the block challenge

i would like to write a lisp extension to autocad, where the user would
input a simple, 2D, multi rectangle layout, and then the extension would
take care of adding the required pieces to the rectangles, as determined by
some programming logic. As an example, consider a simple rectangle that
represents the top view of a single wall. According to the dimensions of the
wall, i would like to automatically place tiles around it that would match
its length and width, and i only have 50cm, 10cm and 5 cm tiles.

Two methods i ve thought:
- using reactors to detect each rectangle as it is drawn, and treat it
seperately
- run the extension once all rectanlges have been drawn

Now for both cases i would need a way of retrieving the rectangle
coordinates in order to proceed, and this is where i m pretty much stuck,
after going through many tutorials. It seems that this is a very simple
task, yet all the tutorials i ve looked assume u already know such basic
stuff.