Radon protective measures and your planning application

Radon protective measures planning questions usually have a short answer: radon is a building regulations matter, not a planning one. No national policy requires a radon report with a planning application, and most councils do not ask for one. What radon does change is the substructure, and that can change the things planning does care about, such as finished floor levels, the overall height of a house on a tight plot, and the position of external vent terminations. Deal with it at design stage and it costs very little. Discover it after the slab is designed and it costs a redesign.

How radon potential is established for a site

Radon potential in England and Wales is mapped by the British Geological Survey with the UK Health Security Agency, based on geology and on measured indoor concentrations.

A Radon Affected Area is one where 1 per cent or more of properties are estimated to exceed the Action Level of 200 becquerels per cubic metre. The Target Level, the concentration below which a home should ideally sit, is 100 becquerels per cubic metre. Those two numbers run through the whole subject.

For a site you need a property-specific report rather than the free postcode-level check, because the mapping is at 1 kilometre grid resolution and a single plot can sit on a boundary between bands. The BGS and UKHSA report gives the protection level required for that address, which is what your building control body and your structural engineer will work from.

Order it early. The fee is small, it arrives quickly, and the answer determines whether the ground floor is a simple beam and block build-up or something with a sump and a fan.

What basic and full protection involve

BR 211, the BRE guidance on protective measures for new buildings, sets two levels and Approved Document C of the Building Regulations points to it.

The word that matters in both is continuous. Radon enters through gaps rather than through the membrane itself, so the detailing at service entries, at the cavity tray, at internal walls and around a stairwell is where protection succeeds or fails.

Full protection does not mean a fan is installed on day one. The usual arrangement is a sump and pipework brought to a capped terminal, so that if post-construction testing shows a problem a fan can be fitted without breaking out the floor. That is low-cost insurance and it is the part most often omitted on self-build sites.

Membranes, sumps and ventilation

A radon barrier is a specific product rather than a repurposed damp-proof membrane, and the specification should be checked against BR 211 rather than taken from a merchant's catalogue.

• Sheet membranes need taped laps to the manufacturer's specification, with top-hat units at every pipe penetration rather than tape wrapped round a pipe.
• A sump is typically a void former or a perforated chamber in the hardcore, connected by a sealed duct to an external terminal or to a position where a fan can be mounted.
• Ventilated subfloor voids work well with suspended floors, and they need cross-ventilation that will not be blocked by later landscaping, which is a planning-adjacent detail because it constrains external levels.
• Where a ground-bearing slab is used, the barrier usually sits above the slab, which raises the build-up and therefore the finished floor level.
• Terminals and any future fan housing have to go somewhere visible, so put them on the elevations now rather than explaining them to a planning officer later.

That last point is the practical link between radon and the planning application. A terminal on a principal elevation of a house in a conservation area is a design issue, and it is far easier to resolve on a drawing than on site.

Basements and the additional measures

Basements change the problem rather than adding to it. The floor and the walls below ground are both potential entry routes, the surface area in contact with the ground is much larger, and the pressure differential that draws radon in is greater.

BR 211 treats basements as a separate case and full protection is the normal starting point, typically with an actively ventilated sump rather than a capped provision. The waterproofing design under BS 8102 and the radon barrier have to be coordinated, because two membranes performing different jobs in the same build-up is where workmanship problems appear.

For planning purposes a basement in a Radon Affected Area strengthens the case for submitting ground information with the application. Where the council requires a Basement Impact Assessment, the ground gas and radon position sits naturally inside it, and the ground investigation that supports a Geotechnical Assessment can be scoped to cover both. The escape, light well and flood interactions are covered separately in our note on basement light wells, escape and flood risk.

Who signs it off

Nobody certifies radon protection for planning purposes. The building control body inspects the installation, and the critical inspection is before the barrier is covered, which means the site programme has to allow for it.

Post-construction measurement is the only way to know whether the protection worked. Testing uses passive detectors left in place for three months, analysed by a validated laboratory, and a shorter screening test is indicative rather than conclusive. For a self-build that you intend to live in, the three-month test over a winter is worth doing even though nothing obliges you to.

If the result exceeds the Action Level, the remedy on a building with full protection is usually to fit the fan to the existing sump. On a building with basic protection only, the options are more disruptive, which is the argument for specifying the sump provision wherever the site report is close to the boundary between levels.

Conversions and extensions

An extension in a Radon Affected Area is building work, so the contaminant requirement in Approved Document C applies to it, and the barrier has to be tied into the existing construction in a way that does not leave a gap at the junction.

A material change of use, such as converting an agricultural building or an office into dwellings, brings the contaminant requirement into play for the whole building in many cases. The extent depends on the type of change and on what is reasonably practicable in an existing structure, so confirm the position with your building control body before you price the works rather than afterwards.

Barn conversions deserve a specific mention. Older agricultural buildings frequently have no floor slab worth retaining, and the floor is being rebuilt anyway, so the marginal cost of a barrier and a sump is small. Retrofitting either one later is not.

Where radon shows up in the application and in conditions

Honestly, usually nowhere. Radon rarely appears as a reason for refusal and rarely attracts a condition, because the regulatory route is building control.

The exceptions are worth knowing. Some local validation lists require a desk study for ground conditions on greenfield or previously developed sites, and radon belongs in that document. Where a council attaches a contaminated land condition requiring a scheme to deal with ground gases, radon is a ground gas and the condition catches it. And where a scheme is already submitting ground investigation for foundations or a basement, including the radon position costs nothing and heads off a later request.

One honest uncertainty. Council practice on whether radon belongs in a validation-stage document varies, and the only reliable way to find out is to read the authority's own validation checklist before you submit.

Enter your postcode on PlanScope to see which ground and structural reports your site 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. Ground gas and geotechnical work should be prepared by suitably qualified and experienced people, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.