DH is a draftsman for a leading Perth building company and this is his

It depends on the builder. A company that markets to first home builders will generally offer very few changes, if any, because they make their money by having streamlined plans. A mid range builder will allow more changes. A top end builder will allow you to design your own house, but this will always cost more.

The best idea is to find a plan thats the closest to what you want, and then ask if you can change it a bit.

HTH