I’m a structural engineer/surveyor but I don’t cover your area I’m afraid. Anyway, as nobody has replied I thought I’d add my twopenneth worth.
It sounds as if there is damp ingressing under the door itself. Most likely caused by the terrace not draining correctly (does rainwater run off it ok?) or inadequate sealing around the door frame or an inadequate threshold between inside and outside. All the above can be exacerbated if there is no roof guttering above the door in question. Hope this may be of some help!