To think they peed in the wall??

[StarFruit11]

I live in a new build(council) and there is a smell in one of the rooms, basically like old urine. I have tried finding out where it's coming from and it seems to be one area within the wall. Could a builder have weed in there before putting the wall there? is this a thing builders do?

That sort of smell can be a sign of problems with the electrics.

My first concern would also be the electrics.

Possibly.
But also maybe a problem with the mortar, that often smells urine like when crumbling due to water ingress.

I'd say electrics too. We have a hot water Switch for our electric shower and I noticed it smelling quite fishy and getting hot, Turns out it's was a wiring problem and had to have a new switch. No more fishy smells