Fines for not applied for building licence

When we renovated our old house in 2017,that then had only a wc with access through the garden,but not separate from the house,we were told by the mechanic that we could apply afterwards for a building licence. The builders made an extention on the house,about 3sqm,to make the wc slightly larger and also in direct connection to the house,no longer having to first go outside and then enter the wc.

After a few years,we spoke to a real estate agent about selling and contacted the mechanic, who said she hadn’t applied for the building licence and that it would cost about €3000 and there would very likely be a fine. (Yes,I was furious.) Is there anyone in here with a similar experience or someone who can guide me through this experience and hopefully get a positive outcome?