G'day mate. Basically, renno's cost nearly 2 x the cost of building new. Adding to a existing house takes a lot of time and effort to "make good". The only thing I can suggest is to make everything standard sizes and keep your fitting and fixtures down to sensible choices. You could do a bit of labouring but it wont make much difference at the end of the day. Painting it yourself will save you a fair bit though.