www.ultimatehandyman.co.uk

Hi, does anyone know what this might be?
It’s been there since I brought my house a couple of years ago, it’s on a kitchen wall that backs onto the lounge. The kitchen extension was built around 10 years ago I think and another builder suggested it might be left over condensation, but I think it might be getting worse
Thanks

Its not condensation its penetrating damp from somewhere.
Any signs of damp in the lounge?
Is that wall the original rear elevation wall?
Is it a cavity wall?
Can you post photos of the outside wall around the opening, and photos of the kitchen roof?