This is too good NOT to necro, especially as this is the first time I've seen anything like it.
As it is, mining is easy--you just "paint" in two "colors", it's dead-easy to go modular by leaving "walls" to steer the miners away from halls & rooms you don't want emptied (yet), and you can even do it with the mouse. Having a Planning Mode would give us this convenience for nearly every single other aspect of fort design. Just arrange a toggle to go from the dwarf-reality Existing Mode to the Planning Mode. Planning Mode would be most useful for aboveground constructions, as you could "paint" with tiles of whatever you want: Click & drag over a swath of tiles to turn them all into Diorite Block Wall--the mouse will continue to be Diorite Block Wall until you set it to something else; Marble Block Floor, Microcline Up/Down Staircase, Magma, Open Space, what have you. There are no restrictions: You can "build" with material you don't have yet, and even designate walls & towers hanging in space if you want (though naturally that will cause problems, when it comes time to actually build the thing). Sure beats having to designate things no more than 10 tiles at a time, choosing materials at every step! Underground, the Planning Mode would work much the same as regular Mining does now: You can still be surprised by obstacles like aquifers, magma, and caverns. I'm unsure about allowing the user to place things like furniture and workshops in Planning Mode, above ground, at least ("Can I put a birch wood Bed on this tile that currently has no floor? And if I do put a floor there later, will that click overwrite the Bed?"), but I *do* know that I want the player to be able to designate stockpiles, especially stone stockpiles, in rooms that don't exist yet. It would be SO NICE for a Miner to dig out a stone and just have it fall, right there, without generating any Hauling job at all.
This would enable the player to design the entire fort, and more importantly check that design, right from Day 1. No more having to tear down part of your grand tower because you just found out that the site's ceiling is too low for the peaked roof you wanted to put up. And the best part is, to make your dream a reality, all you have to do is toggle back into Planning Mode and flag which tiles will be part of your next phase of actual construction.