GOV.UK says a flood risk assessment is required for all development, including minor extensions, that needs a planning application in Flood Zones 2 or 3, and it treats a householder extension of up to 250 square metres as a “minor extension”.

When a flood risk assessment is needed

GOV.UK’s guidance for planning applicants in England says you should complete a flood risk assessment (FRA) for all development, including minor development, proposed in Flood Zones 2 or 3 or within Flood Zone 3b. It also applies in some Flood Zone 1 cases, such as sites of 1 hectare or more or where the flood map for planning shows surface water risk. The local planning authority may refuse an application without a satisfactory FRA. The guidance is written for people applying for planning permission, so ask your council whether your conservatory needs an application and check the flood map for planning first.

GOV.UK adds that if your site is in Flood Zone 1 but the council’s strategic flood risk assessment or the flood map for planning shows increased future flood risk from rivers or the sea, you should complete an FRA and apply the guidance as if the site were in Flood Zone 2. It also mentions areas with critical drainage problems, currently designated only in Devon and Cornwall, where local standing advice applies and can be obtained from the council.

What counts as a minor extension

GOV.UK defines a minor extension as a householder or non-domestic extension with a floor space of no more than 250 square metres. For a minor extension in Flood Zone 2, 3 or 3b, applicants should follow the Environment Agency’s standing advice for minor extensions, covering what to include in the FRA, floor levels, extra flood resistance and resilience measures, and other permissions or consents. The guidance says a simple, low-risk development such as a house extension may be something you can assess yourself, but if you are unsure you should contact the Environment Agency for advice.

What the assessment should contain

The standing advice says an FRA should be proportionate and include your site address, a description of the development, the estimated flood level taking climate change into account, finished floor levels, flood resistance and resilience plans and any supporting drawings. It suggests using the Environment Agency’s FRA template. The design flood is a river flood with a 1 in 100 annual probability or a tidal flood with a 1 in 200 annual probability, each plus a climate change allowance, and a surface water event of 1 in 100 plus climate change. Levels should be stated relative to Ordnance Datum, and you may need a land survey by a qualified surveyor if that information is not available.

Floor levels and resilience

The advice recommends floor levels at least 600mm above the estimated flood level, with flood-resistant materials used up to at least 600mm above it. It says this may be reduced to 300mm if there is a high level of certainty about the estimated flood level, and increased where there is particularly high uncertainty. If your design aims to keep out more than 600mm of water, get advice from a structural engineer, because flood water puts pressure on buildings. Where you cannot raise floor levels, you need extra flood resistance and resilience measures, aiming to exclude flood water where possible and speed recovery if water gets in, and your plans must show how surface water and groundwater flooding will be managed.

Functional floodplain, off-site risk and other consents

The advice says to check with the local planning authority whether your minor extension is in an area of increased flood risk because of multiple minor extensions, and if so to assess off-site flood risk. In Flood Zone 3b, the functional floodplain, table 2 of the planning practice guidance says highly vulnerable, more vulnerable and less vulnerable development should not be permitted. You may also need a separate permit or consent if the work is within 20 metres of a main river, flood defence or flow control structure, directly affects a watercourse, or involves floodplain activity that could divert or obstruct floodwater. GOV.UK stresses that getting planning permission does not guarantee a flood risk permit or consent, so check early.

FAQs

Do I need an FRA if my conservatory is permitted development?

The GOV.UK guidance addresses planning applications. If no application is needed, ask your council and the Environment Agency what applies, and check for other consents near watercourses or defences.

Does this apply in Wales, Scotland or Northern Ireland?

The guidance is for England, referring to the Environment Agency and English planning practice guidance. Other nations have their own flood risk policies.

How do I find my flood zone?

GOV.UK says to use the Environment Agency’s flood map for planning to find whether the site is in Flood Zone 1, 2 or 3 and whether it is at risk of surface water flooding.

Bottom line

If your English conservatory project needs a planning application and lies in Flood Zone 2 or 3, expect to submit an FRA that follows the Environment Agency’s minor extension advice, including estimated flood levels and floor levels or resilience measures, and check for river-related permits. Ask your planning authority early. This is general information, not planning or engineering advice.

Sources

This guide draws on the following primary sources, current as of 20 September 2026:

• GOV.UK, “Flood risk assessments: applying for planning permission”
• GOV.UK, “Preparing a flood risk assessment: standing advice”

Related guides

Compare Conservatories is an independent guide. We may earn a fee from some links; this never affects what we write. This article is general information, not legal or planning advice on any specific property — always confirm requirements directly with your local planning authority.