I assume you have signed a standard HIA Lump Sum Building Contract?

If so you are obligued to make payment within 10 working days of the home being "Practically Complete", the builder is also obligued to rectify all items on the Practical Completion Inspection sheet (you did sign one eh?). Practical Completion has generally been interpreted by the legal fraternity to mean fit for occupation without major siruption or inconvience to the occupier.

All this crap about trade shortages is BS, the construction market has flattened out and there is not a problem with trades like there were 8-9 months ago.

As for the plans showing the Solar Pamels on the western side, the builder will argue that the home is built to the plans which you have signed, this is not likely to hold up at a Building Dispute Tribunal Hearing, unless there is a very clear reason for doing this, as the builder is holding themselves out to be expert in design/construction.

I work in the project building industry and have for 10+ years, I am not a legal expert but have gotten involved in more than the odd dispute. If you want any further info PM me your details and the builder - if it is one I am related to not much more I can say.