I think to do it in one pick is too complex a lisp for someone to generate. It could be done but may not work very well.
Below is a very old routine I modified for your use. You will need to pick the points forming the external layer path & hit enter. Then pick which side to offset.
This routine is a close as I am willing to get to the solution.
;; DrawOffset.lsp
;; Created by C. Alan Butler 2003
;;
;;; Draw a poly line and then offset it & erase the original
;;;
;;; THIS SOFTWARE IS PROVIDED "AS IS" WITHOUT EXPRESS OR IMPLIED
;;; WARRANTY. ALL IMPLIED WARRANTIES OF FITNESS FOR ANY PARTICULAR
;;; PURPOSE AND OF MERCHANTABILITY ARE HEREBY DISCLAIMED.
;;;
;;;
;;
;======================
; Start of Routine
;======================
(defun C:offi (/ pt1 pt2 usercmd str en1 en2 os:off *error*)
;;
;; error function & Routine Exit
(defun *error* (msg)
(if
(not (member msg
'("console break" "Function cancelled" "quit / exit abort" "")
)
)
(princ (strcat "\nError: " msg))
) ; if
(and USEROS (setvar "osmode" useros))
(if (not(equal en (entlast)))
(entdel (entlast))
)
(princ)
) ;end error function
(defun os:off () ; set osmode off & keep settings
(if (<= (getvar "osmode") 15359)
(setvar "osmode" (boole 6 (getvar "osmode") 16384))
)
) ; defun
;; CAB version
;; Make or Update layer ;;(make_lay "Layer11" nil nil)
(defun make_lay (n c lt / l) ;;(make_lay "Layer11" 2 "Continuous")
(if (tblsearch "Layer" n)
(setq l (vlax-ename->vla-object (tblobjname "Layer" n)))
(setq l (vla-add (vla-get-layers (vla-get-activedocument (vlax-get-acad-object))) n))
)
(if c (vla-put-color l c)(vla-put-color l 7))
(if lt(vla-put-Linetype l lt)(vla-put-Linetype l "Continuous"))
)
;; ------- Some Housekeeping ------------------
(setq usercmd (getvar "CMDECHO"))
(setvar "CMDECHO" 0)
(setvar "PLINEWID" 0)
(setq useros (getvar "osmode")
str ""
en (entlast)
)
(setvar "osmode" 175)
;; Draw the pline
(if (setq pt1 (getpoint "/nPick points, Enter when done."))
(progn
(command "PLINE" pt1 (Setq pt2 (getpoint pt1))) ;_ Start the Pline
(while
(progn
(initget "C")
(setq pt2 (getpoint pt2 "\nNext point: "))
(cond
((null pt2) (command ""))
((member pt2 '("C" "c")) (command pt2))
((listp pt2) (command pt2) t)
)
)
) ;_ WHILE
(princ)
(setq en1 (entlast))
(os:off)
(initget 1)
(setq pto (getpoint "\nSide to offset:"))
;; The user can also specify a distance by entering a number in the AutoCAD current distance units format.
;; (setq dist (getdist "\nEnter offset distance:"))
(setq dist 4.25)
(command "_.offset" dist en1 pto "")
(entdel en1) ; remove the user drawn line
(setq en1 (entlast)
filr (getvar "FILLETRAD"))
(setvar "FILLETRAD" 2.5)
(command "_.fillet" "P" en1 "")
(setvar "FILLETRAD" filr)
(make_lay "Internal" 221 "Continuous")
(command "._CHANGE" "L" "" "P" "LA" "Internal" "")
)
)
;;========== Exit Sequence ============
(setvar "osmode" useros)
(setvar "CMDECHO" usercmd)
;; Exit quietly
(princ)
) ;_end of defun
