I am no expert, but do a lot of photomanipulation and so far could not get the gist of using masks (or maybe I do and don't know it

) so what I usually do is the following:
First I make sure that both images have the same resolution if they do and the part I need from "image 1" is larger than the space where I want to insert it in "image 2", I select it (depending on size I give a 3 or 4 px feather or just antialias) copy; paste (as new layer in same image 1); reduce it to the approx. size to fit in the area of "image 2" ; remove all unwanted background px; select the blank area in the new layer, inverse selection then ; copy and paste it in "image 2".
You mentioned that it is a building you need to add; my Photoshop version (CS2) has a Perspective and a Warp option which might be useful in this case.
Hope it helps a little with your question. If it's too obvious just bear in mind that I am a relatively "newbie" with the technical applications in Photoshop.
Rgds - Martha
P.S. Tks Delic for the link about "Masks" will see if I can finally manage to learn this to me "misterious" Photoshop device. 