Radon affected areas and what the maps mean for a new build

A radon affected area is a place where the mapping predicts that one per cent or more of homes exceed the radon action level. If your plot sits in one, the new dwelling will need radon protective measures built in, and the level of protection depends on a property-specific report rather than on the colour of the area on a national map.

The thing most self-builders get wrong is timing. Radon protection interacts with the floor construction, the ground bearing slab or beam and block design, the service penetrations and the sub-floor ventilation. Finding out after the foundations are poured means retrofitting a sump, which is doable and irritating.

How radon affected areas are mapped

Radon occurs naturally from the decay of uranium in rocks and soils, and concentrations vary with geology. The joint mapping produced by the UK Health Security Agency and the British Geological Survey estimates the probability that a home in a given area exceeds the action level of 200 becquerels per cubic metre. A separate target level of 100 becquerels per cubic metre is the figure at which action is worth considering in existing homes.

The published maps are indicative and work at a grid scale. They are not a substitute for a report on your plot, because geology changes over short distances and the mapping shows probability rather than a measured value at your site.

Mapped probability bands determine the level of protection required, and the bands are set out in the guidance that accompanies the maps. What you need is the report for the specific address or grid reference.

Obtaining a radon risk report for a site

A property-specific radon report can be ordered against a postal address or a grid reference, and it states whether the property is in a radon affected area and which level of protection, if any, is required for new construction. The report is the document building control will ask for.

Order it at feasibility stage, alongside the desk study. On a plot with no postal address yet, the grid reference version is what you need, and the surveyor setting out the plot can give you the coordinates.

Radon is also routinely covered in the ground gas section of a Phase 1 desk study, so if you are already commissioning a Contaminated Land Assessment for another reason, ask for the radon position to be stated explicitly rather than mentioned in passing. Where mining, landfill or made ground is present, the ground gas regime and the radon regime interact, and the protection measures may need to serve both.

Basic and full protective measures

Approved Document C to the Building Regulations 2010 sets out the guidance, which in turn draws on the published protective measures guidance for new buildings.

Basic protection is, in essence, a radon-resistant barrier across the whole footprint of the building, including at junctions, service entries and cavity trays, and sealed at every penetration. The workmanship matters more than the specification, because a barrier with an unsealed soil pipe is not a barrier.

Full protection is the barrier plus an active or passive means of depressurising the ground beneath the building. On a ground bearing slab that normally means a radon sump connected to a vent pipe taken to a suitable termination, capable of being fitted with a fan later. On a suspended floor it can be achieved by cross-ventilating the underfloor void instead.

The property report tells you which row applies. Do not infer it from a neighbour's build or from a map extract.

Where the requirement sits in Building Regulations

Requirement C1(2) of Schedule 1 to the Building Regulations 2010 deals with resistance to contaminants from the ground, and radon falls within it. Approved Document C is the guidance on how to meet it.

This is a building control matter, which has two practical consequences. Building control will want to inspect the barrier before it is covered, so build the inspection into the programme rather than treating it as paperwork. And a planning permission granted with no mention of radon says nothing at all about whether protection is needed.

Planning does have a residual interest where ground conditions are a policy issue in their own right, and where a site has both radon and a contamination history a Geotechnical Assessment and a ground gas risk assessment may be conditioned. The distinction is that planning conditions deal with site suitability and remediation, while the radon measures themselves are delivered through building control.

Basements and extensions in affected areas

Basements need particular care. A below-ground room has a larger surface area in contact with the ground, poorer natural ventilation and a tendency to accumulate gas, so the measures that suffice for a ground floor may not be adequate. Where a basement is proposed in a radon affected area, get specific advice rather than applying the standard detail, and consider a design that allows a fan to be added to the sump.

Extensions to existing homes in affected areas are covered by the guidance as well, and the answer depends on the size of the extension relative to the existing house and on whether the existing dwelling has been tested. An extension that seals the ground beneath a previously vented area can raise levels in the existing house even where the extension itself is protected.

Conversions of existing buildings to dwellings, including permitted development conversions, sit in the same territory. A structural survey will tell you about the building, and the radon report will tell you about the ground, and both are needed. Where the conversion involves a historic structure, a Structural Survey and the radon detailing often have to be reconciled, because a continuous barrier is harder to achieve in a building with solid walls and no damp proof course.

Testing after completion

Protective measures reduce risk. They do not guarantee a result, and the only way to know the level in the finished house is to measure it.

Testing uses passive detectors left in place for three months, normally in a living room and a bedroom, and the result is adjusted to an annual average. Three months is the standard period because radon levels fluctuate daily and seasonally, and short-term readings are unreliable.

If the result exceeds the action level in a house built with full protection, the usual answer is to fit a fan to the existing sump, which is why leaving the sump accessible and the vent pipe correctly sized matters at construction stage. In a house with basic protection only, remediation is more involved.

Keep the test result with the house papers. Buyers increasingly ask, and a measured figure is worth more than a statement that the building was constructed to the guidance.

One caution on interpreting results. A reading just under the action level in a house with basic protection is not a reason to relax, because levels vary year to year with weather and with how the house is ventilated. If the figure is close to the threshold, repeat the test in a different year before deciding to do nothing.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. Enter your postcode on PlanScope to see which ground and contamination documents your site is likely to need.