Placing large objects in a 2d grid game


I am trying to develop a simple RTS game in Flash MX 2004, and have a large grid of tiles 40x40 pixels each. I’m currently creating a function which lets me build structures in the game world (C&C-style).

My problem is that the buildings are bigger than the tiles e.g. a barracks is 120x120 pixels and I can’t think of a way of telling Flash how to deal with the building. I don’t want to make the buildings the same size as the tiles because this would make the scale look weird (e.g. when I add units).

How do I implement building placement, and how do I ensure that Flash knows the size and shape of the building so that I can:

a) snap the building to the tile grid
b) prevent building placement outside of the game world
c) count the building as an obstacle once it is placed

Any help would be much appreciated…