You seem to've done everything correctly so far - carry on!
Why should you carry the risk of others unlawful developments?
To add fuel to that fire, you should request them to obtain the permission.
The permission should be a coverall permission for the entire holding, not just the "bits".
This leaves you with a clean Opinion and consequently clean sell-on, when you in your turn sell this property.
You should then request Opinions of Compliance with Planning Permission and Building Regulations on the entire.
They will hugely resist this, because most architecs will not want to issue the latter for work that they have not seen to site.
In relation to the permissions required, permitted development not built in compliance can be enforced up to twelve years after completion.
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.