Setting finished floor levels and freeboard on a flood site

A finished floor level on a flood risk site is the design flood level plus a freeboard allowance, and both halves of that sum need a source you can point to. The design flood level comes from hydraulic modelling, stated in metres above ordnance datum for a named event with a climate change allowance applied. The freeboard is set by the consultee for the site, commonly 300 millimetres and sometimes more. Everything else about the building is then arranged around that number.

Which is why floor levels are a very early decision and not a detail for the technical drawings.

Where the design flood level comes from

Start with the flood map for planning to establish the zone, then get a level rather than a zone. Flood zones tell you the probability of flooding. They do not tell you how deep.

Levels normally come from one of three places. An existing Environment Agency hydraulic model for the watercourse, which the Agency will supply on request, usually as node levels for a range of return periods. A new model commissioned for the site where none exists or where the existing one is too coarse. Or, for tidal and coastal locations, published extreme sea level data combined with an allowance for sea level rise.

Whichever source, the Flood Risk Assessment must state the design event, the model, its date, the node or cross-section used, and the climate change allowance applied over the lifetime of the development. Residential development is normally assessed over a 100 year lifetime.

Climate change allowances are revised periodically, so name the version you used and the date you checked it. An assessment relying on a superseded allowance is the single most common reason the Agency asks for a revision.

Adding freeboard and why the allowance varies

Freeboard covers what the model does not. Survey error, model uncertainty, wave action, blockage of a culvert or bridge, and the difference between a modelled water surface and what actually happens in a real storm.

Three hundred millimetres is the figure most often applied to fluvial sites with reasonable modelling. It rises where the situation is less certain: flashy upland catchments, sites immediately downstream of a structure that could block, locations behind a defence where breach is the mechanism, and coastal sites where wave overtopping is in play. On some sites 600 millimetres or more is expected.

Ask the consultee rather than guessing. The Environment Agency for main river and tidal sites, the lead local flood authority for surface water and ordinary watercourses. A short pre-application enquiry that returns an agreed freeboard is worth more than a paragraph of justification later.

Then record the arithmetic plainly. Design flood level, plus freeboard, equals finished floor level, all three in metres above ordnance datum, on the drawings as well as in the report.

Reconciling raised levels with access, drainage and appearance

A raised floor is easy to draw and awkward to build. Four things push back.

Level access. Approved Document M expects an accessible approach to the principal entrance. A 600 millimetre rise needs roughly twelve metres of ramp at a 1 in 20 gradient, or a regraded plot, or a stepped approach with a separate accessible route. Decide which on the planning drawings rather than at building control stage, because the answer changes the site layout.

Gravity drainage. Raising the building lifts the invert levels of foul and surface water drainage, and on a flat site that can leave you unable to reach the outfall by gravity. Check the connection levels early, and if pumping becomes necessary, say so in the Drainage Strategy with the maintenance implications set out.

Appearance. A house sitting 600 millimetres above its neighbours reads as odd in a street of consistent levels, and officers notice. The usual answers are lifting the whole plot rather than the building alone, using a shallower roof pitch to hold the ridge height, or handling the level change in the landscape design. Explain the reasoning in the Design and Access Statement so the raised level appears as a deliberate response rather than an oversight.

Existing buildings. On a conversion or extension you may be constrained by the existing structure, and a Structural Survey will tell you whether raising an internal floor is feasible at all.

Options where levels cannot be raised

Sometimes the level simply cannot go up. A conversion of a listed building, an extension that must match an existing floor, a site where drainage or access makes it impossible.

The response then has three parts.

1. Put the vulnerable uses above the flood level. Sleeping accommodation on an upper floor is the strongest single measure, and it changes the whole safety argument.
2. Build for water to come in and drain out. Solid floors, closed cell insulation, cement based renders, tiled or resilient finishes, and services and electrical circuits raised above the design level.
3. Provide safety for people. A dry route out, or a refuge above the flood level with a means of raising the alarm, backed by a flood warning and evacuation plan.

Be honest about the hierarchy. Resilience measures are weaker than avoidance, and a scheme relying on them needs the rest of the flood risk case to be strong.

Level for level compensatory storage on floodplain sites

Where a site lies within the functional floodplain or where the development displaces flood water, the volume lost has to be replaced. The convention is level for level and volume for volume: for each horizontal slice of floodplain filled, an equivalent volume must be excavated within the same slice, on land that floods at the same frequency and is hydraulically connected to the same watercourse.

Two practical points. The compensation land normally has to be within the applicant's control and outside the built development, which affects how much of a site is developable. And the calculation should be presented as a table of slices with fill and cut volumes, not as a single headline number.

An illustrative example: a plot with a modelled 1 in 100 year plus climate change level of 12.40 metres above ordnance datum, with a 300 millimetre freeboard agreed with the Environment Agency, gives a finished floor level of 12.70. If existing ground across the footprint sits at 12.15, the building sits 550 millimetres above surrounding ground, the driveway is regraded over 11 metres to give a 1 in 20 approach, and around 40 cubic metres of fill is compensated by excavating the rear of the plot within the same level band.

How the level is secured by condition

Expect a condition fixing the finished floor level at or above a stated value in metres above ordnance datum. Ask for that wording rather than a condition expressed as a height above existing ground level, which invites disputes about what the ground was before the site was stripped.

Two further conditions are common: one requiring the flood resilience measures to be built as approved, and one requiring a flood warning and evacuation plan to be in place before occupation.

Building control checks the constructed level independently, and a surveyed level certificate confirming the finished floor as built is worth obtaining. Lenders and insurers ask for it, and it closes the file properly.

Give both the council and building control the same drawing. Levels shown in ordnance datum on the proposed floor plans, sections and site plan, with the design flood level marked as a dashed line, remove almost all of the ambiguity that otherwise surfaces at discharge of conditions.

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. For specialist technical reports, qualified professional input may be required.