Hi Mayoman2
It is not possible to fit a radon barrier to an existing house as the barrier has to form a perfect seal underneath the sub-structure of the house. Barriers are only effective when fitted to new buildings during their construction. It should be possible however to fit a barrier to the extension. I would also suggest that you also fit a passive radon sump to your extension
When all renovations are complete I suggest that you carry out a radon test to determine if you have a radon problem. Even in high radon areas the majority of houses will not have a problem with radon
If you have a problem with radon you can either activate the existing radon sump, get a radon mitigation company to install a sump or as an alternative increase the ventilation rate in the building which should dilute the radon concentrations. Further information can be obtained from