I have this problem also although with a 10 year old apartment, it's also over a basement carpark and I initially thought it might have been rising damp, I spoke to a very helpful damp specialist over the phone who said it was highly unlikely to be rising damp as it never rises more than 3 foot. He said he would almost guarantee the problem to be caused by human condensation and lack of ventilation and actually there is no proper air vent in the room we have the problem in, just a crappy small vent in the window itself. I bought a dehumidifer a few weeks ago and have asked the tenant to use this as often as she can in order to try and dry out the place, I'm hoping this might do the trick. So, maybe buy yourself a dehumidifer first (I got mine in powercity for €140 approx).