A simple question, to which i couldn't find a definitive answer - how to manage buildings on a tiled map? Should the building be sliced in to tiles or one big image?

EDIT: The game is being built from scratch using C++/SDL 2.0, it will be a turn based strategy, something like Fallout 1 & 2 without the hex grid, a simple square grid, where the Y axis is squished by 50%. Buildings can span multiple tiles, the characters move tile by tile. For now, the terrain is completely flat. Some basic functionality is in place, so I'm aiming to advancing the terrain and levels them selves - adding buildings, gates, cliffs, not sure about the elevation.