Cornwall sits over granite and other rocks that can release radon, a naturally occurring radioactive gas. That does not mean every home is unsafe, but it does mean radon is taken seriously when you build, extend or renovate. Building Regulations require protection in higher-risk areas, and many existing houses benefit from simple testing. This guide gives neutral, practical guidance on when protection is needed, how membranes and sumps work, and what to check on older properties. Always follow current Approved Document guidance and site-specific advice from Building Control or a radon specialist.
Why Cornwall is a high-radon area
Radon comes from the radioactive decay of uranium in rocks and soils. Levels vary street by street, but large parts of Cornwall and Devon fall in the higher bands on UK radon maps. Public bodies publish indicative maps so you can see whether a postcode is in a radon Affected Area. Maps are a starting point, not a guarantee of the level inside a particular house: construction, floors, ventilation and how the building sits on the ground all matter.
If you are buying, selling or altering a home, it is sensible to check the indicative map and, where relevant, arrange a proper test. For new work, Building Control will expect you to design for the risk band that applies to the site.
Building Regulations Part C and when protection is needed
Approved Document C of the Building Regulations covers site preparation and resistance to contaminants and moisture, including radon. In higher-risk areas, new dwellings and many extensions generally need radon protective measures as part of the floor and foundation design. The exact package depends on the risk band and the type of floor construction.
In broad terms:
Basic protection often means a well-installed radon-resistant membrane across the ground floor, carefully sealed at joints, edges and service penetrations.
Fuller protection in higher bands may add a sump (or provision for one) beneath the floor so a fan can be fitted later if needed, sometimes with underfloor ventilation arrangements.
Extensions can trigger protection requirements even when the original house was built before modern standards, particularly where a new ground floor is created.
Details change with updates to guidance and with how your floor is built (beam-and-block, slab, suspended timber and so on), so treat this as an overview and confirm the current approach with your designer and Building Control. Our Building Regulations guide for Cornwall explains the inspection process around a typical project, and our groundworks and foundations service sits at the stage where membranes and sumps are usually installed.
Membranes, sumps and careful detailing
A radon membrane is a continuous gas-resistant sheet laid under or within the ground floor construction. Its performance depends almost entirely on workmanship:
Laps and joints must be sealed to the manufacturer’s system.
Edges need proper sealing to walls and damp-proof courses so there is no path for soil gas.
Pipes, cables and ducts that pierce the membrane need proprietary top-hats or sealed collars.
Damage during following trades must be repaired before it is covered up.
A radon sump is typically a small void or collection point under the floor, with a pipe ready to connect to an extract fan if monitoring shows it is needed. In some designs the sump and pipework are installed from the start; in others the design allows a sump to be added later with less disruption. Either way, the membrane and any underfloor voids need to be thought through together with drainage, insulation and damp-proofing so one layer does not undermine another.
This work is usually done during groundworks and foundations, before the floor is closed up, which is why early coordination on new builds and home extensions matters.
Extensions, conversions and mixed construction
When you extend a Cornish house, the new ground floor will normally be designed to current radon standards for the site. Linking old and new construction needs care so that the membrane and seals continue to work at the junction. Conversions of barns or other buildings that create new habitable ground floors can also bring radon measures into play; see our notes on barn conversions for the wider project context.
Upper-floor work alone does not usually change the radon story, but any scheme that alters ground-floor ventilation, fills underfloor voids or seals a previously ventilated space should be thought through with radon in mind.
Testing existing homes
If you already live in a house in a radon Affected Area, the practical next step is often a test rather than immediate building work. Short-term and longer-term detectors are available; longer measurements are generally more reliable for deciding whether action is needed. Results are usually compared with the UK action level guidance published by the relevant public bodies.
If levels are raised, remedies can include improved ventilation, a positive-input or other ventilation strategy, or a sump-and-fan system that draws soil gas from under the floor and exhausts it safely outside. Many of these can be retrofitted with less disruption than people expect, though the right choice depends on floor type and measured levels. This is specialist territory: use accredited advice rather than improvised sealing that could worsen indoor air quality.
Radon protection FAQs
Does every new build in Cornwall need a radon membrane?
Most sites in higher-risk bands need protective measures under Building Regulations. Confirm the requirement for your plot with Building Control and the current Approved Document C guidance.
Do I need radon protection for a single-storey extension?
Often yes where a new ground floor is formed in a radon Affected Area. The detail depends on the risk band and how the extension joins the existing house.
Can I test for radon myself?
Homeowners can place detectors following supplier instructions. For decisions about major building work or property transactions, follow current public guidance on test length and interpretation.
Will insulating a solid floor make radon worse?
Not necessarily, but changing floors and ventilation can alter how soil gas moves. Design insulation, damp-proofing and radon measures together so seals remain intact.
Talk to us about your project
Whether you are planning a new home, an extension or groundworks where radon detailing matters, we can help coordinate the build side of membranes and sumps with the rest of the programme. Call Cornish Building Company on 01208 816991 or contact us to talk through your site.
