You can write into the contract that the purchase is subject to certain
things. Basically anything your little heart desires. Be sure to have an
"out" where you can get your money back if the finished house is not what
you expected. Best to get an attorney to be on your side - word it so you
can back out for any reason.

Also if house is not built yet, might think about some basic modifications
that you would want. Like they always put the shower head at eye level. Make
them install it higher. Or never enough outlets in kitchen. Have them put in
more. Never outlets in hallway. Have them put outlets in hallway for
vacuuming, etc.