Can 집에서 = "at the house"?
집에서 is "at home" or "from the house" according to the translated in Duolingo, but can it be "at the house"?
How is it different from 집에?
It is also "in the home" and "in the house". Yours is a good question. I think "he's working at that house" would be 그 집에서 일하고 있다, but I would guess 집에 is also used, maybe when the focus is more on the location than the activity.
Where is he working today?
He's working at that house. 그집에 일하고 있다.
Maybe a native speaker will weigh in and help us.