It might be relatively cheap to over-engineer (e.g. digging in an extra meter), but it might not.
FWIW, while researching my Moore's law essay, I found materials claiming that underground construction was more expensive but paid for itself via better heating/cooling. But that was for shallow cut-and-scrape constructions and I suspect 1 meter wouldn't take care of supernova radiation.
I suspect 1 meter wouldn't take care of supernova radiation.
As far as I understand the issue, the danger is mainly from the temporary ozone layer depletion, with the resulting solar UV rays doing most of the damage, and not from any kind of direct supernova radiation. And UV is not hard to shield from.
Article on space.com