It sounds like you are self managing the project without an architect, technologist or engineer being involved.
Your building project will still have to comply with planning, building regulations and perhaps health and safety legislation (refer to an earlier thread which deals with the health and safety topic).
The extent of your knowledge of current building regulation or planning law in Ireland is unclear from your post. Do you realise that the new extension will have to meet in full the requirements of the current building regulations, and that the renovation works may have to comply in part or in full with, for instance, Technical Guidance Document L (Energy Efficiency) amongst others?
Do you require planning permission for the extension or is it exempted development? If you are upgrading an effluent treatment unit, this will require planning permission in any case (and will therefore require site characteristics tests by an engineer/technologist). Will the planning authority take the view that the cottage is no longer a 'house' in planning terms (when was it last lived in?).
What I am trying to do in the above is to point out that you should obtain professional advice before you start. There is a danger of being half way through a building project of this type and then finding out that it doesn't comply with all the regulations that it is supposed to, and then one may be faced with rework and extra expense.
In terms of certificates from professionals, it may be possible to appoint an architect or engineer at the end of the project to do one of those 'i didn't inspect during construction, but from what i can see now that its finished, it probably looks ok'.
Bottom line - I would strongly urge you to talk to someone before you start, if only to obtain clear guidance on what you will have to comply with.
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