New Building Dampness

[Sow]

Hi All,
Ours is a brand new building just occupied 2 months ago. First floor area has bathroom and toilet on a sunken slab. All flooring and wall joints are (paper joint) and some areas not grouted properly.
Recently, we noticed dampness in Ground floor ceiling/ slab which is underneath this sunken portion.
Now, my question :
1) Will regrouting with epoxy ( creating spacer in floor tile by skilled labour) be sufficient?
We are looking at this option since it's a new house and redoing the entire wet area isn't an option.
2) Will an injection grout or waterproof chemical done to slab help?What chemical should be used and how does this work?
3) Also, there is some dampness in FF wall which is above the sunken slab? Does this mean that the sunken slab is filled with water and dampness is rising on the wall?

Please provide your expert suggestions and recommendations. Would also appreciate if anyone can refer someone who can fix this type of problem.


Many thanks
Sowmya