We had a similar thing when the plumbers came to put a boiler on a wall. Underneath 4 alternating of layers of tiles and plaster board you could move the wall with your hand.
Has made us think about a two story extension like the one upthread which most people hated but we quite like.
I'm reading the work you've done with a mixture of admiration for it being done 'right', and trepidation having recently bought a semi detached nearby in not dissimilar condition, complete with some old bomb damage.
Had that with the back wall of our place - Victorian mid-terrace. It was hidden behind wooden panelling in the kitchen. No footings on back wall so complete rebuild - happy days :(
This week's four figure surprise.
Where the extension meets the house at the roof point is coming away.
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