Do resources for construction get assigned immediately?

I am wondering if i create a huge project above ground at once (say, on pause) with several stone stockpiles. Will my dwarfs use the stones from the (near) stockpile while these get refilled? Or do the stones get designated to the construction and being "forbidden to move for other purposes"? So the stones designated to the construction will stay in place till a mason picks them up for constructing a section.

Motivation: I want to construct a above ground castle with the help of quickfort. So the complete thing will be designated at once and will require 100's of stones. I don't want my dwarfs to walk 100 Z levels down to collect the stone by hand though. I normally build one or several stone stockpiles outside and near the construction site so dwarfs will use carts to get the stones up. Since normally i construct a couple of walls at a time they will always pick the stones from the stockpile if they are near and contain the correct stones.

Solution 1:

In fact, you designate the exact materials for construction.

If you hit the x key during the item selection phase of construction you'll notice that instead of each material grouped by type and sorted by distance of the nearest item of that type you see every individual item sorted by distance. These items are then locked in when you finalize the construction order. The construction will use those exact items (and cancel if one of said items vanishes for whatever reason while being assembled).

My recommendations:

1. Place a large stockpile that accepts only stone blocks of the type you want near your construction site. Remember that one unit of stone becomes four units of stone blocks nowadays, to compensate for the fact that mining non-rare rock only drops a stone 25% of the time.
2. Build several masonry workshops near where the stone type you want can be found a lot.
3. Place a stone stockpile that accepts only the stones you want between the masonry shops, then designate it to feed stone to those masonry shops. This will prevent the masonry shops from using other stone types. (Hint: Assign a few wheelbarrows, this will greatly speed up the rate at which the stockpiles get refilled)
4. Set the masonry shops to produce nothing but rock blocks.
5. Designate another stockpile right next to the masonry shops that takes only rock blocks and only from the masonry shops, puts everything in bins, and gives to the stockpile mentioned at 1.
6. Make sure stockpile 1 has a few wheelbarrows at its disposal.

7. Spend some time mucking around elsewhere, carving out new corridors so as to make sure the workshops have enough raw material until the first stockpile has a nice big collection of bins full of rock blocks of the type you want. (One bin holds 10 multiple blocks, saving a great amount of floor space) EDIT: There've been a few changes in the recent version involving how blocks are handled; no longer 100% certain of the exact count.

8. Start building.

Solution 2:

The stones are assigned to the construction at the time they are designated, and are effectively forbidden for any other use until that construction is built or cancelled. This can be quite annoying if a stone designated for construction is in the way of some other second building (construction, workshop etc), as it cannot even be moved out of the way by another dwarf. This "obstruction" can then cause that second building to be suspended.