Radon affected areas and the protective measures

Radon is dealt with through the Building Regulations, not planning policy. If a site falls in a radon affected area, the response is a set of physical protective measures built into the floor and substructure under Approved Document C, and the decision on whether basic or full protection applies is driven by the probability band published for that location. A radon affected area is defined as one where more than one per cent of homes are estimated to be above the action level, and the mapping is produced jointly by the British Geological Survey and the UK Health Security Agency.

Planning officers will rarely mention it. Building control will.

How radon affected areas are mapped

The mapping combines the underlying geology with measured indoor radon results from houses across the country, and the output is a probability that a dwelling at that location exceeds the action level of 200 becquerels per cubic metre as an annual average.

Those probabilities are banded, and the bands determine the construction response. Below one per cent, no radon measures are needed. Between one and three per cent, basic protection. Above three per cent, full protection. The precise band boundaries and the construction requirements are set out in the BRE guidance that Approved Document C references, and a site-specific report is the only reliable way to get the band for a particular plot.

The geology matters because radon comes from the decay of uranium in rock and soil. Granite in the south west, some Carboniferous limestones, parts of the Northamptonshire ironstone and certain phosphatic shales all produce higher readings, which is why affected areas are patchy rather than regional. Two plots 300m apart can sit in different bands.

Finding the probability band for a site

Do not rely on a postcode-level lookup for a construction decision. Postcode centroids are coarse and the bands change over short distances.

The sequence that works is straightforward. Start with the free indicative map to find out whether the area is affected at all. If it is, buy a site-specific radon report for the plot, which returns the band for a defined area or address and is the document building control will want to see. Keep the report with the construction file, because the band is the justification for the measures you install.

Where a site is large or the boundary of a band runs through it, the report should be commissioned for the building footprint rather than the site centre. On a single plot that distinction is academic. On a five-unit scheme spread across a sloping field, it is not.

Basic and full protection in new buildings

Basic protection is a radon-proof membrane across the whole footprint of the building, lapped and sealed at joints, dressed into the cavity tray and sealed around every service penetration. It is a continuous barrier, and continuity is the entire point. A single unsealed soil vent pipe penetration undoes a correctly specified membrane.

Full protection adds a means of removing radon from beneath the slab. In practice that is either a radon sump with a capped pipe brought up to an accessible position so a fan can be fitted later, or a ventilated subfloor void. The sump is installed during construction, and the fan is only fitted if post-completion measurement shows the level is still above the action level.

The detailing is where self-build projects fail. Membranes get punctured by scaffold boards, service trenches are dug after the membrane is laid, and the perimeter detail at the cavity tray is left incomplete because the blockwork gang and the groundworkers are on site in different weeks. Photograph the membrane before the slab goes on.

Extensions, conversions and change of use

An extension in an affected area generally needs the same protection as a new building over its own footprint, and the junction between the new membrane and the existing structure is the hard part. Where the existing house has no membrane, the new work cannot be sealed into anything, so the detail becomes a sealed upstand and a dressed joint to the existing wall.

A loft conversion needs no radon measures, because nothing is being built in contact with the ground.

A material change of use that creates a dwelling, for instance an office to residential conversion or a barn conversion, brings radon into scope under the change of use provisions of the Building Regulations. That is the case that catches landlord-investors, because the planning route may be prior approval with no mention of radon anywhere, and the radon obligation arrives with the building control application. On a conversion the practical options are a new floor slab with a membrane, or measurement after completion followed by remediation.

Measurement and remediation in existing homes

Measurement is done with passive detectors left in place for three months, in a living room and a bedroom, to give an annual average. Short-term tests are indicative only, because radon levels swing with weather, heating and ventilation patterns.

If the result is above the action level, remediation in an existing house is usually a retrofitted radon sump outside the building connected under the slab, with a small continuously running fan. Positive pressure systems and improved underfloor ventilation are alternatives for suspended timber floors. Costs are modest relative to most building work, and the fan runs continuously, so there is a small standing energy cost.

Retesting after remediation is the step people skip. Install, retest, keep the certificate.

Who signs off the measures

Building control, not the planning department. The approved inspector or local authority building control body checks the specification against the band in the radon report and inspects the membrane before the slab is poured, which means the inspection has to be booked at the right moment.

No specialist licence is needed to install a membrane, though the work should follow the manufacturer's details and the BRE guidance. A ground gas specialist is a different matter: where a site also has landfill gas or a contamination issue, the radon membrane and the gas protection system interact, and the design should be prepared by someone competent under BS 8485 rather than treated as two unrelated layers. Land contamination reports sit in the same territory, and the contaminated land assessment will say whether a combined gas protection design is needed.

Where it affects the planning submission

Mostly it does not, and claiming otherwise in a planning statement wastes a paragraph. There are three exceptions.

First, where the site also has a ground gas issue, the planning condition on contamination will require a verification report, and the radon membrane will usually be verified in the same document. Second, where a basement is proposed, radon accumulation in a below-ground room is a design consideration that belongs in the Basement Impact Assessment alongside groundwater and ground movement. Third, where the proposal is a change of use to residential, flagging the radon position early tells the council you understand the building regulations consequence, which supports the overall case.

Beyond that, keep it in the construction file. Checking the radon band at the same time as the other site constraints is sensible housekeeping, and it sits naturally alongside work such as source protection zones and development over groundwater and development inside an air quality management area, which are the other desk checks worth doing before a design is fixed.

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.