Consequences for no permit will be at the municipal level. Where I am, the owner of the house is responsible for any permits regardless of when work was done. My city will require a retro active permit, it will cost 1.5 times what the permit would have cost if applied for before the work was done. There will be an inspection, which may require taking stuff apart, (plumbing or electrical), any wrong stuff will need to be fixed.
Having said that the likelihood of the city ever finding out about interior work where modifications can’t be seen from outside are extremely low, such as basement development or kitchen remodel. Where people get caught is on additions, garages, decks, oversize fences, etc… The reason they get caught is not because the city has permit police but because banks want a real property report for financing, (essentially a survey), the real property report is submitted to the city, who compares it to the one they have on file, any changes and no permit, you’re busted.
Meanwhile in the small towns or rural districts around the city, it’s mostly “do whatever you want to do so long as it doesn’t frighten the cows” So, in order to advise people about permits or no, you need to pick up the phone and call the ahj.