Whether you intend selling or not is not the issue.
You will either sell or pass on and then your estate will have to dispose of it.
The solicitor sounds like and experienced type of person and knows well the pitfalsl fo letting this slide.
Right now everything is fresh in everyone's mind, documents should be available, and the persons who did the work/the design are presumably still around and in business or at least contactable.
Your role, as the procurer of the works, is not complete until the paperwork certifying the development was carried otu in compliance with planning permission and building regulations is in place.
Given that you say you have several previous "builds" to your name, you should know the correct procedure by now and the benefits of striking while the iron is still hot.
You should also obtain a BER Certificate in case you have to sell or let the building as such is now requried by law for those purposes.
ONQ.
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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.