I take it you are not on 2006 or later, because this is built into the RECTANG command now.
If not give this a try:
Code:
(defun c:thicken-slab ( / ANG1 PNT0 PNT1 PNT2 PNT3 R0 R180 R270 R90 SIZE X)
(setq r0 0.0
r90 (* pi 0.5)
r180 pi
r270 (* pi 1.5)
size (getdist "\nWidth of footing: ")
pnt0 (getpoint "\nSlect start point of footing: ")
pnt1 (getpoint pnt0 "\nSlect end point of footing: ")
ang1 (angle pnt0 pnt1)
x (/ size 2)
pnt2 (polar pnt1 (+ ang1 r90) x)
pnt3 (polar pnt0 (+ ang1 r90) x)
)
(command "-layer" "m" "S-Foun-Footing" "c" "3" "S-Foun-Footing" "l" "hidden"
"S-Foun-Footing" ""
)
(command "pline" pnt0 pnt1 pnt2 pnt3 "_C")
)