TrueBlue1 wrote:...wonder if anyone has any idea of how the project is valued for tax purposes as it sounds like we have to pay this even if it is a DIY project.
So far as I know, unless it's changed, you have to pay the taxes (IKA or whatever it's now called) regardless of who does the work, DIY or otherwise. All the costs should be calculated by your CE as they form part of the documentation required for planning approval.
Electrical work should be done by a Greek qualified electrician unless outbuildings are exempt. When we had a holiday apartment built the electricity schematic had to be drawn up by an electrician and approved by his union before it was presented to the building office. The first thing you need is the building licence without which you can do nothing (at least not legally).
Another added cost might be an increase (hopefully small) in your property tax bill if outbuildings are included.