Do I need a flood risk assessment for an extension?

It depends on the zone, the size of the proposal and the local validation requirements. Many modest household extensions outside mapped zones do not, while anything in a higher risk zone usually will. Checking before submission avoids an invalid application.

How close can a soakaway be to the house?

There is a minimum separation to protect foundations, and the exact figure depends on the design and the ground. The important point is that it is a design decision with a stated distance, not something to be positioned wherever there is space left.

My neighbour says my extension will flood their garden. Are they right?

They may be, if the new roof and paving discharge downhill without attenuation. Showing a designed route and a soakaway or attenuation solution usually resolves the objection because it replaces a fear with a drawing.

Does building in a flood zone affect insurance?

It can affect availability and premium, which is a good reason to establish the true risk early rather than after the design is fixed.

Site · Cornwall

Flood risk and surface water in Cornwall, the checks that decide whether a scheme is viable

Cornwall floods in two different ways and they need two different answers. Rivers and tidal creeks flood slowly and predictably enough to be mapped. Surface water floods fast, from steep ground and saturated fields, and it arrives in places no map has ever shown as risky. Planning treats both seriously, and on many valley and coastal sites the drainage strategy shapes the design more than the architecture does.

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Flood zones and what they mean for consent

Mapped flood zones drive whether a proposal needs a flood risk assessment and whether the use you want is considered appropriate in that location. A modest extension to an existing home is treated far more sympathetically than a new dwelling in the same place. The practical questions are the same in both cases: where does water come from, how deep does it get, where does it go, and how do people leave safely if it happens.

Check the mapped zone for the site and for the access route, which is often the weaker link
Establish finished floor levels early, because they are difficult to change once the plan is set
Consider flood resilient construction rather than only flood exclusion
Confirm whether the risk is fluvial, tidal, surface water or a combination of them

Surface water is the everyday problem

Most Cornish drainage disputes are not about rivers. They are about a new roof and a new area of hard standing sending water somewhere it did not go before, usually towards a neighbour or a lane. Any extension adds impermeable area, and the design has to show where that water is being taken. Soakaways work well in some ground and not at all in others, which is why a percolation test in the actual garden beats an assumption every time.

The drainage hierarchy

The expected order is simple and worth knowing before you design. Infiltrate into the ground where the ground allows it. If it does not, discharge to a watercourse where one is available and the discharge can be controlled. Only where neither is possible should surface water go to a sewer, and then usually at a restricted rate. Permeable paving, rainwater harvesting and attenuation crates are all ways of keeping a scheme inside that hierarchy rather than arguing with it.

What this means for the drawings

A drainage layout showing gullies, falls, inspection chambers, the soakaway location and its distance from buildings and boundaries answers most of what a planning officer and building control will ask. Where the site sits on a slope, cut off drainage at the uphill boundary is often the cheapest single intervention available and it belongs on the drawing rather than in a conversation on site.

Frequently asked

Straight answers.

Do I need a flood risk assessment for an extension?: It depends on the zone, the size of the proposal and the local validation requirements. Many modest household extensions outside mapped zones do not, while anything in a higher risk zone usually will. Checking before submission avoids an invalid application.
How close can a soakaway be to the house?: There is a minimum separation to protect foundations, and the exact figure depends on the design and the ground. The important point is that it is a design decision with a stated distance, not something to be positioned wherever there is space left.
My neighbour says my extension will flood their garden. Are they right?: They may be, if the new roof and paving discharge downhill without attenuation. Showing a designed route and a soakaway or attenuation solution usually resolves the objection because it replaces a fear with a drawing.
Does building in a flood zone affect insurance?: It can affect availability and premium, which is a good reason to establish the true risk early rather than after the design is fixed.

Worried about water on your site?

Tell us the address and what you are planning. We will tell you which drainage questions your application will have to answer.

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