Need legal advice - bought home with shop marketed as 1/1 unit

[Michael Meyer]

Mom bought a 3/2 home (California) with a detached one bed, one bathroom unit, that we believed to be permitted as such, as the MLS listing "description" described as "The guest house is a 1/1 and includes it's own indoor laundry, single car garage and updated kitchen with granite countertops," however, it turns out the 1/1 was only permitted as a shop with a bathroom, and was never permitted as an Accessory Dwelling Unit. We naively believed what the listing had read was true and rented out the "guest house" only to have neighbor complain and county come out and fine us for renting out non-ADU. Now we're spending ~$10,000 to get the unit permitted as an actual ADU. Is there a way to seek legal recourse from the prior owner/listing agent for negligently marketing the extra unit as a guest house when it was in fact, not?
Any legal information on this much appreciated.