Selling a House with High Radon Levels

Radon is a colourless, odourless radioactive gas that forms naturally from the decay of uranium in rocks and soil. It seeps up through the ground and can accumulate inside buildings, particularly in ground-floor rooms, cellars, and spaces with poor ventilation. The UK Health Security Agency (UKHSA, formerly Public Health England) estimates that radon exposure is the second largest cause of lung cancer in the UK after smoking, responsible for around 1,100 deaths per year.

If you are selling a property in a radon affected area, or if previous testing has shown elevated levels, this guide explains what you need to do. The good news is that radon mitigation is well understood, relatively affordable, and highly effective. With the right preparation, high radon levels do not need to derail your sale.

What is radon and why does it matter?

Radon (Rn-222) is produced by the radioactive decay of uranium-238, which is present in small quantities in most rocks and soils. As uranium decays through a chain of intermediate elements, radon gas is released. Because radon is a gas, it can move through pores and cracks in the ground and enter buildings through gaps in floors, around pipes, through cracks in solid floors, and via suspended timber floors.

Outdoors, radon disperses quickly into the atmosphere and concentrations are very low. Indoors, it can accumulate to levels that pose a long-term health risk. When radon decays further, it produces tiny radioactive particles that can be inhaled and lodge in the lungs, increasing the risk of lung cancer over prolonged exposure. The risk is proportional to the concentration and the duration of exposure.

For property sellers, radon matters because it is one of the environmental factors that a buyer's solicitor will investigate during conveyancing, particularly through the environmental search. If the property is in a radon affected area, the search results will flag this, and the buyer's solicitor will want to know whether testing has been carried out and whether levels are within acceptable limits.

UK radon levels: action level and target level

The UKHSA sets two key reference points for radon in homes:

It is important to understand that there is no legal requirement to test for radon or to remediate if levels exceed the action level. The action level is a recommendation, not a statutory limit. However, in the context of a property sale, failing to act on known high readings can create problems with buyers, surveyors, and mortgage lenders.

The average indoor radon concentration in UK homes is around 20 Bq/m³. In radon affected areas, concentrations can be many times higher, with some properties recording levels above 1,000 Bq/m³ before mitigation.

Radon affected areas in the UK

Radon levels are determined primarily by the underlying geology. Areas with granite, limestone, and certain other rock types tend to have higher concentrations of uranium, and therefore higher radon levels. The most significantly affected areas in England include:

• Cornwall — The granite bedrock across much of the county produces the highest radon levels in England. In some parishes, more than 30% of homes exceed the action level.
• Devon — Particularly the granite areas of Dartmoor and surrounding parishes, as well as parts of north Devon with Devonian limestone.
• Somerset — The Mendip Hills and parts of the county overlying carboniferous limestone are notably affected.
• Derbyshire — The Peak District and surrounding areas, where carboniferous limestone is prevalent.
• Northamptonshire — Parts of the county overlying ironstone and limestone formations have elevated radon levels.

Other affected areas include parts of Gloucestershire, Oxfordshire, Wiltshire, Herefordshire, Shropshire, and the Yorkshire Dales. In Wales, parts of Powys and Pembrokeshire have elevated levels. The UKHSA publishes an interactive radon map (UKradon.org) that shows the estimated percentage of homes above the action level for any postcode area.

If your property is in an area where 1% or more of homes exceed the action level, it is classified as a radon affected area. This classification is relevant both for property sales and for building regulations governing new construction.

How to test for radon

Radon testing is straightforward and inexpensive. The standard method recommended by UKHSA involves two small passive detectors placed in your home for three months.

The three-month test

You can order radon detectors directly from UKHSA via the UKradon website. Two detectors are supplied: one for the main living room and one for a bedroom (usually the most-used bedroom). The detectors are small plastic devices about the size of a bottle cap that sit on a shelf or windowsill. They require no power and make no noise.

After three months, you return the detectors to the laboratory in a prepaid envelope. Results are typically issued within two to three weeks. The cost for a standard two-detector test is approximately £50 to £60, including analysis.

Three months is the minimum recommended measurement period because radon levels fluctuate with weather conditions, season, and building ventilation patterns. A shorter measurement gives a less reliable snapshot. UKHSA advises that measurements taken during winter (October to March) can be adjusted using a seasonal correction factor to estimate the annual average.

