Can I Do My Own Flood Risk Assessment for Planning?

If your planning application needs flood risk information, it is fair to ask whether you can do your own flood risk assessment rather than pay a consultant. You can, but only for a genuinely minor, low-risk proposal. What looks like a simple technical form is really a policy-and-data exercise, and planners and their statutory consultees scrutinise it closely.

Whether you can sensibly take it on yourself comes down to two things: how much flood risk your site carries, and how much analysis that risk demands.

For most applications a self-prepared report, or one built from a downloaded template, costs more in delay and revision than it ever saves in fees.

When can you do your own flood risk assessment?

For a genuinely minor, low-risk proposal, such as a small householder extension, you may be able to prepare a basic assessment yourself and lean on the Environment Agency's standing advice. In those cases standing advice can support the application without bespoke modelling.

The Environment Agency treats a householder or non-domestic extension of no more than 250 square metres as minor.

Even then, a compliant assessment is rarely as simple as it looks. The Environment Agency's guidance expects you to state the estimated flood level for the relevant design event, which is a 1% annual probability (1 in 100-year) river flood or a 0.5% (1 in 200-year) tidal flood, then add the appropriate climate change allowance. Finished floor levels are normally set at least 600mm above that level, reducible to 300mm only where there is high certainty about the flood level. Every level has to be referenced to Ordnance Datum, which usually means a measured topographical survey.

What counts as acceptable also depends on the development's flood risk vulnerability classification and the flood zone it sits in. So even a small extension can turn on a detail of policy a non-specialist would not think to check. Beyond a minor proposal, the analysis rises quickly, and so does the chance a self-prepared report falls short.

Is there an official flood risk assessment template?

This is where a lot of DIY attempts begin. Search for a "flood risk assessment template" and you will find several, including one published through the Planning Portal for use with the Environment Agency's standing advice. A template can be a useful skeleton, and for the smallest low-risk schemes it may be enough to structure a basic submission.

The problem is that a template only sets out the questions. It does not supply the answers, and the answers are the assessment. A blank field asking for the design flood level or the finished floor level tells you nothing about how to derive that figure for your site, which datasets to trust where they disagree, or when the published mapping is too coarse to rely on.

A template is a form, not a flood risk assessment: it helps you present the work, it does not do the work.

What a planning-ready flood risk assessment has to cover

Once a proposal is anything more than minor, a robust flood risk assessment normally has to consider every relevant source of flooding, not just the nearest river:

• Rivers and the sea — the fluvial and tidal sources most people picture first.
• Surface water — the source now behind most flood-related planning constraints.
• Groundwater — rising water tables that the Flood Map for Planning does not show.
• Sewers and drainage systems — capacity and surcharge risk on and around the site.
• Reservoirs and other artificial water bodies — residual risk from a breach or failure.

On top of that, it has to apply the correct climate change allowance, demonstrate safe access and egress during a flood, set finished floor levels, and show the development will not increase flood risk on neighbouring land. Many applications also need a drainage strategy alongside the assessment. Where the published data is too coarse, hydraulic modelling and GIS analysis may be needed to show what the Flood Map for Planning cannot. If you want the full method, we set it out in our guide on how to do a flood risk assessment for planning.

Why a report or template from a year ago may already be out of date

Flood risk policy and data have moved noticeably over the past eighteen months, and a report, or a template, built on the old position can fail on arrival. Three changes matter most:

1. The update to Planning Practice Guidance published on 17 September 2025 confirmed the sequential, risk-based approach applies to all sources of flooding, including surface water and groundwater, and to future risk under climate change, not just rivers and the sea.
2. The Environment Agency's second National Assessment of Flood Risk (NaFRA2) has fed updated river, sea and surface water data into the Flood Map for Planning, so a site's mapped risk can have changed since it was last checked.
3. From April 2026 the planning appeals process places more weight on the quality and completeness of what is submitted first time, which raises the cost of getting the flood evidence wrong.

The Environment Agency now estimates around 6.3 million properties in England are at risk of flooding from rivers, the sea or surface water, rising towards 8 million by the middle of the century.

National planning policy is tightening in response, and the datasets behind an assessment are revised more often than most applicants expect. Reading the current NPPF and PPG position correctly, and applying the latest climate change allowances, is now a moving target rather than a fixed checklist.

What goes wrong with DIY or non-specialist assessments

A self-prepared report, or one produced by an architect or general consultant offering flood risk as a sideline, can pass validation and still come unstuck later. The common failings are familiar:

• Outdated evidence — relying on superseded policy or old flood data.
• Misread mapping — getting the flood map or the site levels wrong.
• Wrong allowance — applying the incorrect climate change allowance.
• Local requirements missed — overlooking a specific local authority or Lead Local Flood Authority requirement.
• Missing analysis — leaving out modelling the site actually needs.

Flood and drainage submissions are reviewed by specialist consultees: the Environment Agency, the Lead Local Flood Authority and, in some areas, an Internal Drainage Board. If the report does not meet their expectations, they can object or ask for more information, which usually means commissioning a revised assessment mid-application.

On a marginal site, a weak flood case can put the whole scheme at risk, and the apparent saving at the start is often spent several times over later.

How much does a flood risk assessment cost, and is DIY actually cheaper?

The instinct to write your own report is almost always about cost, so it is worth being honest about the sums. For a straightforward site, a professional flood risk assessment is a modest fixed fee set against the value of the scheme. A DIY report that draws an objection is a different equation: a fresh topographical survey, a commissioned assessment, and weeks or months of delay while the application sits unresolved.

Insurers paid out £1.2 billion in weather-related property claims in 2025, up 14% on the year before — part of why consultees are less forgiving of a thin flood case than they once were.

The cheapest report at the outset is rarely the cheapest outcome. Our note on what a flood risk assessment costs explains why the price of the assessment and the cost of getting it wrong are two very different numbers.

Who can write a flood risk assessment?

There is no single licence required to write a flood risk assessment, which is part of why the DIY question comes up so often. In principle anyone can submit one. In practice, the assessment has to satisfy the Environment Agency and the Lead Local Flood Authority, and those bodies expect it to be prepared by someone who understands hydrology, planning policy and the local datasets, and who can defend the method if it is challenged.

That is why most credible assessments are written by chartered flood risk or water and environmental specialists rather than by the applicant or a general design consultant. The value is not the document itself but the judgement behind it: knowing which flood source governs the site, when the published mapping can be relied on, and how current policy applies to this particular proposal. If you are still weighing whether your scheme even needs one, our guide on when you need a flood risk assessment is the place to start.

Getting expert input at the right stage

Flood risk and drainage are now specialist parts of the planning system, and the cheapest way through them is usually to get them right at the outset. An experienced consultant can read the site constraints early, interpret the flood map and local datasets correctly, apply the current policy and climate change guidance, and produce an assessment that holds up when the Environment Agency or the Lead Local Flood Authority reviews it.

If you are preparing a planning application and need a flood risk assessment for planning or a drainage strategy, contact Unda to talk it through with a consultant who works on this every day. We will tell you honestly whether a self-prepared report will do, what a full assessment has to cover, and how to keep the flood evidence from becoming the reason your application stalls.

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