Simply place a building there, model a terrain under it and texture it afterwards? What I find myself doing a lot is that I have dirt as the first texture, I place buildings on it and when that is done, then I actually start texturing a grass. But nothing prevents you from placing a building on a grass and drawing a dirt under it afterwards.
https://youtu.be/LQbdyl1_3bY?t=3m29s (and around 5:20 is the second part of working with terrain around buildings)
EDIT: If you've got trouble outlining something accurately, it is also possible that you don't know that you can change your brush radius by pressing ALT key, left clicking with mouse and dragging to the left or to the right (you can do the same with SPACE key to change pressure btw). This makes resizing brush much faster, easier, but most importantly, much more accurate. Don't use UI for changing brush size, its pretty terrible and useless in this case.
https://www.youtube.com/playlist?list=P ... FxyclGlCKsCZ commentary, but you can learn a lot just by watching.