Site-specific flood assessment against the strategic one
The council's strategic assessment and your own site-specific one do different jobs. How to use the first to cut the cost of the second, and where its data runs out.
A site specific flood risk assessment is your document, paid for by you, about your plot. The strategic flood risk assessment is the council's document, paid for from the plan-making budget, about the whole borough. Both matter to a small residential application, and they do different jobs. The strategic one tells you which zone you are in, how the authority defines the functional floodplain locally and which climate change allowances it applies. The site-specific one has to answer the question the strategic one cannot: what happens on this piece of ground at these levels.
What each document is for
A strategic flood risk assessment underpins the local plan. It maps all sources of flooding across the authority's area, applies climate change allowances, identifies critical drainage areas, records historic flooding, and sets out how the authority will apply the Sequential Test when it allocates sites and determines applications. Level 1 assessments are desk-based screening exercises. Level 2 assessments add modelling where the plan proposes development in higher-risk areas.
A site-specific assessment supports an application. It identifies the sources of risk affecting the site, sets finished floor and threshold levels, demonstrates safe access and egress, shows that the development will not increase risk elsewhere, and deals with residual risk after mitigation.
Neither substitutes for the other. Turning up with a print of the strategic mapping and calling it a flood risk assessment gets the application refused as invalid, and quoting your own levels without reference to the strategic document invites the authority to say you have ignored its evidence base.
Thresholds that trigger a site-specific assessment
| Situation | Site-specific assessment needed |
|---|---|
| Any development in Flood Zone 2 or Flood Zone 3 | Yes |
| Flood Zone 1, site of one hectare or more | Yes |
| Flood Zone 1, smaller site, other sources of risk present | Yes, proportionate to the sources |
| Flood Zone 1, smaller site, critical drainage area | Yes, focused on surface water |
| Flood Zone 1, smaller site, no other sources identified | Not usually, though a drainage strategy may still be required |
| Change of use to a more vulnerable classification | Yes, whatever the zone |
The one-hectare threshold catches people who assume Flood Zone 1 means no assessment. So does the other sources limb, which covers surface water, groundwater, sewers, reservoirs and canals. A plot of a third of an acre in Zone 1 sitting in a surface water hotspot needs an assessment addressing that source, even though the river is a mile away.
The Sequential Test runs alongside and is a separate exercise. Where the site is in Zone 2 or 3 and the authority has not already allocated it, expect to demonstrate that no reasonably available site at lower risk exists, as set out in our note on the Sequential Test for flood risk.
Using strategic mapping as a starting point
Download the strategic assessment before you commission anything. It is published on the council's website as part of the local plan evidence base, and it usually saves money.
What to take from it:
- The flood zone at the site and the authority's local definition of the functional floodplain, which is set in the strategic assessment rather than nationally.
- Historic flood records for the area, including incidents that never reached the national mapping.
- Surface water hotspots and critical drainage area boundaries, with the authority's expectations for discharge rates.
- Groundwater emergence areas, where the strategic assessment covers them.
- The climate change allowances the authority applies, by river basin district and development lifetime.
- Any site-specific commentary, since strategic assessments often include a schedule of allocated and potential sites with notes attached.
Cite the document and its date in your assessment. An officer who can see you have read their evidence base reads the rest of the submission more generously.
Where strategic data is too coarse
Strategic mapping is produced at a scale suited to a borough, and it shows its origins when you zoom in on one plot.
The national flood zone mapping is generally undefended, so it shows the extent that would flood if defences were absent, which can put a well-protected site into Zone 3 on paper. Zone boundaries derived from broad-scale modelling do not pick up a half-metre rise across a garden. The Zone 3a and 3b split is frequently indicative in a Level 1 assessment. Surface water mapping is modelled without drainage systems and overstates risk in places where a highway gully does its job, a point developed in our note on reading the surface water flood risk map without overstating it.
Groundwater is the biggest gap. Many strategic assessments say little about it, and where they do the mapping is regional. Site evidence is the only answer, as described in our note on evidencing groundwater flooding in a flood risk assessment.
