mwhich,
I'm afraid I know of no-one who has successfully argued for a reduction based on economic hardship due to the recession, but other posters here may contribute something in this regard.
OTOH, you are about to spend hundreds of thousands on a self-build - do you think your position will be viewed as credible by the Local Authority or a Court of Law?
Perhaps you could successfully agree staged payments - in that case, see below.
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Levies and Contributions normally arise through the wording of a Condition of a Grant of Planning Permission.
Conditions are usually non-negotiable unless you've appealed your permission to An Bórd Pleanála successfully, or unless the wording of the Condition allows for alternative payment arrangements.
Where I have seen these, you are likewise bound by the terms set out on the Condition, so read your permission carefully.
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.