Rockhopper knows, bang on.
Pay attention to the bit about soil up against the outside, as this is sometimes the only bit you can’t fix fully. Our old house had a single skin brick extension that was about 1m under the ground level at rear, after a few years of living there noticed the carpet was wet, totally soaking. Pulled it up and found a steady supply of water coming under the plasterboard…
Removed the bottom half of plasterboard and uncovered the ‘fix’ by a major UK damp expert, that was to lay a plastic sheet up against the bricks and cement it in at the bottom. Behind that was a tangled web of damp mould and a soaking wall. Dug it back on the outside and found they had built the extension on bed rock (it’s on a quarry hill, so pretty solid!) With no foundations. A constant stream of water was draining off the hill and pooling against the back wall, and fast. If you dried it off it would be a pool again in 10 mins.
Had a few independents round to look at it but none could give a decent solution. Everyone ran a mile or just gave terrible ideas about blocking the inside again.
So… We built a small brick retaining wall underground and chucked a perforated drain pipe in front of it, back filled with pea gravel and took the water off ‘elsewhere’, left it open for six months or so and it was a lot drier behind the wall. Had the back end lime rendered to evaporate anything left and re did the inside. All in all I think it took about three years start to finish!.
TLDR, don’t buy houses dug into the ground.
Current house is 200yr old solid brick cottage, old owners hated the damp and the cold, got it for a steal. Whole place covered in cement render, largely blown. Lime render, stuck the fire back in, bosh. Lovely house. Nice and toasty with 55% average humidity. Good ventilation under the house helps.
Really odd thing, when we went to view it agent said we were the only people that had been up in the loft to check the roof, insane
