errant atlas
As it stands whenever you build something it appears in a translucent blue ghost and the drones start construction.
I think the best way to implement a planning mode would be to have a flag on the construction tools, maybe it would appear beside the category name, that is a toggle for planning mode ON or OFF.
All the construction ghosts would probably need to have a flag added, so that the drones ignore them until the planning flag is cleared.
When planning mode is toggled ON, the buildings and conveyors and wires placed automatically get the planning flag.
Then we would need another tool on the toolbar that sets/unsets the planning flag. I think it would work best if there was a PLAN brush (similar to mining mode) that sets the ghost planning flag, a BUILD brush that unsets the planning flag, and maybe a button for PLAN ALL and BUILD ALL that set or unset the flag for every current ghost on the currently displayed map layer.
I'm not sure how much extra this would add to the game development or what changes would need to be made, but it seems this implementation would make use of most of the systems currently in the game without needing a lot of extra interactions added.
vague sage
slender chasm
hallow sky