Damp internal wall - opinions on solution please

[cloughie100]

Have extended house in last year since then understairs cupboard smells really musty ( didn't before the work) builder says wall in cupboard damp so proposing to knock down wall ( non-supporting) and add damp membrane - does this sound right? Should I get guarantee for work?

We had a damp wall in an internal cupboard in the hall.

It turned out that a radiator pipe was producing a very fine spray under the floor and making the wall wet. You need to find the source of the damp.

yes.

does this cupboard have a wooden floor? Lift a board and have a look and a sniff underneath.

Did the extension block airbricks, preventing airflow under the floors?

Have you got a water meter?

Yes has wooden floor and the wall in question is the one underneath the stairs. I will have a look at boards tonight, it's possible that an air brick has been blocked up.
We do have a water meter - why?

if your peer at the meter with a torch (wipe the glass clean) you will see a bubble in the middle, which spins when water is passing through, even if so little that you can't see the digits change.

when all your taps are turned off, and you have not flushed a WC for a few minutes, the bubble should stop. Unless you have a leak.