Flood risk assessment for a householder extension

If your house is in Flood Zone 2 or 3, your extension will very probably need a flood risk assessment with the application, and for a householder extension that assessment is a short document you can often write yourself. Two or three pages, a plan, the flood zone confirmed from the official map, your floor levels, and a list of the resilience measures you are building in. Not a modelling exercise.

The reason it is short is that householder development counts as minor development in national flood risk guidance. Minor development is exempt from the Sequential Test and the Exception Test, and the Environment Agency is not normally consulted on it, relying on standing advice instead. So the audience for your assessment is the case officer, and sometimes the council's own drainage engineer.

When one is needed

Three triggers matter for a house extension.

The site lies in Flood Zone 2 or Flood Zone 3, as shown on the Flood Map for Planning. Print the map with the date on it and put it in the assessment.

The council's local validation list asks for one, which some authorities do for any development in an area with recorded surface water flooding even outside the fluvial zones.

The proposal includes a basement, a self-contained annexe or a change to the number of dwellings, any of which takes the proposal out of the simple householder category and into something needing more care.

Flood risk covers more than rivers and the sea. Surface water, groundwater, sewer flooding and reservoir risk all appear on the long term flood risk service, and if the council has a critical drainage area or a surface water flood history at your address, say so and address it. A short paragraph on a known source that the officer already knows about is worth more than a silence.

The proportionate approach for minor development

National guidance is explicit that the assessment should be proportionate to the risk and the scale of the development. For a rear extension, that means the following structure.

1. Describe the site: the address, the plot, the existing house, ground levels and the existing internal floor level, with a levels drawing if you have one.
2. Identify the sources of flood risk, naming each one and the evidence for it, including the Flood Zone and any surface water risk shown on the official maps.
3. State the proposed floor level of the extension, and its relationship to the existing floor level and to any known flood level.
4. Set out the flood resistance and resilience measures being built in.
5. Explain the effect on flood risk elsewhere, including whether any floodplain storage is lost and how surface water will be dealt with.
6. Describe the residual risk and what the occupiers will do, including signing up to flood warnings.

That is it. A Flood Risk Assessment for a house extension does not need modelled hydrographs, and an assessment padded out with generic national policy text irritates the person reading it.

Floor levels and resilient construction

The textbook answer to flood risk is to raise the floor above the design flood level with freeboard. On an extension that is usually impossible, because the new floor has to meet the existing floor, and stepping down into an extension from a hallway is not a sensible outcome.

So the answer is resilient construction: accepting that water may enter and designing so that the damage is limited and the building dries out.

• Solid concrete floor slab rather than a suspended timber floor.
• Closed cell insulation that does not absorb water, in the floor and in the lower part of the walls.
• Water resistant render or lime plaster in the lower part of the wall rather than gypsum plasterboard to floor level.
• Electrical sockets, consumer unit and boiler controls raised well above the likely flood level.
• Tiled or sealed floor finishes at ground level, with any timber elements removable.
• Kitchen units on plinths or of water resistant construction where the kitchen is in the flooded zone.

Resistance measures sit alongside those: demountable door barriers, covers for airbricks, and non-return valves on the drainage so that a surcharged sewer does not back up into the house. The government's guidance on flood resilient construction sets out the principles, and naming the specific measures on the drawings is what turns an intention into something the council can condition.

Loss of floodplain storage

This is the point where a householder extension can actually fail. If your garden currently floods and holds water, an extension built on it displaces that water, and the volume has to go somewhere.

In Flood Zone 3a the usual approach is to keep the footprint modest, to raise the extension on a void or open undercroft where practical, and to show that the loss of storage is very small in the context of the floodplain. In Flood Zone 3b, the functional floodplain, the position is harder, because that is land where water has to flow or be stored in a flood, and councils will look for the extension not to reduce storage at all.

Level for level compensatory storage, which is the standard solution on a development site, is rarely practical in a domestic garden. Be honest about that in the assessment rather than proposing something you will not build.

Where the case turns on whether the Exception Test applies, the two limbs of that test are set out in exception test evidence: the two limbs in practice, although for householder development proper it should not be engaged at all.

What the council will and will not ask for

Councils vary, and the pattern is reasonably consistent.

If you are asked for modelled flood levels, you can request flood risk data for your address from the Environment Agency rather than commissioning a model. That request gives you the levels the Agency holds, which is normally enough to set a floor level or to specify resilience measures to a defined height.

Where a basement or a lower ground floor is proposed, the assessment becomes a different document, and the reasons are set out in basement development and the flood risk objection.

Writing it yourself, and when not to

For a rear or side extension in Flood Zone 2, or in Zone 3a with a small footprint and no storage loss, a careful homeowner or their architect can write the assessment. Use the official maps, describe the measures precisely, and keep it to the point.

Bring in a flood risk consultant when any of these apply: the site is in the functional floodplain, the council asks for compensatory storage, a basement or below ground room is proposed, the extension is large relative to the plot, or the council's drainage engineer has raised an objection you cannot answer with the data you hold.

The Sequential Test (Flood) is worth understanding even though it does not apply to your extension, because officers occasionally ask for one in error, and being able to point at the minor development exemption in the guidance saves a month.

One honest caveat about climate change allowances. The allowances used to set design flood levels are updated periodically and vary by river basin district and by the lifetime of the development, so take the current figures from the Environment Agency guidance rather than from an older report on a neighbouring property.

Enter your postcode on PlanScope to see whether your site falls in a flood zone and which reports the extension is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice, so check the flood risk requirements for your address with your local planning authority before you submit.