Follow Complainer's advice.
The neighbour appears to be in clear breach of the permission received.
At the very least, the density of dwellings on the site has just doubled without permission.
If these are now deemed to be a commercial development, other statutory approvals may be required
I haven't looked at the legislation in a while, and Residential typically gets a few easements, bur they could need both -
(i) A Fire Safety Certificate
and
(ii) A Disability Access Certificate
If the Local Authority are not moving on this, remind them that anyone can take a Section 160 Action under the Planning and Development Act 2000 as amended.
A private citizen taking or even threatening such an action will tend to embarrass them into at least visiting the premises, after which matters should take their course.
Please report back and let us know how you get on.
ONQ.
[broken link removed]
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.