Rising damp?

Bendy_House

That is almost certainly proper 'damp', either rising, or from an internal source such as a leaking pipe (where the water ultimately ends up on the floor in that area, and THEN rises!)

I think condensation can be completely ruled out.

So, the obvious, as suggested above. Go around and examine any possible water source; pipes mostly, but don't dismiss a roof leak unless you really can .

What was on that outside wall before the extension? A rainwater gulley? An overflow pipe sticking out the wall? Was there a kitchen, ute, or loo there that might have had a live pipe terminated?

Consider the highly unlikely. Do you have a dawg or toddler that p..... No, that's going too far.

NornIronRose

Love the idea of a toddler peeing. I needed that giggle. 
There hasn't been anything up against the wall in those areas. And nothing leaking down from up above. It was originally the back wall of the property. The sink would have been on the adjacent wall. 
I was cleaning and noticed something leaking along the floor near the washing machine. Only a little bit but definite wetness. That's about 10ft away from that damp. 

Need to try to get the kick boards off and the washing machine out to see what is going on there. 

Nifty_Purse

The vast majority of damp is from water leaking somewhere. It doesn't need to be immediately above or below the damp wall as the water can travel away from the source. 
A concrete floor can't absorb water. So the water can only be displaced until it gets to a material that will absorb it and allow it to evaporate. It is very common for leaking water to travel across a concrete floor and then up the walls. 
You do need to investigate any leaking from the washing machine.
It will take a few months for the wall to dry out completely even after the source of water is stopped.  

NornIronRose

Thank you Nifty Purse. That makes sense. Will try to pull the washing machine out today. Or at least investigate behind the kick boards. 

Bendy_House

This is promising, and would be the best outcome.

In-laws had a slowly leaking W/M in their kitchen which had Khars oak flooring. One day, stepping on it caused spurts of water to squelch up between the T&Gs - it had become a true floating floor. The assessor thought it must have been leaking for weeks.

NornIronRose

Hadn't thought of it that way. But there is no sign that the leak has gone across to where the damp is going up the wall. 
Been working so will wait til it is light tomorrow and have a good look. 

Bendy_House

NornIronRose said:

But there is no sign that the leak has gone across to where the damp is going up the wall.

Floor. Leak at one end. Damp at t'other.

There you go - a 'sign'

I hope it's the case as it's easily sortable, and  - apart from some cosmetic make-up - you might get off with it lightly. 'Rising' damp  - a damaged DPC, for example - not so much.

A thought - if this is due to the W/M, and the damage to the floor is more extensive, this could be a valid insurance claim.

NornIronRose

I've found the leak. Pulled everything out of the cupboard behind the wm and there was water dribbling down a pipe. It's been happening for a while as there is evidence of warping of the shelves and the things on the bottom shelf were wet. There is a wee stop valve on the pipe and it has stopped it instantly. 
The thing I'm most concerned about is that floor had under floor heating. Hoping that hasn't been ruined. 
Is the best plan to get a plumber? I don't think the issue is with the wm itself. Looks as if this is a cold water feed pipe. 

NornIronRose

This is yhe offending pipe. 

Apodemus

That's a good result!  Easy fix to install a new washing machine feed pipe or perhaps just tightening up the blue end on the (grey) pipe will do it!

Is the underfloor heating a wet system that is part of your central heating or an electric element with a separate switch?  If it is part of the CH system, I would turn it on for a few days to dry everything out and check that it is working.  If it is electric, then I think I would keep it switched off until you can get it checked out properly.

EDIT:  Oh! And leave everything to dry out for a few weeks before sanding and repainting the wall!