Good ideal Clanad. There could be many reasons why: Was there a cat/dog in the previous home, it may smell like a potty area, make sure and clean it all the way to the carpet pad with an enzymatic cleaner that has active enzymes in it to take the odor away, if it is an old spot it may have to be treated quite a few times to get it removed. You may need to re-train your kitty over again on litter tray, unless this house is built the exact way your old home was she may be a bit stressed, she won't show it, cats don't like to show weakness, and if she sees another cat she may feel threatened, make sure she feels comfortable where YOU want the tray, if SHE doesn't like where you put it she may find a safer/less traffic/easier place So figure out why, cats who have had good potty habits then change their litterbox habits it's for a reason, they don't do it out of spite or revenge.
Check out:
http://www.catsinternational.org/
click on "articles" and click on "Housesoiling"
there's lots of info there, good luck. ~/:\~