Radon Gas and Building in London: A Homeowner's Guide

Introduction

Radon is a naturally occurring radioactive gas that can accumulate in buildings, posing a long-term health risk to occupants. In most of London, radon risk is considered low compared to high-risk areas such as Devon, Cornwall and parts of the East Midlands. However, building regulations require consideration of radon risk for new extensions and habitable spaces in basement and ground-floor construction, and homeowners planning significant works to ground floors or basements should understand the regulatory position.

This guide explains what radon is, the risk levels in London, the building regulations requirements, and what protective measures may be needed in extension and renovation projects.

What Is Radon?

Radon (Rn-222) is a colourless, odourless radioactive gas produced by the natural decay of uranium in rocks and soils. It enters buildings through cracks and gaps in floors and walls in contact with the ground. In well-ventilated spaces, radon disperses without reaching harmful concentrations. In poorly ventilated spaces — particularly ground floors and basements — it can accumulate to levels that represent a significant lifetime cancer risk.

The UK Health Security Agency (UKHSA) sets an Action Level of 200 becquerels per cubic metre (Bq/m³) — the level above which action is strongly recommended to reduce concentrations — and a Target Level of 100 Bq/m³. Occupational exposure limits also apply in some contexts.

Radon Risk in London

London is predominantly a low-radon area. The geology of most of the capital — London Clay, river gravels and chalk — does not generate the elevated radon levels found in granite-based regions. The UK Radon Association and UKHSA mapping shows that across most of inner and outer north London (Camden, Barnet, Haringey, Islington, Hackney, Harrow, Enfield), the percentage of homes likely to exceed the Action Level is below 1% — typically below 0.3%.

This does not mean radon can be entirely ignored, particularly for basement construction or for ground-level habitable spaces with limited ventilation. Localised variations in geology — such as areas overlying chalk or disturbed fill — can create higher-than-average concentrations. The UKHSA's interactive radon map allows any specific postcode to be checked.

Building Regulations Requirements

Approved Document C (Site Preparation and Resistance to Contaminants and Moisture) addresses radon as a site contaminant for building regulation purposes. For extensions, new ground-floor construction and basement conversions, the building regulations require:

• In Radon Category 1 areas (less than 1% of homes above Action Level) — no specific radon protective measures required beyond standard good construction practice
• In Radon Category 2 areas (1–3% of homes) — basic radon protection measures required (radon-resistant membrane and sumps for possible fan installation)
• In Radon Category 3 areas (3–10% of homes) — full radon protection required including a continuous barrier and provision for active sub-slab depressurisation
• In Radon Category 4 areas (over 10% of homes) — enhanced radon protection required

Most of north London falls within Radon Category 1. However, building control inspectors will check the category for specific postcodes when approving ground-floor and basement construction, and designers should confirm the applicable category at design stage.

Radon Protective Measures

Where radon protection is required (Categories 2–4), the standard measures for new construction include:

Radon-Resistant Membrane

A continuous low-permeability membrane installed beneath the ground-floor slab, designed to prevent radon ingress. The membrane must be lapped and sealed at all joints and penetrations to form a continuous barrier.

Sub-Slab Ventilation Void or Radon Sump

A radon sump is a void or collection point beneath the slab to which a fan can be connected if post-construction monitoring shows elevated radon levels. The sump allows active sub-slab depressurisation to be installed without breaking up the finished floor.

Positive Pressure Ventilation

In occupied buildings where elevated radon is measured, positive input ventilation (PIV) units can dilute radon concentrations in the living space by maintaining a slight positive pressure relative to the ground.

Testing for Radon in Existing Properties

Radon testing in an existing property involves placing passive radon detectors (typically alpha-track detectors) in the ground floor and bedroom for a period of 3 months. The detectors are then returned to a laboratory for analysis. Testing kits are available from the UKHSA for approximately £50–£70.

Testing before undertaking a basement conversion or ground-floor extension is sensible best practice even in Category 1 areas, particularly where the property has a basement or ground floor with limited ventilation. If elevated levels are found, protective measures can be incorporated into the design before construction.

Radon in Basement Conversions

Basement conversions are the scenario where radon merits the most careful consideration in north London. A basement space — below ground, with limited natural ventilation and direct contact with the surrounding ground — is inherently more susceptible to radon accumulation than an above-ground room. For any basement conversion to habitable use, radon testing of the existing space (or risk assessment based on the site geology) and incorporation of appropriate protective measures is good practice, regardless of the formal regulatory category.

Conclusion

Radon gas is a low but non-trivial risk factor for ground-floor and basement construction in London. Most of north London falls within Radon Category 1, where standard construction practice is sufficient. For basement conversions, habitable spaces at or below ground level, and projects in areas of slightly elevated geological risk, testing and protective design measures are advisable. An architect managing the building regulations design for an extension or basement project will confirm the radon category for the specific site and incorporate appropriate protective measures in the ground-floor construction specification where required.