Finished floor levels and freeboard above the flood level
The design flood level is not the colour on the flood map. How to establish it, what freeboard to add, and every opening that has to sit above the result.
On a site at risk of flooding, the finished floor level freeboard your council expects is usually 300mm above the design flood level, and both numbers have to be stated in metres above Ordnance Datum on the drawings. Get either one wrong and the application either fails at consultation or returns as a condition you cannot meet once the slab is cast.
The design flood level is not the level on the flood map. It is the modelled water level for a stated event, in a stated year, with a stated climate change allowance, at your plot rather than at the nearest gauging station.
Establishing the design flood level
Work through four steps in order.
- Check the flood zone on the Environment Agency flood map for planning, and note that the map shows zones, not levels. A site in Flood Zone 3 needs a level, not a colour.
- Request the modelled levels. The Environment Agency holds detailed modelling for most main rivers and will supply node levels under a product request, usually for a fee and with a turnaround of a few weeks. Ask early, because this is the item that delays applications.
- Identify the relevant event. For fluvial flooding that is normally the 1 per cent annual exceedance probability event, the 1 in 100 year flood. For tidal flooding it is normally the 0.5 per cent event, the 1 in 200 year flood.
- Add the climate change allowance for the right river basin district, epoch and allowance category. The Environment Agency publishes peak river flow, peak rainfall and sea level allowances, and they are revised, so take them from the current published tables rather than from an older report.
Where no modelling exists, the level has to be derived, which means a hydraulic model or an agreed conservative approach using topographic survey and historic flood evidence. That is specialist work, and a Flood Risk Assessment prepared on that basis should be authored or reviewed by someone suitably qualified.
Why freeboard is asked for
Freeboard is an allowance for the things the model cannot capture: survey error, model uncertainty, blockage of a culvert or bridge, wave action and the simple fact that water arriving at a building piles up slightly against it.
The usual figure in England is 300mm above the design flood level for the finished floor. Larger allowances are asked for where the modelling is coarse or old, where flooding is rapid onset, where a defence or a culvert is relied on, and in tidal locations where wave action adds to still water level. Smaller allowances are rare.
Be explicit in the submission. State the design flood level, the allowance, and the resulting finished floor level, all to Ordnance Datum and all to two decimal places. A statement that floors will be set "above the flood level" is not a level and will come back.
| Item | What to state |
|---|---|
| Design event | 1 per cent annual probability fluvial, or 0.5 per cent tidal |
| Climate change allowance | Allowance category, epoch and river basin district |
| Design flood level | Metres above Ordnance Datum at the plot |
| Freeboard | Allowance in millimetres and the reason for it |
| Finished floor level | Metres above Ordnance Datum, to two decimal places |
Setting levels on a sloping or infill plot
On a sloping site the problem is rarely the flood level. It is reconciling a raised floor with the ground outside.
The practical moves are to step the building down the slope with split levels so that only the lower part needs raising, to use the natural fall to bring external levels up to the threshold on one side, and to set the entrance on the uphill elevation. Each of those is better than a ramp bolted on at the end.
On an infill plot in an established street the constraint is the neighbours. Raising a floor by 600mm pushes ridge height up by the same amount unless internal heights are reduced, and that affects daylight to adjoining windows and the street scene. Where the plot sits between two houses with matching eaves, the honest conversation with the designer is about whether the plot can take a raised floor at all.
And one thing that catches self-builders out: raising ground levels in a fluvial floodplain displaces flood storage. The Environment Agency will expect compensatory storage, excavated on a level for level and volume for volume basis within the same floodplain cell, which needs land you control. Check that before committing to a raised platform.
Where raising levels conflicts with other requirements
Three conflicts come up repeatedly.
Access. Building Regulations expect a level or gently sloping approach to the principal entrance, and a raised floor makes that harder. Long ramps are poor design and they eat garden space. A shallow external terrace with a graded approach usually works better than a ramp.
Heritage. On a listed building or in a conservation area, raising internal floor levels can harm historic fabric and alter window reveals and ceiling heights, and in many cases the works themselves need listed building consent. Here the answer is usually resistance and resilience measures rather than level raising, along the lines described in the note on residual flood risk and property flood resilience measures.
