months because the right flood report was missing, or because the drainage was left until it was too late to change. This guide walks through the planning system in plain English and shows exactly where flood risk and drainage attach at each step.

The short version: flood risk is a matter of principle. It helps decide whether your development belongs on the site at all, so it has to be settled early, not bolted on at the end. Drainage is partly principle and partly detail, which is why some of it is fixed at the start and the rest is finished later under a planning condition. Miss the timing and the fix is expensive; get it right and flooding barely slows you down.

Flood risk reaches almost every planning decision through a single door in national policy: the development should be steered away from the highest-risk land and, where it must go there, made safe for its whole lifetime without pushing flooding onto anyone else.

First: is your project even "development"?

Planning permission only bites on "development", and the law (section 55 of the Town and Country Planning Act 1990) splits that into two things: building or engineering works, and a material change in how land or a building is used. If what you are doing is neither, you need no permission, and no flood test applies to it at all.

That sounds like a way out, and sometimes it is. But the catch matters for anyone near water: raising ground levels, forming an attenuation basin, laying a new surface water sewer, building an outfall or culverting a ditch are all engineering operations. They are development in their own right, even when they are only there to serve a scheme, so they carry their own flood and drainage questions rather than riding along for free.

• Usually not development: internal works that do not change a building's external appearance, and use of a house's garden for something incidental to living there.
• Usually not development: a water company or council inspecting, repairing or renewing an existing sewer, main or pipe, but not laying a new one.
• Development in its own right: regrading a site, raising levels, forming basins or ponds, and constructing outfalls or culverts, which are the very things a drainage scheme is made of.

The routes to planning permission, at a glance

Most proposals reach a decision by one of a small number of routes, and the route you take sets the timescale, the fee and, crucially, when flood risk has to be resolved. You rarely get to pick freely; the nature and scale of the project usually decide it for you.

Whichever route applies, the flood and drainage questions do not go away; they simply attach at a different point. The rest of this guide follows them stage by stage.

Where flood risk and drainage fit: the short version

Two documents do most of the work. A site-specific flood risk assessment (FRA) shows how likely the site is to flood, how bad it could be, and how the development is made safe without increasing risk elsewhere. A surface water drainage strategy shows where the rain that lands on the site will go, how fast it is allowed to leave, and who maintains the system afterwards. On many sites you need both, and they are the two items most often named on a council's validation list, the checklist an application has to pass before it is even registered.

Behind those two documents sits national policy: Chapter 14 of the National Planning Policy Framework (the December 2024 edition), backed by the Government's Planning Practice Guidance on flood risk. It is worth knowing that an FRA has to address six sources of flooding, well beyond the rivers and sea shown on the familiar flood map. All six count: rivers, the sea, surface water (rainfall that cannot soak away), groundwater, sewers, and reservoirs or other artificial sources.

Full planning applications: everything is judged at once

On a full application the whole scheme is decided in a single go, so all of the flood and drainage evidence has to be there up front. In a flood risk area that means the FRA, plus (where they are triggered) the two policy tests that decide whether the development should be in that location at all.

The first is the sequential test, which asks whether there is a reasonably available site at lower flood risk that could take the same development. The second, the exception test, only comes into play for more sensitive uses in higher-risk zones, and asks two things: do the wider benefits to the community outweigh the flood risk, and will the scheme be safe for its whole lifetime? These are questions of principle, and once a full permission is granted they are settled.

Alongside the FRA, a full application in most cases needs a surface water drainage strategy showing the discharge point, the rate water is allowed to leave the site, the storage needed to hold back the rest, and how the system is maintained once it is built. If you want the detail of what a council expects to see, our guide to the drainage information needed for planning sets it out.

Outline permission: why flood risk cannot be saved for later

Outline permission settles the principle of a development and leaves the details for a later "reserved matters" application. It is a common and sensible route for larger sites. But there is a trap that catches a lot of people: flood risk is not one of the things you are allowed to reserve for later.

The law fixes exactly five reserved matters: access, appearance, landscaping, layout and scale. Flood risk and drainage appear nowhere on that list. Because flood risk goes to whether the development should be on the site at all, the sequential test, the exception test and the question of which uses are acceptable in the flood zone all have to be answered at the outline stage. They cannot be revisited once outline permission is granted.

The practical reason this matters is land. Attenuation basins, buffer strips and flood-storage areas all compete for the same ground as the buildings. Fix a dwelling count at outline without a drainage strategy and you may have fixed a number the site cannot actually deliver once the drainage is drawn. The most valuable time to size the drainage is before the outline pins the numbers down.

