Finished floor levels and freeboard: justifying the numbers

A defensible finished floor level flood freeboard statement has three parts: the design flood level in metres above Ordnance Datum, the freeboard allowance above it with a stated reason, and the resulting floor level shown on a drawing that a reviewer can check against a levels survey. Assessments fail on the first and third far more often than on the second. A floor level expressed as 150 mm above existing ground, on a site with no levels survey, cannot be assessed at all.

Everything below is about producing numbers someone else can verify. That is the whole job.

Establishing the design flood level

The design flood level is the water level in the event the development has to be safe against, with an allowance for climate change over the lifetime of the development. For fluvial flooding the usual design event is the 1 per cent annual exceedance probability flood, and for tidal flooding the 0.5 per cent event, each with the relevant allowance applied.

Three sources, in descending order of what a reviewer will accept.

1. Site-specific hydraulic modelling, where the catchment is small, the mapping is coarse, or the scheme is large enough to justify it.
2. Modelled levels from the Environment Agency's detailed river network model for the reach, obtained as a data request, including node levels and the model extent.
3. Levels from the authority's Strategic Flood Risk Assessment, where the modelling behind it covers the site at an adequate resolution.

What is not a source is the online flood map alone. The Flood Map for Planning shows extents and zones, not levels, and an assessment that infers a level from a flood outline has no number in it. Request the modelling data early, because turnaround takes time and the answer sometimes reveals that no model covers the reach at all.

Climate change allowances are set by river basin district, by epoch and by allowance category, and they are revised. Take them from the current published guidance, state which category you have applied and why, and state the lifetime assumed for the development. A residential lifetime of 100 years is the conventional assumption, and the allowance epoch follows from it.

Choosing a freeboard allowance you can defend

Freeboard is an allowance for what the model does not capture. It is not a safety factor dropped in from habit, and a reviewer who asks why you chose 300 mm deserves a reason rather than a convention.

The components to reason from are model uncertainty, the resolution of the topographic data, wave action and wind set-up on open water, the potential for blockage of a culvert or bridge upstream, sediment accumulation, and the consequences of the ground settling or the channel changing over the life of the building.

State each component and then state the total. A freeboard of 600 mm explained as 300 mm for model uncertainty and 300 mm for blockage is a number a reviewer can engage with. A freeboard of 600 mm with no explanation reads as padding, and it will be challenged on a site where 600 mm of extra height matters to the elevation.

Site levels, thresholds and the external ground

Raising the floor level is not free. Raising the ground with it displaces flood water, which reduces floodplain storage and can increase risk elsewhere, so compensatory storage on a level-for-level basis is normally required in fluvial floodplain. That is why the levels strategy and the flood modelling have to be developed together rather than sequentially.

Where the floor is raised but the ground is not, the threshold has to be resolved. That is where Part M and flood risk pull against each other: an accessible approach wants a level or gently sloping route to a threshold with a minimal upstand, and flood risk wants the floor 600 mm above the ground outside.

The workable solutions are ordinary.

• A ramped approach within the plot at a compliant gradient, with a level landing at the door.
• An external raised platform or terrace at floor level, with the change in level taken further out.
• Regrading a limited area around the entrance, where compensatory storage allows it.
• Accepting a stepped secondary entrance while providing an accessible principal one.

Say which you have adopted, and draw it. A plan that shows a floor level 600 mm above the adjacent path and no means of reaching the door is a plan that will be picked up at building control and changed.

Where raising levels is not possible

On a constrained urban plot, or where compensatory storage cannot be found, raising is not available. The alternatives are a change in layout, a change in use at ground level, or a form of construction that accepts water below the habitable floor.

A void or undercroft beneath a raised floor, with openings sized so water passes through, keeps the floodplain storage while lifting the habitable accommodation. Undercroft parking works on the same principle and treats the parking as a sacrificial use, which needs a flood evacuation arrangement for the vehicles rather than just for the people.

Two constraints apply. Highly vulnerable uses such as basement dwellings are not acceptable in the higher flood zones, so a void cannot be used to create habitable space below the design flood level. And the void has to be designed to fill and drain, which means openings, a floor that can be left wet, and services routed above. Where the flood zone requires it, the wider policy case runs through the Exception Test, and the evidence expected on each limb is set out in the note on exception test evidence.

Safe access and egress

Levels do not stop at the building. The assessment has to address whether people can leave the site safely during the design event, along a route where depths and velocities stay within the accepted thresholds, to a place of safety above the design flood level.

Where dry access cannot be achieved, the fallback is safe access with a flood warning and evacuation plan, and the authority will want to know who issues the warning, who acts on it, and how long the route remains passable. Write the plan as a document a management company can operate rather than an appendix nobody reads.

Presenting the levels on the drawings

This is where good assessments are distinguished from ones that get a holding objection.

1. A topographical survey to Ordnance Datum, with spot levels across the site and along the boundary.
2. A levels plan showing existing and proposed ground levels, finished floor levels and the flood extent.
3. At least one section through the building, the threshold and the ground, annotated with the design flood level, the freeboard, the finished floor level and the external ground level.
4. A table listing, for each building or block, the design flood level, the freeboard, the finished floor level and the lowest external threshold, all in metres above Ordnance Datum to two decimal places.
5. The access route drawn, with depths along it at the design event.

Every number in metres above Ordnance Datum. No relative datums, no levels expressed against a temporary bench mark that nobody can find, and no discrepancies between the architect's drawings and the flood assessment. The full package sits in the Flood Risk Assessment, with the surface water design in the Drainage Strategy, and the companion note on setting finished floor levels and freeboard on a flood site covers the practical sequence of doing it.

Enter your postcode on PlanScope to see which reports a site in a flood zone is likely to need.

This is general guidance, not legal advice. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.