I live in a Victorian terr house. In the front room there is a slight damp smell and nightly visits from slugs from under the floorboards so it it obviously damp below. The walls are quite dry, there is a dpc but the damp seems to be contained below the floor. but why is it so wet ? the soil below feels wet - there are air vents which are clear, some joists have been replaced because they were rotting. The floor in the back addition has been replaced in the past with a concrete flr. Where might this moisture be coming from ? I have redirected the front down pipes away from the 2 front drains as they weren't draining quickly but doesn't appear to make any difference.
You need to check that there are air vents which will provide a cross-flow of air under the floor. If the back extension has a solid floor vents may have been cast into the floor therefore no cross ventilation.