Should I test for radon in my London renovation?

Yes if: basement conversion (any borough), ground-floor extension in NW/SE higher-risk postcode, existing house in higher-risk postcode without prior test. No if: upper-floor flat (radon decays before reaching upper floors), Inner London zone 1 with no basement. UKHSA radon map provides postcode-specific probability — check before deciding.

Can I retrofit radon mitigation?

Yes — PPV (loft-mounted positive pressure fan) is retrofitted in 1–2 days, £950–£1,800. Sub-slab depressurisation requires drilling through slab + ducting — more disruptive, £1,800–£4,800, 1 week. Membrane retrofit only at floor slab replacement (major renovation only). Test first to confirm need.

Does radon affect property value?

If untreated >200 Bq/m³ with documented test: yes, devalues 2–8% (buyer concern, mortgage valuer flag). If mitigated to <100 Bq/m³ with retest evidence: no effect. UKHSA radon test certificate (Radon Certificate) demonstrates compliance — useful at sale in higher-risk postcodes.

Radon London Renovation 2026 | Builderr

Is Radon a Risk in London Renovations and How Is It Tested?

Radon risk across most of London is low — UKHSA radon map shows <1% probability of exceeding 200 Bq/m³ action level in most boroughs. Higher-risk pockets exist in NW London (Hillingdon, Harrow), south-east London (chalk areas of Bromley, Bexley), and basement conversions in any borough. Test with passive alpha-track detector £45–£185 across 3-month exposure. Mitigation (positive pressure ventilation, sub-slab depressurisation) £950–£4,800. Radon is the second leading cause of lung cancer after smoking.

What radon is

Radon-222 is a naturally occurring radioactive gas formed by decay of uranium-238 in rock and soil. Colourless, odourless, tasteless. Decays to radioactive 'radon daughters' (polonium-218, polonium-214) which lodge in lung tissue and emit alpha radiation — primary cause of lung cancer in non-smokers, second leading cause overall (after smoking). UKHSA estimate 1,100 UK deaths/year from radon-induced lung cancer. Radon enters buildings through gaps in ground-floor slab, cracks, service penetrations; concentrates in basements and ground-floor rooms where ventilation low. Outdoor concentrations 4–8 Bq/m³; indoor average UK 21 Bq/m³; problem homes 200–2,000+ Bq/m³.

London risk and testing

UKHSA radon map (radonmap.com): most of London <1% probability above 200 Bq/m³ action level. Higher-risk pockets: NW boroughs Hillingdon, Harrow, parts of Barnet (proximity to Chiltern chalk); SE boroughs Bromley, Bexley (Kentish chalk extending); central London generally low (alluvial clay/sand subsoils). Basement conversions: any borough — basement depressurises during use, draws radon-bearing soil air into the building. Test mandatory for basement conversion regardless of borough. Test: passive alpha-track detector (REM, Bowser-Morner, RSSL via UKHSA-approved labs) — small canister placed in living room + main bedroom for 3-month exposure (covers seasonal variation), returned to lab, result in Bq/m³. Cost £45–£60 per detector for domestic 2-room test, £185–£385 for larger surveys. Action level 200 Bq/m³ (legal trigger); target <100 Bq/m³ (UKHSA guideline).

Mitigation

If test >100 Bq/m³: increase ventilation in lower rooms — MVHR or positive-pressure ventilation (PPV — fan in loft pressurises dwelling, displaces radon-bearing air out). PPV cost £950–£1,800 install. If test >200 Bq/m³ or 100–200 Bq/m³ persistent: sub-slab depressurisation (active radon system — fan extracts air from beneath ground floor slab, vents to outside above roof). Most effective; cost £1,800–£4,800 install + ongoing electricity. Slab membrane: gas-resistant DPM (Visqueen GasGuard, Triton Radonbarrier) under new floor slabs in renovations — required spec for basement conversions, recommended for ground-floor extensions in higher-risk postcodes. UK Building Regs Approved Document C requires radon protection in radon-affected areas (basic protection — sub-slab gas-resistant membrane). New basements always full mitigation (sub-slab fan + barrier).