PART C · Site preparation and moisture
Do I need radon protection for an extension
Whether an extension needs radon protection depends on the location, ground conditions and the applicable design assessment. Radon is a ground gas risk addressed by the Part C guidance. Other ground gases may require different measures.
How should the risk be checked
Start with current site information and a competent assessment of the relevant ground gas risks. Location-based radon information can assist, but it should be used appropriately for the site and type of work.
A former industrial site may raise questions beyond radon. Ask the designer whether contamination or other gases need investigation.
What protection might be proposed
The design may include a suitable barrier and, where appropriate, a means of extracting or dispersing gas. The correct solution depends on the identified risk and the guidance being followed.
A general damp proof membrane should not be assumed to perform as a specified gas barrier. Product suitability, jointing and installation details need checking.
Why are extensions awkward
New construction meets an existing building that may have a different floor arrangement. The design needs to explain that junction, including how the barrier is continued or terminated.
Service entries, corners and damaged sections deserve particular attention. A barrier can look satisfactory across the middle of a room while remaining ineffective at its edges.
What should you ask before construction
Ask what assessment supports the proposal, which products are specified, how junctions will be formed and what inspection or verification is required. Include any specialist work in the quotation.
Keep the assessment and installation records after completion. If a monitoring or testing strategy is advised, understand who arranges it and what results require further action.
The practical aim is a complete, verifiable system. Buying a membrane described as radon resistant does not, by itself, demonstrate that the finished extension provides the required protection.