On Nov 4, 10:52*pm, wrote:
Sorry, the dividing wall is between the kitchen and the extension, both have the concrete floors.

I had a chemical DPC done by a plasterer a few years back.

It is very likely condensation especially if it is worse on the
kitchen side and the extension is a cold room..
I'm surprised you don't also have condensation on the outer wall(s)
of the kitchen but some surfaces don't show it or maybe are insulated.

Do you have a cooker hood to take away cooking vapours?
If not, that is the likely cause of your troubles.
The chemical DPC should be good if properly done.