thanks :-) but not perfect yet :-(
the mod now properly gives back stone tiles and also road tiles when a depot is built with the "caution" on those tiles, a road tile is automatically placed for free under that "caution" tile and on deconstruction this new road tile is also removed without giving it back. everything ok at first sight, BUT this only works as described when not done on or near a road ...
when the depot is removed and the "caution" tile has any road tiles next to it on any of the three neighboring tiles (it doesn't matter whether they were there when building the depot or whether they were placed later), the mod does NOT remove the road tile, probably to avoid interrupting an existing narrow road. but since a road tile was returned on construction, you now end up with one more road tile than before building and deconstructing the depot.
to solve this (small) problem, there are several options, but none are easy :
maybe either (1) somehow remember the fact that there was a road tile (probably complicated and can cause other conflicts/problems), or (2) the road tile would have to be removed always (which might interrupt a road), or (3) remove a road tile from the inventory in addition to the depot that is built (this might be impossible or cause lots of other problems since i know no items that are built using more than a single item from the inventory), or finally (4) require depots to be built with the "caution" tile on top of an existing road (require to have a road or build a road first, then place the depot with the "caution" on top of it, similar-but-opposite to how other buildings may NOT be built on roads).
but as already said above: this is only a small problem. as it works now, people won't accidentally run out of floor tiles or road tiles, some times it works well, and to get more road tiles by cheating, people would have to do quite some timeconsuming work of building and removing a depot lots of times to get a single floor tile each time.