Bathroom extractor fan

Last year I had a new bathroom and ensuite installed. I live in a ground floor flat and there is a ‘loft’ space between my floor and the 2nd floor. From the end of last year water has intermittently been coming in through the light fitting. I couldn’t understand why as there are no pipes up there. The problem has continued to get worse and now comes through the gaps in the mermaid boards on the ceiling. I asked the builder earlier this year to investigate which he did. He said he removed the extractor fan and saw that the floor above had dried water marks. Long story short I recently decided to have a look for myself and discovered that there’s no ducting from the extractor fan so I’m assuming all the water is condensation (?) The ensuite also has a similar problem except this water appears in a gap between the mermaid boards behind the toilet. This extractor fan does have ducting that runs across the bedroom to the outside. However it appears that something isn’t right. The bathroom and ensuite are back to back and share a communal loft space. My questions are: Is this illegal; do I have any come back from the builder and what would be the best remedy going forward? I’m presuming the ensuite extractor fan isn’t getting the steam out sufficiently along the horizontal ducting. I hope to get this fixed before the ceiling collapses!!