Question the cost of EVERY building material

Build costs can easily and quickly spiral out of control if a tight grip is not kept on them. 

You need to be questioning every cost you are quoted. Once your design is finalised ensure you arm yourself with a detailed, and we mean detailed, build take off, in essence a list of every single item and it's quantity - number of, square meter of etc, you need. Work your way through the list getting at least three quotes for each item and then question the cost. Ensure the best price includes for all elements of that item. Does it include delivery, is it the correct size and specification. Watch out for the small print stating what is not included. The cheapest price may not be the best price when the exclusions are calculated.

It's always worth the time to make call and see if there is any movement on cost. If you don't ask you will pay the highest price.

This is time consuming and can seem never ending but is well worth the effort in the end. Keep a spreadsheet of savings as you make them, as the saving goes up use this as your inspiration to get a discount on the next material. 

The cost can be reduced but you also need to consider if the material can be changed in any way to ensure the same quality and spec is achieved but in an alternative way. Again hours of research will pay off and at the end of the build it will all be worth it.