Vulnerability of use. Where floors cannot be raised, the fallback is to keep the more vulnerable accommodation above the design flood level. No sleeping accommodation at ground floor level in higher risk locations, safe refuge above the level, and the basement position set out in the note on basements, flood risk and sleeping accommodation.
None of this removes the need to pass the sequential test first, and the approach to that is covered in the Sequential Test (Flood) requirements.
Thresholds, air bricks and service entries
A floor set 300mm above the design flood level is undermined by an air brick at 150mm. Water finds the lowest opening.
Everything below the design flood level plus freeboard has to be either sealed or deliberately resilient:
- Door thresholds, including patio doors and french doors, which are often set lower than the front door.
- Air bricks and sub-floor ventilation, which need automatic closing devices or relocation above the level.
- Service entries for water, gas, electricity, telecoms and drainage, each sealed where it passes through the wall.
- Soil and vent pipes, gullies and rodding eyes, and any below-level drainage that can surcharge back into the building.
- Cavity trays and the wall construction itself, where a sump and pump in the sub-floor void may be appropriate.
Mark every one of them on a section drawing with its level. That drawing is what persuades a lead local flood authority that the strategy is designed rather than asserted.
Recording levels to Ordnance Datum
Site datums cause more trouble than any other single item on these applications.
Commission a topographic survey tied to Ordnance Datum, either from an Ordnance Survey benchmark or by satellite positioning with a stated accuracy. Put the datum statement on every drawing. Show existing ground levels, proposed ground levels, threshold levels and finished floor levels, all in metres above Ordnance Datum, and keep the same figures across the site plan, the sections and the flood risk assessment.
Inconsistency between documents is the single most common reason a flood consultee asks for a resubmission, and it is entirely avoidable.
Conditions that secure the levels
Expect a condition in the form that the finished floor levels shall be set no lower than a stated level above Ordnance Datum, and that the development shall not be occupied until a level survey carried out by a suitably qualified person has been submitted and approved.
Take that condition seriously at the slab stage. A floor built 80mm low is a real problem: the options are an expensive remedial build-up, a revised application that the Environment Agency may object to, or a long argument. Have the levels checked by the surveyor before the concrete is poured, not after, and keep the survey as part of the handover pack along with the approved drawings.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.
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Frequently asked questions
How much freeboard is required above the flood level?
The figure most commonly applied in England is 300mm above the design flood level for finished floor levels. Larger allowances are asked for where the modelling is coarse or old, where flooding is rapid onset, where a defence or culvert is relied on, and in tidal locations where wave action adds to the still water level.
Where do I get the design flood level?
From Environment Agency modelled node levels, requested as a data product for the relevant watercourse, usually for a fee and with a turnaround of a few weeks. The flood map for planning shows zones, not levels. Where no modelling exists, the level has to be derived by hydraulic modelling or an agreed conservative method.
Which flood event is used?
For fluvial flooding, normally the 1 per cent annual exceedance probability event, often called the 1 in 100 year flood. For tidal flooding, normally the 0.5 per cent event, the 1 in 200 year flood. In both cases the relevant climate change allowance is added for the river basin district, epoch and allowance category.
What if I cannot raise the floor level?
Then the strategy shifts to resistance and resilience measures plus keeping vulnerable accommodation above the design flood level, with no sleeping accommodation at ground floor in higher risk locations. That is the usual outcome on listed buildings and in conservation areas, where raising floors would harm historic fabric.
Does raising ground levels cause a problem?
Yes, in a fluvial floodplain. Raising levels displaces flood storage, so the Environment Agency will expect compensatory storage excavated on a level for level and volume for volume basis within the same floodplain cell. That needs land you control, so check it before committing to a raised building platform.
How are levels recorded?
In metres above Ordnance Datum, from a topographic survey tied to an Ordnance Survey benchmark or satellite positioning with a stated accuracy. Put the datum statement on every drawing and keep existing, proposed, threshold and finished floor levels consistent across the site plan, the sections and the flood risk assessment.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.