Hi beetroot,
It is generally bad practice for any designer to base a new design on drawings that are over thirty years old.
Many things could have happened to the house during that period, including extensions, demolitions and additions and a proper survey should be carried out.
The original drawings may be useful for comparison purposes, but a new extension should always start with very recent "as built" [not planning] drawings or a current survey drawing.
Of course, you will have to pay him to survey the house, but you'll have to pay him to read whatever file and make sure its the right set of drawings.
You may find that certain conditions have not been met in relation to your property and he may have to rectify matters under planning law.
Equally, the time for enforcement may have passed and you may be well-advised to let sleeping dogs lie - but follow his advices.
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That having been said, in relation to the information you are looking for three place spring to mind.
1. The original architect
Architects are a collegiate lot and would not mind forwarding copies of drawings of that vintage to a newer practice.
2. The Local Authority
I am researching a file going back to 1985 at the moment and have been told that this information may be available in electronic form at the local authority offices.
3. The Architectural Archive
Some firms have deposited drawings there for posterity, and they persist even when the original architects files have disappeared or been shredded.
I searched for and found a set of drawings there for an extension of a similar vintage to your house and they showed the full extent of the works including notation for the builder on construction drawings.
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For the record, there is no such thing as an architect/builder.
You can have architect-developers and architect-project managers, not architect-builders.
The RIAI Form of Contract places the architect as the administrator of the contract between the Employer [the Client] and the Builder [Main Contractor].
Thus, for him to act as a builder would be a conflict of interest and against code of conduct.
Perhaps Docarch or Threebedsemi could clarify if I am misleading you here.
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.