depending on the "obstruction", if its pretty small and would appear under the house base, shouldnt be a problem. but if its bigger, it would appear inside your house! xD
As for placing house in "special" area (over water ect.) its way more expensive... can look up my house in Fel, just n-w of Wind over the small outpost (1226.842 in Fel), I had two 18x18 house + outpost for 1k ED (had lot of luck at that price, was meant to go at auction) but it costed me 500ED to add an another 18x18.
