I live in an old cottage built 1840. It has thick stone walls and downstairs is tiled throughout. Not sure whether it has a damp course! I have always had a bit of damp at the rear (ground level is higher) and a bit above the bedroom windows. The problem there is leaky stone guttering which I am getting fixed. Other than that it has always been a dry house.
About 3 months back I had a new kitchen put in. The kitchen is part of the lounge. It could be a coincidence, but since then I have had high levels of humidity downstairs with resultant mould on one wall, damp sofa cover etc. Even the porch has got damp. I have a dehumidifier going non stop to dry everything. None of the pipework seems to be leaking and underneath the units/appliances it is very dry. Could it be condensation coming from the built in freezer and fridge?