A householder flood risk assessment from us starts at from £375. It is written to the Environment Agency's standing advice for minor extensions, and it goes with your planning application. No council fee applies to the assessment itself.
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Many houses do not need one. The ones that do sit in an area the government's flood map marks as Flood Zone 2 or 3, or in an area with critical drainage problems, and for a small extension the assessment is a short document. This post explains how to tell, and what the document must contain.
When does a planning application need a flood risk assessment?
Three cases, set out in the government's guidance. Development in Flood Zones 2 or 3. Development in Flood Zone 1 on a site of one hectare or more. And development in areas with critical drainage problems (GOV.UK). The guidance says these apply to all development, including minor development and changes of use.
Check your address on the Environment Agency's flood map for planning before you commission anything. If you are in Zone 1 on a normal-sized plot and your council has not flagged a drainage problem, the answer is usually no. If you are unsure, our constraints check reads the maps for an address.
What is a minor extension, and why does it matter?
It is the category that makes a householder assessment short. The guidance defines a minor extension as a householder or non-domestic extension with a floor space of no more than 250 sq m, and says to use the Environment Agency's standing advice for it in Flood Zones 2 and 3 (standing advice).
That means the assessment follows a set method instead of a bespoke study, which is why the fee can be fixed. A larger extension or a new dwelling is not a minor extension and needs a fuller assessment, which we quote separately.
What does the standing advice require?
Floor levels first. The standard recommendation is to set floor levels at least 600mm above the estimated flood level. Where the level is known with high certainty that may be reduced to 300mm, and it may need to be higher where there is more uncertainty. Flood-resistant materials should also be used up to at least 600mm above the estimated flood level (standing advice).
Where floor levels cannot be raised, the standing advice asks for extra flood resistance and resilience measures protecting the property to at least 600mm above the estimated level. The wider guidance adds that design flood and residual risk estimates must include an appropriate allowance for climate change. Our assessment sets out those levels and measures against your drawings.
What does it cost, and what else is there?
Our fee starts at from £375. It rises for a larger or more complex site, and we confirm the total in writing before we start. The council does not charge for the assessment, and it is not a separate application: it is a validation document.
You may also need the planning drawings and the application itself, which the extension plans package prices end to end. Councils can only make an assessment a validation requirement if it is on a local list published on the council's website (MHCLG guidance), so check the list as well as the map.
Frequently asked questions
Is a flood risk assessment the same as flood insurance advice? No. It is a planning document that shows how the extension is designed for flood risk. Insurance is a separate matter with your insurer.
Do I need one for a loft conversion? Usually not, because a loft does not change floor levels or footprint. If your address is in a flood zone, ask the council whether it wants one.
Can the assessment stop my extension? It can show that floor levels must rise or resilience measures be added, which changes the design and the cost. We would rather you knew that before drawings are finished.
What if my address is in Flood Zone 1? Then a householder extension normally needs no assessment unless the site is one hectare or more or in a critical drainage area. The council's local list says what it asks for.
