Captain Z,
Its good to see your project is progressing and you appear to have done all you reasonably can to contact your neighbours.
Building on a boundary is now the subject of
the Land and Conveyancing Law Reform Act 2009.
Chapter Three refers but there are other laws and regulations involved too I understand.
Make sure your architect is competent to take on the role of planning consultant.
I've been doing Appeals since shortly after I qualified but this isn't unversal.
Not all architects do this work nor are they competent to advise on it.
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If your architect is competent to do so he should advise.
There is no rule that says your architect must write the letter for you BTW.
Some local authorities seem to prefer the owner/occupier to write their own letters.
If the Observation from you is calm, courteous, accurate and fair it can give a good account of you.
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A planning appeal is a different kettle of fish in my experience.
Here a professional acting on your behalf may give a better impression to the An Bórd Pleanála Inspector.
Most Inspectors of whom I am aware seem to be drawn from the ranks of planning, building or legal professionals.
If an appeal is expected its as well to involve a planning consultant at an early stage and take his advice on the observation.
Best of luck with it and you might let us know how this turns out Captain Z.
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.