Great advise! I would have said exactly the same!There is so much I wish I had done! Only said to my DH last night I would love to build this house again ... he on the other hand would rather poke his eyes out than build again ... considering I did everything I don't know what he was whinging about.
Here's a few things:
The price listed for the house is usually for bare bones, with basic light and powerpoint in each room on a 'perfect' site. By the time we allowed for site fees and a few changes we were already $50k over the base price without even adding electricals/kitchen upgrades etc.
You can never have enough powerpoints ... it is heaps cheaper to put them in at build stage than after.
Really think about window/door openings ... ie we've got a cavity slider in the ensuite, it slides to the right' shower is on the right, towel rails can't go on that wall due to the cavity slider ... no issues in summer, but very chilly in winter!
Whatever storage the plan has, add more!
Increase the length of the bath tub ... in our case I didn't think of it and there was room for a full length bath, but they put a shorter with a hob at the end.
Project builders seem to be reducing the size of double garages ... we increased ours to 6x6 but with two big cars we can't get the kids in/out if both cars are in the garage. Again ok if weather is lovely. We really should have gone 7m wide at least, especially as we have pushbikes in there too.
Check what paint they offer as well. Alot are dropping to two coals of 'quality' paint, but the undercoat is counted as one ... one topcoat is not enough.
Think about heating/cooling and have it all added in at build stage, much cheaper! We didn't like air conditioning system the builder offered, so we arranged with the builder for our own sub contractors to supply our preferred system during the build, saved us thousands.
Look at the way appliances open and their placement in the room ... ie. do you need to swap where the fridge is placed? Front loader washing machine, make sure the door swings the right way in the room.
They say allow for 10% buffer/contingency ... should really allow 20%+
Go with a builder that is happy for you to visit the site daily, don't accept it when they say laws don't allow you onsite ... alot of builders supply construction keys now so you can keep any eye on the build.
Google the builder you are thinking of going with ... you'll come across some great Australian building forums that are a wealth of information.