Retroactive permit in the bay

I just bought a house with an extra bathroom unpermitted in San Bruno. I am planning to install a water softener in the garage.

1. Will the city check for the unpermit bathroom inside the house if my request is just to do some plumbing and install the softener in the garage?
2. How hard is it to get permit for the bathroom in San Bruno, CA? I believe the extra bathroom is done up to the code but I am willing to fix anything not done right. I am hoping they don't ask me to tear down the whole bathroom and rebuild again. Anyone had experience with getting a retroactive permit before?