Finished floor levels, freeboard and the right datum

A finished floor level condition is the most commonly imposed and most commonly un-dischargeable condition in flood risk work. The reason is almost always the same. The level in the condition was taken from a flood risk assessment that quoted metres above Ordnance Datum, and the architect's drawings were set out on a site datum of 100.000. Getting finished floor level freeboard and datum right is a five minute decision at concept stage that costs weeks at discharge of conditions stage if you get it wrong.

Fix the datum first. Everything else follows.

Why levels are the most conditioned item in a flood submission

Levels are the only flood mitigation an authority can verify with a tape measure. Attenuation volumes are buried, resilient construction is inside the wall, and an evacuation plan is a document. A floor slab is either at the right height or it is not.

So where a site is in Flood Zone 2 or 3, expect a condition fixing the finished floor level, and expect the authority or the Environment Agency to want it verified before occupation rather than afterwards. That makes the level a programme item, not a detail.

It also makes the level a design constraint with knock-on effects on access, drainage, ceiling heights and the street elevation. Treat it as one of the first decisions on the site rather than something to resolve after planning.

Design flood level, freeboard and the resulting floor level

Three numbers stack up, in this order.

The design flood level is the modelled water level at the site for the relevant event, usually the 1 per cent annual exceedance probability fluvial event or the 0.5 per cent tidal event, with the appropriate climate change allowance added for the development's lifetime. The allowances are revised periodically, so take them from the current Environment Agency guidance rather than from a report written three years ago.

Freeboard is then added to cover modelling uncertainty, wave action, blockage and construction tolerance. A 300 mm allowance is the common starting point for a fluvial site, with 600 mm sought where the modelling is less certain or where the site sits behind a raised defence. There is no nationally fixed figure, so agree it in writing with the Environment Agency or the lead local flood authority and record the agreement in the Flood Risk Assessment.

The finished floor level is the sum. State it as a single figure in metres above Ordnance Datum, to three decimal places, in the assessment and on the drawings.

Those figures are illustrative. Use the model output for your own site.

Stating levels to ordnance datum rather than site datum

A site datum is a convenience for setting out. It is not a level, and it cannot be audited. A condition requiring a floor at 12.750 mAOD cannot be discharged by a drawing showing a floor at 100.300 unless somebody can prove the relationship between the two, and on a site where the temporary benchmark was a wooden peg that nobody can prove six months later.

Specify the survey properly. A topographic survey tied to Ordnance Datum, by levelling from a published benchmark or by GNSS observation transformed to the national grid and datum, with the control points listed and at least one permanent benchmark installed on site. A bolt set into a retained kerb or a mass concrete block outlasts the groundworks. A peg does not.

Then carry the datum through every drawing. Site sections, drainage layouts, external works and the architect's floor plans should all read in mAOD, and the drainage design should use the same levels as the flood assessment. Mismatched datums between the architect's set and the Drainage Strategy is one of the most common reasons an officer sends a submission back.

Topographic survey accuracy and its consequences

Ask for a stated vertical accuracy in the survey specification and check it in the deliverable. Levels good to 15 mm are achievable with ordinary levelling and are appropriate where a 300 mm freeboard is being relied on. Levels taken from a drone photogrammetry model without ground control are not.

Where the existing ground level matters, for example because the flood level sits close to it, get spot levels at a density that captures the real surface rather than a 10 m grid across a sloping garden. On a sloping site the question of where the flood water actually goes is a design question in its own right, and runoff on a steep site behaves nothing like runoff on a flat one.

One practical point. Survey before demolition if you can, and record the levels of existing thresholds, air bricks and manhole covers. Those are the datum checks you will want when the slab is poured and somebody questions the height.

Raised levels, access gradients and step free entry

Raising a floor level solves a flood problem and creates an access problem. Approved Document M expects a step free approach to the principal entrance of new dwellings and buildings, and a ramp cannot be steeper than the gradients the Approved Document sets out.

The arithmetic is unforgiving. As an illustrative example, raising a threshold 600 mm above the external ground level needs roughly 12 m of 1 in 20 ramp, plus landings, plus a level platform at the door. On a tight urban plot with a 6 m front garden that geometry does not exist, and the answer has to come from somewhere else: a stepped and ramped approach from a higher point on the street, a split level plan with habitable rooms raised and the entrance at grade, or a regrading of the whole plot.

Decide this at planning stage and show it on the submitted drawings. An approved scheme that cannot physically deliver step free access at the conditioned floor level is a reserved matter nobody has reserved.

Where parts of the building must sit below the design flood level, design them for water. Flood resistant and resilient construction guidance in BS 85500 covers the detailing, and the principle is simple: nothing below the design level should be ruined by immersion.

Verification before occupation

Plan the discharge submission while the slab is being set out, not when the keys are due.

1. Have a registered surveyor record the as-built finished floor level in mAOD, from the same control used for the original survey.
2. Produce a short certificate naming the control, the method, the date and the measured level, with a plan showing the survey points.
3. Submit it with photographs of the slab and the permanent benchmark.
4. Allow eight weeks for determination of the discharge application, and more if the Environment Agency is consulted.

If the as-built level is 20 mm low, say so and explain it. An honest 20 mm discrepancy with an explanation is usually accepted. A certificate that quietly reports the design level rather than the measured one is a different problem altogether.

Wording a level condition you can comply with

Where you are offered a draft condition, read it as the person who will have to discharge it. Good wording fixes a numerical level in metres above Ordnance Datum, names the drawing it relates to, and allows verification by a surveyor's certificate.

Three forms of words to resist. A level expressed relative to existing ground level, because existing ground level is about to change. A level expressed relative to the adjacent carriageway, because the carriageway gets resurfaced. And a condition requiring the floor to be set at a level to be agreed, which simply defers the argument to a point where you have no negotiating room.

Suggest the wording yourself, in the covering letter, with the figure already in it. Officers are generally glad of a condition they do not have to draft, and you will have written one you can comply with.

For specialist technical reports, qualified professional input may be required. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which reports a site in Flood Zone 2 or 3 is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.