Hi everyone,
Apologies for the long post. You helped me previously with window details.

I've just moved into my new build house that I have been constructing over the last few years.
It's an open plan living dining room.
The house is externally insulated apart from one side because a neighbouring wall (3 storey) is only 300mm away from my external wall. There is no space to get anything between the houses. As a consequence, this wall is internally insulated by constructing a timber stud wall with insulation in between. The make up of this side elevation is as follows render, block work, 100mm cavity, breather membrane, insulated stud wall, vcl, insulated plaster board, plaster.

Up until we moved in, no issues had been identified regarding any damp or smell. However, once we moved in a musty smell was apparent coming form the sockets. Taking sockets off, it was clear that builders had cut vcl to sink back sockets. I then bust open the back of sockets and you can clearly smell the damp stale musty air. I could not inspect the cavity between the breather membrane and externall wall. However everything appeared dry on the internal face of the breather membrane.

I have subsequently re insulated and repaired the vcl later. The smell has reduced significantly but it seems like there is still an odour which suggests that there is background leakage of air into the house. The building inspectors suggested drilling 50mm holes from the external wall so that the internal 100mm cavity can ventilate?
If this did not help the it would then require busting open the whole wall and doing an instrusive survey. .

Not sure how good the drainage is between the two houses (the 300mm) gap. Both houses have their foundations and the ground is made up. Possibly the rainfall is not draining away and that the smell is coming from the ground up.

The floor is a block beam with insulation and a screed for UFH.

What would you suggest re next steps? Make a hole to ventilate the cavity between false stud wall and external block work? Rip open internal wall and start again with possible tanking of floor/wall etc but not sure what system to use if this route is recommended?

I feel hugely disappointed and blame myself for not picking up on this earlier. I just did not see it smell anything.
If you cannhelpnwith any thoughts or advice that would be much appreciated.

Is the beam and block underfloor void ventilated?

Is it a drains smell. Could you smoke test the drains?

It would very unusual for a cavity wall to smell, there are millions of them in the UK that don't smell, what is different about yours?

Shower or bath anywhere above?

If it's rainwater down the 300mm gap that's adding to the problem, an extreme solution might be to attach a 300mm roof over the gap? It might just help.