Okay, on the 'neat trick' point, imagine that I've got my heart set on building a 'room' such as the following plan, to which I am not
yet guaranteed any wall-tops (and maybe not ever), just currently the stairwell access, "X", indicated...
#######
#+++++#
#+++++#
#+++++#
#+++++#
###+###
X
This is how I'd "fire and forget" the building process. Here "W" and "F" are 'set to build this turn', "#" and "+" are 'in thevqueue from a previous phase' and "c" is designations you cancel.
WWWWWWW
WWWWWWW
WWWWWWW (a 7x7 block of walls,
WWWWWWW which is impossible to build in one go
WWWWWWW but perfectly possible to /place/)
WWWWWWW
WWWWWWW
X
#######
ccccccc
ccccccc
ccccccc (Cancel all but the back wall)
ccccccc
ccccccc
ccccccc
X
#######
FFFFFFF
FFFFFFF
FFFFFFF (put floors in the gap)
FFFFFFF
FFFFFFF
FFFFFFF
X
#######
+++++++
ccccccc
ccccccc
ccccccc (Cancel all but the /entire/ back floor)
ccccccc
ccccccc
X
#######
+++++++
WWWWWWW
WWWWWWW
WWWWWWW (Wall block)
WWWWWWW
WWWWWWW
X
#######
+++++++
#ccccc#
#ccccc#
#ccccc# (Remove the inner wall designations,
ccccccc even back to the lower row,
ccccccc plus a bits of the corners)
X
#######
+++++++
#FFFFF#
#FFFFF# (Fill with floors)
#FFFFF#
FFFFF
FFFFF
X
#######
+++++++
#+++++#
#+++++#
#+++++#
ccccccc
ccccccc (Remove these again)
X
*If* you don't have 'corridor' access, yet sideways from the X
#WWWWW#
X
...then
###c###
X
...then
###+###
X
But if you *do*, you can' modify that sequence to skip quicker to the final state, which is (designated to be) the following:
#######
+++++++
#+++++#
#+++++#
#+++++#
+++++++
###+###
X
#+###+#
#+++++#
#+++++#
#+++++#
#+++++#
#+++++#
#+#+#+#
X
This gives you a square room designated (during one 'paused instant') that you can forget about and mostly ignore as you manage everything else. Except of course that when it is
ctually built (or sufficiently so, for our purposes, i.e. the corners built from their' tab floors' are constructed, whatever else is currently in process or needs unsuspending from deliberate or accidental actions), designate to deconstruct the tabs and when they are gone replace them with the walls. A quick enough finishing process.
Non-square rooms are possible. You can generally manipulate things to create structures-
in-potentia of up to ten tiles away from a single access point and 19 tiles wide, centred upon that place. By concentrating on constructing floating walkways first (perhaps through the centres of an entire imagined room-system) you can even pre-pre-plan for a whole pre-planned
complex of constructions for which you don't have quite so many existing standing places at the start. It just needs some proportionately simple consideration before you even start.
(If, however, I have already gained the support of an identical level below, already built, I'd designate the central floor and doorway-floor first, then the four corner walls (the 'near corners' can be stepped around upon the wallgtops to build the 'far corners', if necessary), then as soon as any two adjacent corners are wallified I'd designate the wall-top between them as walls. As soon as floors get produced to give a place to stand, these wall-tiles are ready and waiting and able to be built, and become priority over everything except the more
even more recent wall designations, which are probably in the same bit of the grand building plan anyway.)
((Or, as YetAnotherLurker says, there's DFHack, but I don't use that. I am usually personally content with the above techniques.))