Radon
Does Approved Document C mean this house has already failed a radon test?
No. Approved Document C is England’s building-regulations guidance on site preparation and resistance to contaminants and moisture, including where new work has to consider radon. It is not a measurement of an existing living room. The Action Level remains 200 Bq/m3 as an annual average from a proper test.
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 surveyor may mention that an extension or a new floor should have followed that guidance. That is a prompt to ask your solicitor for building-regulations paperwork. It is not a becquerel result. A missing certificate does not invent a level, and a certificate does not tell you today’s average if nobody measured it.
Do not pay for radon-proofing the whole house because an extension paragraph cites Approved Document C. Ask what was built and what the papers say. If you need the level, use the three-month test. Costs of any later membrane or sump are for a contractor who has seen the job. None are stated here.
What should you ask your surveyor?
- Are you citing Approved Document C because of new work, or because of the address?
- Was any test provided?
- Which part of the building is the extension?
What are the typical next steps?
- Ask the solicitor for the building-regulations paperwork on the extension.
- Do not convert a guidance document into a rating.
- Measure if you need a number. Do not membrane a floor on a hunch before exchange unless you have instructed that work with your eyes open.
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 Approved Document C apply in Scotland?
It is England guidance. Scotland has its own building standards. Do not paste the English document onto a Scottish house.
Is a radon barrier the same as a damp-proof membrane?
They can be specified together in new work. A damp-proof membrane you can see is not proof it was detailed as radon protection.