Are you at the planning stage? Well, once you press the button to get quotes and commence construction you will find that any changes or revisions will trigger the Alterations Clause. What is that? you ask. There always has to be some flexibility for alterations, but it comes at a cost - and sometimes at a surprising cost. In fact many people complain that simple alterations, or alterations that even reduce the scope of works can seem unreasonably high.
The best way to avoid this issue is to plan well and minimise the chances of expensive alterations. Do not get rushed into going ahead with plan that are "near enough", as near enough is not good enough when it comes to approaching your build smartly.
Considerations like your future needs (growing family perhaps?) or realisations that you have not utilised natural light well are all avoidable with a bit of planning. Do not fall into the common trap of commencing construction and then realise you did not plan enough void space for the cooling and heating ducts.