Radon affected areas and what the application addresses

A radon affected area is one where at least 1 per cent of properties are estimated to be above the radon action level of 200 becquerels per cubic metre. If your plot falls in one, the new dwelling needs protective measures built into the floor construction, and the requirement comes from the Building Regulations rather than from planning.

That split catches self-builders out. The planning officer may never mention radon. The building control body certainly will, and by then the ground floor detail is already priced.

What a radon affected area means

Radon is a naturally occurring radioactive gas produced by the decay of uranium in rocks and soils. It disperses harmlessly outdoors and accumulates indoors, entering through gaps in the ground floor, service penetrations and construction joints.

The UK Health Security Agency sets an action level of 200 becquerels per cubic metre for homes, above which remedial action is recommended, and a target level of 100 becquerels per cubic metre as the level to aim for. The mapping, produced jointly with the British Geological Survey, estimates the proportion of properties in each grid square expected to exceed the action level.

Geology drives it. Granite areas, some limestones, certain sandstones and parts of the country underlain by uranium-bearing strata carry the highest estimates, and the pattern does not follow administrative boundaries at all. Two plots 300 metres apart can fall in different bands.

Checking the mapping for a specific site

The public map viewer gives an indication by area and is enough to tell you whether radon is a live issue. It is not enough for the building control submission.

For that you need a site-specific radon risk report, ordered against the address or the grid reference, which states the estimated percentage of properties above the action level for that location and therefore the level of protection required. It costs modestly, arrives quickly, and is the document building control will ask to see.

Order it at the design stage, not when the foundations are being set out. The difference between no measures, basic measures and full measures changes the floor build-up, the service entry details and in some cases the foundation design.

Where a site is close to a band boundary, the report will say which side it falls on. Do not interpolate from the public map.

Basic and full protective measures

The guidance is set out in the BRE document on radon protective measures for new buildings, and it divides provision into two levels keyed to the estimated percentage of properties above the action level.

Basic protection means a continuous radon-proof barrier across the whole footprint of the building, lapped and sealed at joints, carried through the cavity and sealed around every service penetration. In practice it is a membrane detail, and it fails at the details rather than in the middle of the floor.

Full protection means the barrier plus a means of reducing the pressure beneath the floor: a radon sump with a pipe to a capped stack that can be fitted with a fan later, or a ventilated subfloor void where the construction allows. The sump is usually installed and left passive, with the fan added only if post-completion testing shows it is needed.

Verify the current bands against the site-specific report rather than relying on a table, because the guidance has been revised and the report states the requirement for that address.

Which regime applies, planning or building control

Approved Document C to the Building Regulations covers site preparation and resistance to contaminants, and it is the route through which radon protection is required. That applies to new dwellings, to conversions creating new habitable accommodation, and to extensions that create habitable rooms in an affected area.

Planning does not normally control radon at all. There is no policy test, no consultee and no standard condition, and a planning permission says nothing about whether the floor detail is adequate.

The two regimes do meet in one place. Where a site also has a ground gas issue from previous use, methane or carbon dioxide from made ground or a former landfill, the gas protection design has to work alongside the radon measures, and the two are often delivered by the same membrane and sump arrangement. That is a contamination assessment matter, and a contaminated land assessment must be prepared by a suitably qualified and experienced person rather than assembled from a template.

Basements and other situations that need more

A basement sits in the ground on all sides, has a larger surface in contact with the soil, and ventilates poorly. Radon concentrations in basements are routinely higher than in the rooms above.

Where habitable basement accommodation is proposed in an affected area, treat the protection as a design problem rather than a detail. The waterproofing system, the ventilation strategy and the radon measures have to be resolved together, because a fully tanked basement with mechanical ventilation behaves differently from a drained cavity system with a sump.

Two other situations deserve attention. Buildings with suspended timber floors over an unventilated void, where blocking historic airbricks during a refurbishment has made things worse. And extensions that seal a previously ventilated external wall, which can change the pressure regime under the original house.

When a council asks at application stage

Some local validation lists ask for a radon statement or a copy of the site-specific report with applications for new dwellings in affected areas. It is not a national requirement, so check the authority's own list before submitting rather than after.

Where the council does ask, the answer is short: the report, the band it places the site in, and a sentence saying which level of protection will be provided and that the detail will be submitted to building control. A paragraph in the Design and Access Statement covers it.

Radon rarely changes a planning decision. Checking it early alongside the other site constraints avoids a late redesign of the floor, and the same discipline applies to the whole constraints exercise, as the note on ordering constraint checks in the right sequence sets out. Other unmapped or non-statutory issues behave similarly, as covered in the note on non-statutory designations that still stop schemes.

Recording the check and keeping the evidence

Keep the site-specific report, the design detail showing the membrane and any sump, photographs of the installation before it was covered, and the building control sign-off. That package is what a buyer's surveyor will want in ten years.

Testing after completion is the only way to know what the house actually achieves. Detectors are placed for three months to average out seasonal variation, and if the result is above the action level with a passive sump already installed, fitting a fan is a modest job. If no sump was installed, it is not.

One honest limitation. The mapping is an estimate derived from geology and measured results in the area, not a measurement of your plot. A site in a low band can still produce a high reading, which is why testing after occupation is worth doing wherever the geology suggests any risk at all.

Enter your postcode on PlanScope to see which site constraints and reports your project is likely to need. This is general guidance, not legal advice; check the position with your local planning authority, your building control body or a qualified professional before relying on it.

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