What the zones mean
The August 2026 NPPF defines Flood Zone 1 as low probability, Zone 2 as medium probability and Zone 3a as high probability of river or sea flooding. Zone 3b is the functional floodplain: land needed to carry or store floodwater. The council's Strategic Flood Risk Assessment identifies its local extent. [1]
The basic zones do not account for the benefit of flood defences or the possible effects of climate change. Read them alongside the other Environment Agency data and the council's assessment. A house being outside the coloured river-flood area is not evidence that water cannot collect around its entrance or flow into a basement. [1] [2]
When a Flood Risk Assessment is needed
Under policy F4 of the NPPF published on 17 August 2026, a site-specific assessment should accompany development proposals in Zones 2, 3a and 3b. In Zone 1 it is required for sites of one hectare or more, land identified by the Environment Agency as having critical drainage problems, and land identified in the relevant mapping or Strategic Flood Risk Assessment as at risk from any source now or in the future. The assessment should be proportionate to the proposal. [3] [4]
This is why a modest extension can need a flood document even though it is not a new housing scheme. Check the location, application type and council's requirements before deciding that the report is unnecessary. [5]
“Critical drainage area” can mean different things
Distinguish an area notified by the Environment Agency as having critical drainage problems from a Critical Drainage Area identified through local surface-water studies. Kingston's guidance expressly explains that these are not the same designation. A local designation can still be relevant to the council's drainage and planning requirements. [6]
Ask which document and map the council is using, what problem it identifies, and what evidence the application needs. Do not infer a national exemption or requirement merely from the initials “CDA”.
The sequential test is a separate question
A Flood Risk Assessment examines the site's risks and how they would be managed. The sequential test considers whether development should instead be directed to reasonably available lower-risk sites. Policy F5 exempts householder development from that test, but this does not cancel F4's assessment requirements. [4]
A proposal creating a separate dwelling should not be described as an ordinary house extension simply to use that exemption. Basement dwellings are classified as highly vulnerable in the NPPF's flood-risk table; assess the actual proposed use and relevant zone compatibility, rather than assuming waterproof construction makes the location acceptable. [1] [4]
What to prepare
Collect a site plan, existing and proposed levels, the intended use of each space, access arrangements and the relevant flood information. The assessment may need to address future flood levels, safe access, resistance or resilience measures, drainage and whether risk would increase elsewhere. Agree the scope with the appropriate adviser and council before commissioning detailed modelling. [2] [5]
Use the official long-term flood-risk service for an initial check, then the planning-specific evidence for the application. Keep the maps and assessment date in the project record so the design is based on the information actually reviewed. [7] [2]