Can I ask why you want to build and not buy established? For is it was to get a house plan to fit our family's needs (which standard plans did not) and if you want this, many standard builders plans won't actually suit you. Which means you'll probably want to make changes to their standard plans which can add time and cost, so clarify if that will be the case with your builder.
Areas where you get stung with extra costs include:
- electrical plans (most builders only include one double PowerPoint per room and only 1 way switches
- kitchen appliances - builders can use really cheap appliances (noisy range hoods, inefficient dishwashers etc)
- internal wall insulation is often not included (makes for a noisy house)
- no eaves on the house (not very energy efficient - eaves will cost extra
- ceiling heights - often 2.4 metres is standard (which is really quite low)
- only 1 hot water system and small, you may need to add another one or a larger capacity one if you're a family of 4 or more.
-internal wardrobe fixtures are often just a shelf and rail
Our builder included quite generous electrical, however when we had a think about what we needed for our house it cost us am additional $4,500 and that was still using standard switch plates, not the fancy modern pushbutton ones (they actually cost $40 MORE each than the standard white old school ones.
I could go on and on, but I won't more you - ask any questions you want, this is the third house we are building so we've learnt from using mediocre builders and mistakes along the way.