Freeboard in a flood risk assessment, explained

Freeboard is the height added above the design flood level when setting finished floor levels, and it exists because the design flood level is an estimate rather than a measurement. Models carry uncertainty, defences settle, culverts block, wind pushes water up against a bank. Freeboard buys margin against all of that. On fluvial sites the Environment Agency commonly looks for 300 millimetres above the modelled level for the design event including a climate change allowance, with more where wave action, surge or blockage is in play.

What the allowance is actually covering

It helps to name the uncertainties rather than treat freeboard as an arbitrary rule of thumb.

Hydraulic models are calibrated against limited gauge records and simplify the floodplain. Channel roughness changes with vegetation and season. Survey of bank levels carries its own tolerance. A trash screen on a culvert can block during the event that matters most. On tidal and coastal sites wave run-up and wind set-up add height that the still-water level does not capture. Defences can settle or be overtopped.

None of those is a failure of the assessment. They are the reasons an assessment cannot produce a single exact number, and freeboard is the honest response to that. And the size of the allowance should track the size of the uncertainty, which is why a figure copied from another site is a poor starting point.

What freeboard is not is a substitute for the sequential test. Raising a floor by 300 millimetres does not answer the question whether the development should be in that location at all, and an assessment that leads with floor levels before addressing the sequential approach will be sent back.

Establishing the design flood level

Everything depends on the level you are adding to, so state it precisely.

For fluvial sites the usual design event is the 1 per cent annual probability flood, with the appropriate climate change allowance for the river basin district and the development's lifetime. For tidal sites it is commonly the 0.5 per cent annual probability event with a sea level allowance.

The level itself comes from a modelled source. That might be a detailed hydraulic model held by the Environment Agency, a model built for the site, or, on smaller sites, product data supplied by the Agency. Say which, give the model reference and the date, and state the node or cross-section the level was taken from.

Climate change allowances are revised, and an assessment quoting an allowance from an older report is the commonest reason for a consultee holding objection. Check the current published allowances at the time of writing, name the epoch and give the percentage applied.

Typical freeboard and where it comes from

There is no single national figure, which surprises people. The requirement is set for the site by the Environment Agency or the lead local flood authority, informed by the nature of the flooding.

The pattern is consistent even so. A fluvial site with a reliable model and no structures upstream attracts a modest allowance, commonly 300 millimetres. A site behind a defence, downstream of a culvert or trash screen, or exposed to wave action attracts more. A site where the model is coarse or the data thin attracts more again, because the uncertainty being covered is larger.

Ask the question early, in writing, as part of a pre-application enquiry. An email from the Agency naming the figure is worth more than a consultant's assumption, and it takes a fortnight rather than a month. The answer changes the section drawings, the access levels and sometimes the viability of the plot, and it is not a good discovery to make at consultation stage.

Raising floors has consequences

Adding 300 or 600 millimetres to a floor level is easy on a drawing and awkward everywhere else.

Level access has to be maintained, which means a longer approach, a gentler gradient or a ramp with landings, all of which need space that a small plot may not have. Approved Document M expectations do not relax because the site floods.

Ridge height rises with the floor, and on an infill plot between two existing houses that can turn an unremarkable design into a scale and massing objection. Where a local plan or a design guide caps ridge height relative to neighbours, the freeboard eats into the storey heights.

Drainage gets harder. A raised floor may still sit below the level of the connection, and gravity discharge can become a pumped system with a maintenance obligation attached. Garden levels, driveway falls and the relationship with the pavement all shift.

Work these through at the same time as the flood levels rather than afterwards. The structure of a flood risk assessment for a home in Flood Zone 2 shows where the level-setting section sits within the wider document, and the practical business of setting finished floor levels and freeboard on a flood site is where the design consequences get resolved.

Where the freeboard cannot be achieved

Sometimes the required level is impossible: a conversion where the floor cannot move, a site constrained by an adjoining highway, a listed building.

The response is a package rather than a single measure. Flood resistant construction to keep water out for a period, flood resilient finishes so that a property that does flood can be dried and reoccupied quickly, raising vulnerable services and sockets, and safe refuge at a higher level with a flood warning and evacuation plan. Where the development is more vulnerable in flood risk terms, the exception test may also be engaged, and the evacuation plan then has to satisfy the emergency planning officer as well as the Agency.

Say plainly in the assessment that the standard freeboard cannot be met, explain why, and set out the residual risk. An assessment that quietly adopts a smaller allowance without saying so loses credibility on the first read, and the relationship between finished floor levels, flood risk and freeboard is the part a consultee checks first.

Recording the levels on the drawings

Get this right and the condition discharges itself later.

• Express all levels in metres above Ordnance Datum, not as relative levels.
• Annotate the design flood level, the climate change allowance applied and the freeboard separately, so the arithmetic is visible.
• Show finished floor levels on plans, sections and elevations, and make sure the three agree.
• Show existing ground levels and the proposed site levels, including any raising of external areas.
• Mark the access route and its levels, including the point where it falls below the design flood level.
• Note the source and date of the flood level data on the drawing itself.

Councils commonly condition finished floor levels as approved, so any later change needs an application. Building to a level 150 millimetres different from the approved drawing is a breach that surfaces at warranty or sale, long after the site team has moved on.

The levels evidence belongs in the Flood Risk Assessment rather than in the design and access statement, because that is the document the Environment Agency reads. Enter your postcode on PlanScope to see which flood and drainage reports your site is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.