This depends very much on the soil composition. Some areas might have almost no radon, whereas others do. Here's a high-level view of radon probably by county (WA state), but you can find local maps that show it at the neighborhood level. We looked at the maps when buying our house, had it tested, and radon levels were high so we installed a mitigation system in our garage (which is the bottom level of our house).

https://www.epa.gov/sites/default/files/2014-08/documents/wa...