A householder flood risk assessment costs from £375 at Fixed Fee Plans. We check whether your address needs one, and if it does we write it to the Environment Agency's standing advice, with the floor levels and flood resilience measures the council expects to see for a minor extension.
Most owners learn they need one the hard way, at validation: the council writes that the site is in Flood Zone 2 or 3 and the application is incomplete without an assessment. It is a short document, but it has to follow a defined structure, and a wrong flood level is the kind of error a planning officer, or the Environment Agency, finds.
Do I need a flood risk assessment for my extension?
If the site is in Flood Zone 2 or 3, yes. The planning practice guidance says a site-specific assessment is needed for development, including minor development and changes of use, in those zones. In Flood Zone 1 you need one if the site is 1 hectare or more, or if the flood map shows a surface water risk now or in future, and the Environment Agency adds that a Zone 1 site facing increased risk in future should be assessed as if it were in Zone 2.
So the answer depends on your address. The flood map for planning gives it in a minute, and we check it before quoting. If the site is clear in every respect, we tell you no assessment is needed, which is the least costly outcome. Our constraints report shows flood zone alongside conservation area and Article 4 status if you want the whole picture before deciding.
What does the standing advice ask for?
The Environment Agency's standing advice for minor extensions applies to a householder extension of no more than 250 square metres of floor space in Flood Zone 2 or 3. It asks for the site address, a description, the estimated flood level including climate change over the extension's lifetime, the finished floor levels, a plan for flood resistance and resilience, and supporting plans.
The headline number is the floor level. The advice recommends finished floor levels at least 600mm above the estimated flood level, with flood-resistant materials to the same height; it can be reduced to 300mm where the flood level is known with high certainty, and increased where it is not. If you cannot raise the floor, you need extra resilience measures instead. Over 600mm of water, it says to get advice from a structural engineer, and our structural calculations service covers the beams and openings that follow.
What does it mean for the design?
Often a raised floor. A rear extension that sits level with the old house may have to step up, which means a step inside, a ramp outside and a different threshold for the garden doors. These are drawing decisions as much as flood ones, so we write the assessment and mark the levels on the plans in one go.
We also look at where the water goes. The standing advice says your plans need to show how the extension will not be flooded by surface water or groundwater, for example by diverting water safely within the site or raising floors above estimated flood depths. For a house that floods from the river, a different design may be better than defending the old one. Where the assessment shows the extension is unwise, the report says that, not what you hoped to read.
What does it cost?
Our fee is from £375, and it is a floor: a larger extension, a site with several buildings or one in a critical drainage area is quoted in writing. The council charges nothing extra for the assessment itself; the planning fee for the application is £548 for a householder extension and is a separate payment.
If you do not yet have drawings, our complete planning drawing service at £645 drawings + £150 survey (£795 total) produces them and the application, and our permitted development checker shows whether a smaller version avoids an application altogether. Permitted development is no escape from flood risk in every case, but an extension that needs no application needs no assessment either, and we say when that applies.
Frequently asked questions
Do I need a flood risk assessment in Flood Zone 1?
Usually not for a house extension. You need one if the site is 1 hectare or more, if surface water flooding is shown to be a risk now or in future, or if the flood map shows increased river or sea risk in future.
What is the 600mm rule?
The Environment Agency recommends finished floor levels at least 600mm above the estimated flood level, with flood-resistant materials to that height. It may be reduced to 300mm with high certainty about the level.
How much does a flood risk assessment cost?
Our fee is from £375. There is no separate council charge for the assessment; the planning application pays the usual fee.
Can I extend in Flood Zone 3 at all?
Often yes, for a minor extension, if it follows the standing advice. The council decides, and a house that floods deeply may still be refused, so the assessment is your evidence.