Radon
The extension is said to have a radon barrier in the slab and the slab was not opened. What can the survey not confirm about that barrier?
The survey cannot confirm that a radon barrier is in the slab, that it is continuous, or that it was detailed at the edges. The extension is said to have one, and the slab was not opened. A statement about a membrane you cannot see is only a statement. The inspection can record the claim and the fact the concrete stayed closed. It has not measured radon, and it has not verified the barrier. UKradon maps and levels do not become a certificate for a hidden sheet. The barrier, if it exists, remains unseen.
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?
Barriers in slabs work only if they are present, joined, and carried through the places where pipes and walls puncture them. None of that is visible from a finished floor. The surveyor should not tap the screed and call the result a membrane. They can note a new extension, a solid floor, and the seller's or the builder's claim. They cannot confirm laps, seals, or even that the product was the one named. Leaving the slab unopened is the right limit of a home survey.
Ask the solicitor for any warranty, building-control note or installer letter that mentions the barrier, and read those papers as claims too until they actually describe what was installed. If you need the air tested, that is a measurement in the rooms, not a hole cut in the floor on a whim. Do not instruct anyone to break out the slab just to satisfy the survey. The survey has already said what it cannot confirm. A document trail is the proportionate next step.
What should you ask your surveyor?
- What claim about a radon barrier did you record, and what of the slab was opened?
- Can you confirm continuity, or only that the floor was not exposed?
- Have you kept this separate from a radon measurement of the rooms?
What are the typical next steps?
- Treat the barrier as claimed and not confirmed.
- Ask for any installation or building-control paperwork that mentions it.
- Do not read an unopened slab as proof the membrane is there.
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
If the extension is recent, is the barrier more likely and therefore confirmed?
Recency might explain why someone claims a barrier. It does not confirm one. The slab was not opened, and the survey still cannot say the membrane is present, continuous or sealed at the edges. The claim is still only a claim.
Does a sump in the cupboard prove the slab membrane?
No. A sump, if you have one, is a different part of a radon system. It does not show what was laid in the extension slab, and that slab was not opened or tested for a membrane. The slab membrane stays unseen.