There are two main types of contract that a builder can use. They are the COST PLUS Contract and LUMP SUM Contract.
Cost Plus Contract is a contract where the builder will charge you the cost of the material and labour used for the build plus either a fixed fee or a percentage on top of that.
Lump Sum Contract simply means that there is only one agreed fix price for all of the work listed in the specification or scope of work. Any changes to the specification will become part of a variation order which you as the client will need to approve and sign off before the builder can do any changes and charge you for those work.
Check out this video for my tips around what to look out for when looking at building contract.
Link to 'Notice for Home Owner' for WA https://www.commerce.wa.gov.au/sites/default/files/atoms/files/notice_for_home_owner_aug2013.pdf