Electronic radon monitors

Continuous electronic radon monitors (CRMs) can provide faster results, but they are more expensive (typically £100 to £300 to hire) and are not a substitute for the standard three-month test in the eyes of most solicitors and lenders. They can be useful as a screening tool or for post-mitigation verification.

When to test

If you are planning to sell a property in a radon affected area, the ideal time to test is before you list. A three-month test takes time, and having the results ready to share with the buyer's solicitor from the outset prevents a common cause of delay. If you have a previous test result within the last five years, this is generally accepted, although the buyer's solicitor may request a fresh test if the result is borderline or the property has been significantly altered.

Mitigation options for high radon levels

If testing reveals radon levels above 200 Bq/m³ (or if you want to reduce levels that are above the 100 Bq/m³ target), several proven mitigation methods are available. The Building Research Establishment (BRE) publishes detailed guidance in BRE Report BR 211 (Radon: Guidance on Protective Measures for New Buildings) and BRE Good Building Guide 74 (Radon in Existing Buildings).

Radon sump system (active or passive)

A radon sump is the most effective and widely recommended method for reducing radon in existing buildings. A small void is excavated beneath the ground floor slab, connected to an external pipe that vents radon above roof level. In a passive sump, natural stack effect draws the gas upwards. In an active sump, a small continuously running fan enhances the airflow. Active sumps can reduce indoor radon levels by 90% or more.

Installation typically costs £800 to £2,000 depending on the property's construction, floor type, and accessibility. The fan uses very little electricity — typically less than £20 per year. Maintenance requirements are minimal, but the fan should be checked periodically to ensure it is running.

Positive input ventilation (PIV)

A PIV system draws fresh filtered air into the property from the loft space, creating a slight positive pressure that inhibits radon ingress through the ground floor. PIV is effective for moderate radon levels (typically up to around 500 Bq/m³) and costs £500 to £1,200 installed. It also improves general air quality and reduces condensation, which can be a useful selling point.

Radon-proof membrane

A gas-resistant membrane installed across the ground floor acts as a barrier to radon ingress. This is most practical during renovation or when floors are being replaced. The membrane itself is relatively inexpensive, but the labour cost of fitting it beneath an existing floor makes it more suitable for properties undergoing refurbishment. Typical cost: £1,000 to £3,000 depending on floor area and construction type.

Sealing gaps and cracks

Sealing gaps around pipes, cracks in solid floors, and openings in suspended timber floors can reduce radon ingress. However, sealing alone is rarely sufficient to bring levels below the action level in properties with significantly elevated concentrations. It is best used as a complementary measure alongside a sump or ventilation system. Cost: £200 to £500.

Post-mitigation testing

After any mitigation work, UKHSA recommends retesting to confirm that radon levels have been reduced to below the target level of 100 Bq/m³. A follow-up three-month test provides the most reliable confirmation. Having documented post-mitigation results is essential for your property sale — it demonstrates to the buyer and their solicitor that the issue has been properly addressed.

Disclosure obligations when selling

As a seller in England and Wales, you are required to complete the TA6 Property Information Form, which asks questions about the condition, history, and environment of your property. If you have had radon testing carried out, or if you know that the property is in a radon affected area, you should disclose this information. For a full overview of your obligations, see our guide on what to disclose when selling a property.

Your disclosure should cover:

• Test results — If you have radon test results, provide copies to your solicitor for inclusion in the contract pack. This applies whether the results are below or above the action level.
• Mitigation work — If a radon sump, ventilation system, or membrane has been installed, provide details of the work, including the contractor's details and any certificates or guarantees.
• Post-mitigation test results — If mitigation has been carried out, provide the follow-up test results confirming that levels have been reduced.

Deliberately withholding known radon information could give the buyer grounds for a claim under the Misrepresentation Act 1967 or the Consumer Protection from Unfair Trading Regulations 2008. Being transparent about radon is always the better strategy, particularly because effective mitigation is available and affordable.

How radon affects surveys and searches

Radon will typically come up at two points during the conveyancing process: the environmental search and the buyer's property survey.

Environmental search

Standard environmental search reports from providers such as Landmark and Groundsure include a radon section. The report states whether the property is in a radon affected area and gives the estimated probability of exceeding the action level. If the result is flagged as amber or red, the buyer's solicitor will raise enquiries asking whether radon testing has been carried out and whether mitigation is in place.

This is also connected to the broader ground stability search, as the underlying geology that produces radon (granite, limestone) can also be relevant to ground condition assessments.

