If you said that to me, I would imagine the exterior of my house. But one of the things I am trying to work out is whether my definition of "imagine" is different. I would think of, and sort of see different details of the house at different times. Sort of like picturing one part at a time, but perhaps more like remembering than picturing. I know the overall layout, but I don't know if I literally "see" it.
I've tried a few times to actually _see_ something in my mind, and there have been moments, usually when I'm close to falling asleep, where I have actually seen something vividly. So much more vividly than usual, that I remember thinking that if this is what other people can do easily, then sketching must be far easier than for me - you'd just copying down what you see in your head!
Edit: I remember thinking the illustration on the Wikipedia page might be a good way to think of it. I picture things with less vividness/detail, so I'm not sure whether I really see them.
Ironically, the Wikipedia image is not loading for me right now, so I can't see that either :) https://en.wikipedia.org/wiki/Aphantasia