Yes, this is a very specialist topic. We mostly know that radon is bad for you; it is the second-most impactful cause of lung cancer after smoking. I have found that radon levels are high in our barn conversion (barn conversion sounds grand but we live in just a part of a barn). While the builders put a radon-proof membrane under the new floors, there is an obvious issue with conversions. Our original walls have two stone outer surfaces and rubble filling the inner gap. Unlike a new build, the radon membrane does not go across under the wall. As it is only installed between the inner edges of the walls and not the outer edges, radon can go around the side of the membrane, even up inside the rubble-filled all cavity, it can penetrate through any cracks or gaps (no matter how tiny), and go around the barrier into the rooms. I have seen no reference to this on the internet and just wanted to make people aware that conversions, even recent ones which include radon protection, are susceptible to radon finding its way into the house. For this reason it is worth getting your conversion checked if your area has any level of radon risk.