I was quoted $650k by ycm before I got this $400k quote fm a small builder.
Seriously my place is 20 yrs old (old but not that old), and for 400k it wid be cheaper to get another place rather than rebuilding, but of cos we love the currently environment.
For 2.5 storey rebuilding, many of the additional space are not very productive space like roof terrace. I actually add jus 1 bedroom.