The fix in each case is the same. Get a topographical survey tied to Ordnance Datum, request the Environment Agency's detailed flood risk data for the site, and compare the modelled water levels against your surveyed levels rather than against a zone boundary on a screen.
Modelling and when it is needed
New hydraulic modelling is expensive and often unnecessary. Work through the sequence before commissioning any.
Start with the Agency's detailed data enquiry, which returns modelled flood levels, flows and extents for the relevant events where a model exists. If the existing model covers your reach, is recent enough, and used topography that reflects the ground as it is now, you may need nothing more than a levels comparison and an application of the current climate change allowances.
Commission modelling where there is no model, where the model predates a change that matters such as a new culvert or a raised embankment, where the proposal will alter flows or floodplain storage, or where the difference between Zone 3a and the functional floodplain determines whether the scheme is possible at all. Agree the scope with the Agency or the lead local flood authority first. Modelling produced without that conversation is frequently redone.
For most single-plot self-build schemes, the honest answer is that a levels survey, the Agency data and a careful Flood Risk Assessment are enough, and the money is better spent on getting the finished floor level right.
Who prepares each and at whose cost
The authority prepares and pays for the strategic assessment, and publishes it free. You prepare and pay for the site-specific one.
That asymmetry is worth using. Ask the lead local flood authority and the council's drainage engineer what they expect before commissioning, because both give informal guidance, and both would rather answer a question early than object to a submission later. Where the authority publishes standing advice for minor development, follow it precisely. Departing from standing advice without explanation is the most common reason a small application picks up a drainage objection.
If the assessment is prepared by a consultant, check that the author has the relevant background and that the report is signed. Councils increasingly expect flood risk work to come from a suitably qualified author, and professional review of any draft is strongly advised.
How the two are cited together
Structure the site-specific assessment so the relationship is explicit. State the strategic assessment's title and date in the sources, quote its zone finding and functional floodplain definition for the site, then set out your own survey data and say where the two agree and where they differ.
Where you depart from the strategic position, justify it with evidence rather than assertion. "The strategic assessment shows the site within Flood Zone 3a; the topographical survey records ground levels between 11.8 and 12.4 metres above Ordnance Datum, against a modelled 1 per cent annual probability level of 11.6 metres including the central climate change allowance, so the developable area lies above the design event." That is the sentence an officer needs, and it is worth more than fifteen pages of policy recital.
Finish with the residual risk and the management response, and make sure the drainage strategy and the flood risk assessment use the same levels. Two documents with different finished floor levels is the error that generates a request for further information more often than any other.
To see which flood and drainage documents your plot is likely to need, enter your postcode on PlanScope and run the constraint check before you commission a survey.
This is general guidance, not legal advice. Check the position with your local planning authority, the lead local flood 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.
Frequently asked questions
When is a site-specific flood risk assessment required?
For any development in Flood Zone 2 or 3, for sites of one hectare or more in Flood Zone 1, for smaller Zone 1 sites where other sources such as surface water, groundwater or sewers are present or a critical drainage area applies, and for any change of use to a more vulnerable classification.
Can I use the council's strategic assessment instead?
No. A strategic assessment informs plan-making across the whole authority area and is mapped at that scale. It tells you the zone, the local functional floodplain definition and the climate change allowances to apply, but it cannot set finished floor levels or demonstrate safe access for one plot.
Does Flood Zone 1 mean no assessment is needed?
Not always. A Zone 1 site of one hectare or more needs an assessment, as does a smaller site affected by surface water, groundwater, sewer or reservoir risk, or lying in a critical drainage area. Even where none applies, the authority may still require a drainage strategy for surface water.
Do I need new hydraulic modelling?
Often not. Request the Environment Agency's detailed flood risk data first. Where an existing model covers the reach, is recent and reflects current topography, a levels comparison with the current climate change allowances may be enough. Commission modelling where no model exists or the proposal alters flows or floodplain storage.
Why do drainage and flood documents contradict each other?
Usually because they were written by different people at different times from different surveys. Fix the finished floor and threshold levels once, in metres above Ordnance Datum, and use the same figures in the flood risk assessment and the drainage strategy. Inconsistent levels are a common trigger for further information requests.
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.