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Work out all your problems on the drawing board. Good detailed design can make life easier on site. Your design should optimise solar gain, be as compact as possible and eliminate cold bridges. Designing to the passive house concept is a great idea and a good investment, especially with whats coming down the line in fuel prices. Also its a bit classier to have an environmentally friendly house with a constant temperature rather than a celtic tiger era McMansion with loads of fancy sticky-out bits, requiring a massive boiler to heat a leaky badly insulated envelope. Its like filling a bathh with the plug out.
With the 2010 part L likely to make our current methods of building (cavity wall and kingspan) obsolete it is worth while building a house that will be sellable in 10 years and livable in 20. You need a consultant you can ring at 10pm as kristian advises. You need a qualified Architect or technologist who understands insulation and airtightness and can anylise condensation risk and thermal bridging. Its no longer possible just to get a draftsman, a surveyor or an engineer to do some rudimentary planning drawings and build from those. You need to take on board a consultant who will sign off on building reg compliance on completion. It is essential also to have a contract.
With higher standards of building and integration of components, the selfbuilder needs to know more than his subbies and be on site full time to check continuity of insulation and that the airtight barrier is not breeched. If you're building a house to live in for the forseeable future, don't accept the standards of the last decade. Push for passive and get a bright comfortable easy to heat home with fresh clean air and lots of light and views.
Wishin you the best of luck with your build doyler. If you had to take one piece of advise its what my granny told me. Measure twice cut once.