Cornwall has rivers that rise quickly after heavy rain, tidal harbours and estuaries, and plenty of sloping ground that sheds surface water towards roads and gardens. If you are extending, renovating or building, flood risk is a different question from everyday weatherproofing. Our guide to weatherproofing coastal homes is about rain, wind and salt on the building fabric. This post is about fluvial (river), tidal or coastal, and surface-water flood risk, and how that can affect planning, design and groundworks.
It is general guidance for homeowners, not a flood-risk assessment and not a substitute for official maps or specialist advice on a particular site.
Check the official flood maps first
Before you sketch an extension or commit to a plot, look up the address on the government's flood-risk services. The long-term flood-risk maps and the flood map for planning, both available on gov.uk and based on Environment Agency data, show risk from rivers and the sea and from surface water. They are a starting point, not a detailed survey of your garden.
At a homeowner level, the useful questions are simple:
Is the site shown as being in an area of flood risk, and from which source — river, tide or surface water?
Would a new extension, replacement dwelling or converted building sit in a higher-risk area than the rest of the plot?
Are there obvious overland flow routes, low spots or historic flooding that the maps may not describe in detail?
Planning policy steers new development towards areas of lower flood risk where that is practical (often called sequential thinking). Where development in a higher-risk area is being considered, extra justification and design measures may be needed (the exception test, in national policy language). You do not need to become a flood-risk assessor to benefit from that logic: if there is a choice about where on a plot to place an extension, the higher, less exposed part of the site is usually the better starting point. For some applications Cornwall Council will ask for a site-specific flood-risk assessment prepared by a suitably qualified specialist.
Harbour and estuary towns where checks especially matter
Flood risk is not only a countryside-river issue. Harbour and estuary towns can be affected by high tides, storm surge and river flows arriving at the same time. Places such as Hayle, Fowey, Looe, Padstow and the tidal stretches of Truro are examples of settlements where it is particularly worth checking the maps early. That is not a claim about any individual street or property. Some homes in those towns sit well above the water; others are closer to quays, creeks or low-lying roads. Our Hayle building guide notes the harbour and estuary setting there as one local reason to look at flood risk before you design.
How flood risk affects planning and design
If a site is at risk, planning officers may look at:
Whether the extension or new building is appropriate in that location.
Finished floor levels, often set above a design flood level with an additional margin.
Flood-resilient or flood-resistant construction details.
Safe access and escape routes.
Whether the proposal would increase flood risk elsewhere, for example by displacing flood water or increasing runoff.
These points may appear as planning conditions. They sit alongside ordinary design questions for home extensions and new builds in Cornwall. Building Regulations also remain relevant, particularly around site preparation, moisture resistance and drainage. Approval under one regime does not automatically satisfy the other.
Flood-resilient construction in outline
Where a building may get wet, design can aim to keep water out for a time (resistance) or to make the building recoverable after water has entered (resilience). Typical resilience measures discussed in national guidance include:
Raising electrical sockets, consumer units and boilers above an agreed flood level.
Choosing floor finishes that can be cleaned or replaced without ripping out the whole structure.
Using materials that dry reasonably well, rather than finishes that fail after one soaking.
Fitting non-return valves on drainage to reduce the chance of sewer flooding backing up into the house.
Planning how walls and insulation will behave if they do get wet.
The right package depends on the depth and source of flooding and on whether the building is historic. Older solid-walled houses need a particularly careful approach so that moisture can still evaporate. This is outline guidance only; a specialist should specify measures for an at-risk site.
Surface water, soakaways and groundworks
Surface-water flooding happens when rain cannot soak away quickly enough, often after intense downpours or where ground is already saturated. New roofs and paving add to the volume of water you have to manage. Building Regulations generally look for surface water to go to a soakaway first where the ground allows, then to a watercourse, then to a sewer.
Soakaways and drainage fields are not always suitable on sites with a high water table, clay soils, contamination or known flood risk. Our guide to septic tanks, drainage and soakaways explains the wider drainage checks, and our groundworks and foundations post covers ground investigation. On some plots, permeable paving, attenuation crates or a carefully designed outfall will be more appropriate than a simple soakaway. Those decisions belong in the early groundworks conversation, not as an afterthought once the extension is already set out.
Flood risk and building work FAQs
Does a flood-map result mean I cannot extend?
Not necessarily. Many existing homes in flood-risk areas have been extended with the right design, floor levels and supporting information. The maps tell you to investigate, not that every project is impossible.
Do I always need a flood-risk assessment?
Cornwall Council and national planning practice guidance set out when a site-specific assessment is expected, often relating to flood zones and the type of development. Check the current guidance for your proposal rather than relying on a rule of thumb.
If my site is not in a flood zone, can I ignore surface water?
No. Surface-water flooding can affect sites outside river and tidal flood zones. Look at the surface-water mapping as well, and design drainage so that extra roof and paving do not simply push water onto a neighbour.
Is this the same as protecting a house from Cornish weather?
No. Weatherproofing deals with rain, wind and salt hitting the walls and roof. Flood risk is about water rising from rivers, the sea or the ground around the building, and it has its own planning and design implications.
Talk to us about your project
If you are planning an extension or renovation and flood risk may form part of the picture, we can help you think through the practical building and groundworks issues alongside the specialist advice you may need. Call Cornish Building Company on 01208 816991 or contact us to discuss your site.