Property survey

RICS surveyors conducting Level 2 (HomeBuyer) or Level 3 (Building) surveys will note whether the property is in a radon affected area and may recommend testing if no results are available. If an active radon sump or ventilation system is visible, the surveyor will note its presence and may comment on whether it appears to be functioning. A surveyor cannot measure radon levels during a standard survey inspection.

Impact on buyer negotiations

If radon is flagged during the sale process and no testing or mitigation has been carried out, buyers commonly respond in one of three ways: they request a radon test before proceeding, they request a price reduction to cover the cost of mitigation, or they use it as a general renegotiation point after the survey. Having test results and mitigation evidence ready before listing removes all three of these scenarios.

Building regulations for new builds in radon areas

If you are selling a property that was built after the early 1990s in a radon affected area, it should already have some level of radon protection built in. Approved Document C (Site Preparation and Resistance to Contaminants and Moisture) of the Building Regulations for England sets out the requirements:

If your property was built with radon protection, having the building regulations completion certificate and any documentation from the original build showing the radon measures installed is useful evidence for the buyer's solicitor. If the property has a passive sump provision (a sump installed but without a fan), and subsequent testing shows levels above the action level, activating the sump by adding a fan is a straightforward and low-cost upgrade.

BRE guidance and further technical resources

The Building Research Establishment (BRE) is the principal source of technical guidance on radon in UK buildings. The key publications for property sellers and their advisers are:

• BR 211 — Radon: Guidance on Protective Measures for New Buildings. The definitive guide to radon protection in new construction, referenced by the Building Regulations.
• Good Building Guide 74 (GBG 74) — Radon in Existing Buildings: Corrective Measures. Practical guidance on installing radon sumps, ventilation systems, and membranes in existing homes.
• BRE Information Paper IP 1/05 — Identifying Radon in Buildings. Guidance for surveyors and building professionals on recognising radon risk.

BRE guidance is widely accepted by surveyors, solicitors, and mortgage lenders as the authoritative reference for radon remediation standards. If you are commissioning mitigation work, choosing a contractor who works in accordance with BRE guidance provides additional assurance to the buyer's side.

Practical steps for sellers

If your property is in a radon affected area, or if you have reason to believe radon levels may be elevated, the following steps will help you prepare for a smooth sale.

1. Check whether your property is in a radon affected area

Use the UKHSA interactive radon map at UKradon.org to check the radon probability for your postcode. If the area shows 1% or more of homes above the action level, the property is in a radon affected area and the issue is likely to come up during conveyancing.

2. Test before listing if you do not already have results

Order radon detectors from UKHSA and carry out a three-month test. Because the test takes three months plus a few weeks for analysis, plan ahead — ideally starting the test at least four months before you intend to list. If your property has been tested within the last five years and levels were below 100 Bq/m³, a fresh test is usually not required.

3. Mitigate if levels exceed 200 Bq/m³

If your test result exceeds the action level, install mitigation before listing. A radon sump is the most effective option for most properties. The work typically takes one to two days and causes minimal disruption. After installation, retest to confirm levels are below 100 Bq/m³.

4. Prepare your documentation

Compile all radon-related documentation for your solicitor: original test results, details of any mitigation work (contractor, method, date), post-mitigation test results, and any building regulations documentation showing radon protection installed at construction. This package answers the enquiries that the buyer's solicitor will raise and demonstrates that the issue has been properly managed.

5. Disclose on the TA6 form

Complete the TA6 Property Information Form honestly, disclosing the radon position and attaching supporting documentation. Being upfront about radon and providing evidence of mitigation is far more effective than waiting for the buyer's side to discover the issue through searches.

Sources and further reading

• UK Health Security Agency (UKHSA) — UKradon.org interactive radon map, radon testing service, and guidance for homeowners: ukradon.org
• Building Research Establishment (BRE) — BR 211 (Radon: Guidance on Protective Measures for New Buildings) and GBG 74 (Radon in Existing Buildings): bregroup.com
• HM Government — Approved Document C (Site Preparation and Resistance to Contaminants and Moisture), Building Regulations 2010: gov.uk
• Consumer Protection from Unfair Trading Regulations 2008 — Material information disclosure obligations: legislation.gov.uk
• RICS — Home Survey Standard and surveyor guidance on reporting environmental hazards including radon: rics.org
• Radon Council — Trade body for radon measurement and mitigation professionals in the UK: radoncouncil.org

Related guides

• Selling a House Near a Landfill Site
• Selling a House on Contaminated Land

Frequently asked questions