Radon protection and where the requirement bites

Radon protection is a Building Regulations requirement, not a planning one. If your plot sits in a radon affected area, Approved Document C requires protective measures in the new building, and building control will check them at foundation and floor stage. Planning officers will rarely ask about radon at all, which is exactly why self-builders discover the requirement after the slab design is fixed.

The measures themselves cost little to install and a great deal to retrofit. A radon barrier laid across the whole footprint during construction costs a fraction of what a remediation sump costs once the house is finished and monitoring has shown a problem.

How radon affected areas are mapped

Radon is a naturally occurring radioactive gas produced by the decay of uranium in rock and soil. Its concentration in the ground varies with geology, so the mapping is geological rather than administrative.

The maps are produced jointly by the UK Health Security Agency and the British Geological Survey, and they express radon potential as the estimated percentage of homes in an area above the action level. That percentage drives the requirement: areas below a lower threshold need no measures, areas in the middle band need basic protection, and areas above the upper band need full protection. Approved Document C and the supporting BRE guidance set out where those bands fall.

Two points about the mapping catch people out. It works at a fine grain, so a neighbouring village can sit in a different band. And the published atlas maps are indicative only, which is why an address-level report exists.

Checking a site address

Get a radon report for the specific address or grid reference rather than reading a regional map. The national radon service provides address-based reports, and the British Geological Survey issues site-specific ground reports that include radon potential alongside other geological hazards.

Do this at feasibility, not at building control stage. The result changes the ground floor build-up, the detailing at every service penetration, and on some sites the choice between a suspended floor and a ground bearing slab.

Where you are already commissioning a desk study for other reasons, the radon screen falls out of it at no extra cost. A Phase 1 contaminated land assessment covers the geological sources anyway, and a competent author will report radon potential as a matter of course.

Basic and full protective measures

Basic protective measures amount to a continuous radon-resisting barrier across the whole ground floor area, carried through the cavity and lapped and sealed at every joint, at the perimeter and around every pipe and duct that passes through it.

Full protective measures are the barrier plus a means of reducing the pressure in the ground beneath the building. On a ground bearing slab that normally means a radon sump formed in the granular fill, connected to a capped pipe brought to the outside, ready to be fitted with a fan if post-completion monitoring shows it is needed. On a suspended floor it can instead mean a cross-ventilated void with adequate openings on opposing walls.

The detail that fails on site is the penetration. A barrier with a torn sleeve around a soil pipe performs no better than no barrier at all, so specify proprietary top hat sleeves and require photographic evidence before the floor is poured.

Basements and radon

Below-ground rooms sit in contact with more soil surface per unit of floor area, and they are harder to ventilate, so a basement in a radon affected area needs particular attention. The barrier has to wrap the walls as well as the floor, and it has to coexist with the waterproofing system.

That coexistence is the hard part. A type C drained cavity waterproofing system deliberately admits water to a drained void, and the radon strategy has to work with that void rather than against it. A type A barrier system can be detailed to do both jobs, but only if the specifier sets out to do so from the start.

Mechanical ventilation of the basement is usually part of the answer, and the ventilation strategy needs to be set out rather than assumed. Where the basement is also the subject of a Basement Impact Assessment for planning purposes, keep the two documents consistent: a groundwater strategy that contradicts the radon detailing will be noticed by building control even if it passes planning.

Where planning and building control divide

Radon sits with building control. The planning system does not normally impose radon conditions, and a council that did so would be duplicating a Building Regulations requirement, which national guidance discourages.

There are two exceptions worth knowing. Some local validation checklists ask for a ground conditions statement covering naturally occurring hazards, and radon appears there alongside ground gas and mining legacy. And where a site has both a radon issue and a ground gas issue from made ground or a former landfill, the protection measures interact, because a gas resistant membrane specified to BS 8485 performance levels may also serve the radon function if it is detailed to do so. That interaction is set out in ground gas protection and BS 8485 measures.

Do not conflate the two regimes in the same specification without checking. A membrane certified for methane and carbon dioxide is not automatically accepted as a radon barrier, and the reverse is also true.

Extensions and conversions

An extension in a radon affected area needs protection in the extension itself. Approved Document C treats an extension as new building work, and the barrier has to be tied into the existing structure in a way that does not simply stop at the junction.

Material changes of use bring the requirement in too. Converting an agricultural building to a dwelling, or a basement to habitable accommodation, triggers the relevant requirement under the Building Regulations even though no new floor area is created. Barn conversions in rural radon areas are a common case, and the works needed to achieve a radon barrier in an existing slab are considerably more intrusive than the conversion drawings usually show. That is the same lesson as foundation design on made ground and former industrial sites: the ground decides the buildability, and it decides it early.

Existing dwellings with no building work proposed are outside the regime entirely. The occupier can test and, if levels are high, remediate voluntarily.

Recording the check in the application

Even though radon is not a planning matter, record the check. Put the address-level radon result, its source and its date into the desk study or the design and access statement, with one sentence stating which level of protective measures the design provides.

Three reasons. It answers the validation checklist question where the council asks one. It prevents the issue resurfacing at building control with the floor design already tendered. And it gives a buyer's solicitor a straight answer to the radon enquiry, which on a self-build plot is one of the questions that holds up completion.

This is general guidance, not legal advice; check the position with your local authority building control body or a qualified professional before relying on it.

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