Aspect, slope, soil and the constraints your block puts on the design.
The block decides more about the finished house than the drawings do. Aspect, slope, soil, easements, setbacks, overshadowing and the neighbours' windows all constrain what can be built before anyone has drawn a line.
These guides cover reading a site properly — what the orientation gives you in the southern hemisphere, what the soil report means for the footings, and which constraints are negotiable and which are not.