Finished floor levels: setting and justifying them

Setting a finished floor level for flood risk is arithmetic with one awkward constraint attached. The level has to clear the design flood level plus freeboard, and it also has to let someone reach the front door without a ramp the length of the garden. Those two requirements pull in opposite directions on most sites, and resolving them is a design exercise rather than a calculation.

Start with the number the flood risk assessment produces, then work back through access, street scene and drainage until the section works. Do it in that order. A floor level chosen to suit the elevation and justified afterwards is visible to any reviewer.

What the level has to clear

The starting point is the design flood level for the relevant source, taken at the building position rather than at the site boundary. For fluvial flooding that is normally the 1 per cent annual probability event with the appropriate climate change allowance applied. For tidal it is the 0.5 per cent event with sea level rise. For surface water and groundwater the source is different and the level usually comes from modelling rather than published mapping.

Two points that get missed. Flood levels vary across a site, sometimes by several hundred millimetres between the front and rear of a plot, so quote the level at the building with its location. And the design flood level is a water level in metres above Ordnance Datum, not a depth, so it means nothing without the existing ground levels surveyed to the same datum.

The whole chain belongs in the Flood Risk Assessment: source, event, allowance, level at the building, freeboard, finished floor level. Six numbers in a row, each traceable.

Freeboard and where the number comes from

Freeboard is the margin added above the design flood level to allow for the things the model does not capture: wave action, blockage, local variation, survey tolerance and model uncertainty.

Three hundred millimetres is the common starting point for fluvial flooding. Some lead local flood authorities and some coastal authorities require 600 millimetres, and a few ask for more where the hydrology is uncertain or the consequences of exceedance are severe. Check the local requirement before you draw the section, because the difference between 300 and 600 changes the entrance arrangement completely.

A larger freeboard is sometimes the pragmatic answer where the modelling is coarse. Offering it up front is more persuasive than defending a marginal level through three rounds of consultation.

Level access pulls the other way

Approved Document M expects a step-free approach to the principal entrance of a new dwelling where the topography allows. Gradients are limited, landings are required at intervals, and a ramp that complies takes far more length than most plots have to give.

Work out the length before committing to the level. A 600 millimetre rise at the shallowest permitted ramp gradient needs twelve metres of run plus landings, and on a plot with a four metre front garden that is simply not available.

The routes out of this are all design moves rather than arguments. Raise the whole plot rather than the building, so the approach rises with the ground and the threshold stays flush. Bring the entrance to the side where the ground is higher. Use a shallow terraced approach with a compliant path routed across the frontage rather than straight at the door. Where the site cannot deliver step-free access because of the flood level, Approved Document M recognises that topography can make it impracticable, and the flood risk assessment and the design and access statement should both record why.

Do not solve it with a lift or a platform on a two-storey house. It reads as an admission that the level was set without thinking about the approach.

Street scene and conservation constraints

A raised dwelling in a terrace of houses at footpath level looks wrong, and in a conservation area that is a refusal reason in its own right.

The usual compromise is to raise the ground floor slab while keeping the ridge at the height of the neighbours, which means the internal floor to ceiling heights absorb the difference. That works for 300 millimetres and becomes difficult at 600.

Where the building cannot be raised, the alternatives are flood resilient construction below the design level with the habitable accommodation above, or a change to the layout so that sleeping accommodation and the escape route sit at first floor. Neither removes the need for safe access and egress, which is assessed separately.

Basements are a special case and the answer is usually no. The reasoning is set out in the note on basement development and the flood risk objection.

What raising the ground displaces

Raising levels across a plot within the functional floodplain removes storage, and that storage has to be replaced level for level if the scheme is not to increase flood risk elsewhere.

Compensatory storage means excavating an equivalent volume at the same range of levels, hydraulically connected to the flood source, on land within the applicant's control. On a single plot in a village that land rarely exists, which is why raising the ground is often available in principle and not in practice.

Surface water drainage is affected too. A raised platform changes the falls, and a drainage design produced before the levels were fixed will need redoing. Where infiltration was assumed, check the depth to groundwater against the new formation level, and keep the Drainage Strategy and the level design in step. The same coordination point applies to a single plot, as set out in the note on drainage evidence for a single new dwelling.

The condition that fixes the level

Councils condition finished floor levels routinely, and the wording repays a careful read before you agree to it.

A well drafted condition states the level numerically in metres above Ordnance Datum, names the drawing, and requires the level to be achieved and retained. A poorly drafted one requires levels to be submitted and approved before development begins, which turns a settled design point into a discharge of conditions application with a fee and eight weeks attached.

Where the level is already shown on the submitted section, ask the officer to condition it by reference to that drawing. Most will, if the drawing is clear.

Expect a further condition requiring flood resilient construction details, and on some sites a flood warning and evacuation plan. Neither is onerous when anticipated and both are irritating when they arrive after the contractor is on site.

Recording levels to a datum

Every level in the submission should be in metres above Ordnance Datum, to two decimal places, tied to a stated benchmark.

Relative levels, assumed datums and levels described as "existing plus 300" all cause problems, because the building inspector, the contractor and the officer discharging the condition each need to measure the same thing. A topographical survey by a competent surveyor with a stated benchmark costs little against the cost of getting this wrong, and the setting out on site should be checked against it before the slab is poured.

One honest uncertainty: published flood levels are revised as models are updated, and a level set today may not match the mapping in five years. That is not a reason to build low. It is a reason to record the source, the model version and the date in the assessment, so that anyone revisiting the decision can see what it was based on.

Enter your postcode on PlanScope to see whether your site is likely to need a flood risk assessment. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

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