As a builder with many years experience and going through the building experience as I write this thread, I feel I should share some thoughts with soon to be home owners.
A common error I find from my clients when it is time to sign contracts is that they have not chosen their PC items. PC items are what I call non structural items such as ovens,tiles,flooring etc...
I strongly recommend to my clients that they go out and see what things cost. If you are going through the building process for the first time you might think that $20square metre can get you a great tile but when it is time to buy the tile you realise that the tile that you saw in the flashy magazine actually costs $60sqm. There goes the tile budget.
This will happen on all items unless you choose or at least have a visited shops to know what the prices are. Also do not think that you might spend less than your budget, in 20 years I have never met any one who has spent less on their PC items than what is allowed for.
This is especially the case with project homes. Go to their display centres or suppliers before you sign your contract. They give you the very basic unless you go with one of their package specials.
Hope this was helpful.