Moisture in cupboard under stairs

[Swwils]

It looks like it's still damp in the living room now and yes very likely related or at least will provide more clues!

What is the exterior construction look like with the neighbor join and how was the previous issue resolved?

 

[RonnieE]

I'd recommend getting a salts test done, it will give you a lot of information on what the source of the water is, not massively expensive either.

Quick google shows this useful explanation - Salts Test - Value For Damp Or Water Leak? | WaterLeak.co.uk

 

[23vc]

Also worth checking whether boiler pressure (if combi) or header tank level dropping, which would indicate a leak in the heating, and if you’ve got a water meter see if it goes up during a period when nothing’s turned on

 

[^woody^]

The damp to the wall behind the settee is showing a classic condensation pattern. Also, rising dampness would tend to leave a brown/yellow/white mark around the perimeter of the dampness.

However, that's not to say that there is some dampness within the under-stair cupboard, but excessive or abnormal dampness would be affecting the stair timber and skirtings and you would notice it. It may well be a cold spot for any other moisture that you are producing around the house - which would also cause that staining behind the settee.

If there was a problem there previously, then you would expect there to be existing mould or signs of previous mould or recent redecoration to hide it. If not, then it may well be related to the change of use and different use of the property

Take the lino up, wipe all the mould with a fungicidal wash and put your dehumidifier in there for a week or so, with the door closed. But you need to consider if you are producing excess humidity in the way you are using the house.