Permitted development and prior approval: the "automatic" routes still have a flood catch

Some changes do not need a full application because permission is granted nationally through permitted development rights: many home extensions, and popular change-of-use routes such as an office to homes (Class MA) or a barn to a house (Class Q). It is tempting to assume flood risk does not apply here. It does.

On these prior-approval routes, flooding is one of the specific matters the council must consider, and a site-specific FRA is still required where the building sits in Flood Zone 2 or 3. There is a widely believed shortcut that catches people out: changes of use are exempt from the sequential test, but that exemption does not remove the requirement for a flood risk assessment. A conversion can be refused on flood grounds alone. We cover the two most common cases in detail: the office-to-residential (Class MA) and barn conversion (Class Q) routes.

Turning an office or a barn into homes raises the flood "vulnerability" of the building, because people sleep there now, so a site that was fine for its old use can need a fresh flood risk assessment for its new one.

Who decides: the Environment Agency and the lead local flood authority

Two bodies do most of the deciding on flooding and drainage, and it helps to know which is which, because they cover different things and are consulted at different points. Neither of them, importantly, can veto your application. They advise the council, which makes the decision.

• The Environment Agency: the statutory consultee on flooding from rivers and the sea. On a major scheme in a flood risk area, a sustained EA objection does not refuse the application, but it can force the council to refer a proposed approval to central government, which means delay.
• The lead local flood authority (LLFA): your county or unitary council's drainage arm, consulted on surface water drainage for major development. In practice the LLFA's response usually becomes the drainage condition on your permission.
• Everyone else advises: water companies, internal drainage boards and the highway authority all have a say on drainage, but none is a general statutory consultee, so their comments carry weight without carrying a veto.

If an objection does land, the first job is to work out what kind it is: a lack of information (usually fixable with a technical note) or an unacceptable level of risk (which needs a design change). We explain how to lift an Environment Agency objection and an LLFA objection separately, and what a statutory consultee actually is.

Conditions: where most drainage work actually happens

Very few permissions arrive with the drainage fully designed. Instead the council grants permission subject to a drainage condition, and the detailed design is submitted afterwards to "discharge" it. This is normal, but it is not a licence to leave everything to the end. The principle (can the site drain, to where, and at what rate) is fixed earlier; only the engineering detail is finished at the discharge stage.

1. Principle fixed: the discharge point, rate and broad land-take for drainage are established at full or outline stage.
2. Permission granted: a drainage condition is attached, usually needing approval before any work starts.
3. Condition discharged: the detailed drainage design is submitted and approved, and only then can the scheme begin or be occupied.

Because so much rests on that condition, the numbers agreed with the LLFA early on, especially the rate at which water may leave the site, quietly shape the whole scheme. They size the storage, which in turn sizes the land the storage takes up.

The consents planning permission does not give you

Here is the point that surprises almost everyone: a planning permission with the drainage condition discharged still does not let you touch a river, a sewer or a highway drain. Those are separate regimes, each with its own body, its own fee and its own clock. A permission built on a strategy that cannot get its downstream consent is worth very little.

• Environmental permit (flood risk activity): from the Environment Agency, for building an outfall or any works in, over or close to a "main" river.
• Ordinary watercourse consent: from the LLFA or an internal drainage board, for culverting or obstructing a smaller ditch or stream.
• Sewer connection and adoption: from the water company, for connecting to or building over a public sewer.
• Highway drainage agreements: from the highway authority, where roads and their drains are involved.

None of these is granted by your planning permission. The good news is that they are all solvable at modest cost if you line them up early. The expensive version is discovering the outfall needs a permit after the layout is set in stone.

How to keep flood risk and drainage from derailing your application

The thread running through all of this is timing. Flood risk is a question of principle, so it is settled early; drainage is a question of land, so it has to be sized before the layout is fixed; and the downstream consents have their own queues, so they should be started, not left. Deal with them in that order and flooding rarely holds a good scheme back.

If you are not sure whether your site needs a flood risk assessment or a drainage strategy, the quickest first step is a screen on the flood map and the council's validation list. You can start with our postcode flood risk map and our guide to when you need an FRA. When you are ready for the reports themselves, Unda prepares flood risk assessments for planning and drainage strategies that are written to the current national standards and to your council's local requirements.

Frequently asked questions