Several things here to consider:
Planning:
I would be of the same opinion as other posters, that the four walls as they currently stand do not represent compliance with planning permission. check the expiry date, if it has not yet past (although you mention 2006, which would suggest that it has expired) you can apply to extend the duration of the permission.
The above post relating to potential restrictions and conditions on the original permission is also important.
Compliance with new regulations:
You are likely to have to apply for a new planning permission for somethig like 'completion of partially constructed dwelling house'. Note that if this is the case, you will have to comply with current Building Regulations. the house was presumably designed to comply with the regulations in force in 2006.
This is likely to have the most impact with regard to insulation levels, which are now considerably higher that they werein 2006. You will also now have to comply with the new Safety and Health (Construction) regulations.
You might also check the current EPA regulations in terms of effluent treatment, if the property is 'off mains'.
Quality of existing works:
If this is the last structure constructed by a builder who was going bust, it is possible that things may have been skimped on (steel in foundations, etc). Have the structure inspected by a good structural engineer in this regard.
Is it worth it?
What you would be buying in effect is a site with a potential 'head start' in terms of a previous planning, and hopefully €15-25k of works already completed to a satisfactory standard.
Ask yourself if the layout of the existing structure is suitable, or are you going to want to remodel it to a large degree? If you are, then the value of the existing structure will quickly vanish.
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