"I was not at my house today."
Translation:Ich war heute nicht zu Hause.
'zu Hause' is a set phrase meaning 'at home'. I think it would be better translated as 'I was not at home today', rather than 'I was not at my house today', but I'm not a native speaker so I could be wrong.
@ajdedinburgh : "nicht heute" would mean 'not today', and the sentence is trying to say "nicht zu Hause" as in 'not home'.
@SelphieB : I believe 'zu Hause' and 'nach Hause' are set expressions, and we can use 'Haus' and 'Hause' freely in other contexts. Hope native speakers will correct me if I'm wrong.
The expression is 'zu Hause'. You might also hear 'zu Haus' but this is a colloqualism, I believe, not textbook German.
@levi when do you use nicht at the end of a sentence and when do you use it in the middle ?
@lvarriale : The position of nicht: http://german.about.com/od/grammar/a/The-Position-Of-Nicht.htm