Radon
Nothing on the visit sampled a radon membrane, opened the join at the extension or tested continuity looking cannot prove. What has to stay undecided?
The membrane, the join and its continuity all stay undecided. Nothing on the visit sampled a radon membrane, opened the join at the extension or tested continuity, and looking cannot prove that a sheet is complete. An extension is a common place for a new slab to meet an old one. The survey can note that a membrane was mentioned or partly seen. It has not measured radon and it has not decided that the barrier runs unbroken under the new floor.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
A radon membrane is a sheet in or under the floor meant to limit soil gas entering the rooms. It only works as a system if joints, service penetrations and the junction with the older house are sealed. Those details are covered once finishes go down. The eye cannot see continuity through a screed. This visit did not open the join and did not test the air. Mentioning a membrane is not a demonstration that the extension is joined to it.
Ask the solicitor for the extension drawings, any building-control completion paper and any radon report that names the membrane. Those papers may describe what was intended, which is still not a test of what is continuous now. A radon specialist can advise on measurement. Tell a lender no continuity test and no radon measurement were done. Do not treat a specified membrane as a proved seal at the extension join.
What should you ask your surveyor?
- Was the membrane join at the extension left unopened?
- Can looking at the floor prove continuity, or was that left untested?
- Was any radon level left unmeasured?
What are the typical next steps?
- Leave membrane continuity at the extension undecided.
- Ask the solicitor for drawings and any radon report.
- Do not tell a lender the join was proved sealed.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Does a sump in the old house prove the extension membrane?
No. A sump is a different mitigation measure, usually a void and a fan. It does not show that a sheet under the extension is continuous at the join. This visit opened neither the sump performance nor the membrane. The two systems must not be merged into one untested success.
If the extension is recent, is the membrane assumed continuous?
No. Recent work can be well detailed or poorly lapped, and the finishes hide the join either way. The survey did not open it and did not measure radon. Age of the extension is not continuity. Ask the solicitor for the completion papers, and keep them as design records rather than as a test result.