I am still not sure what happens if you shoot through a Floor tile in a house , Do you fall to the lower level and get stuck or do you just float over the hole.
If it will work like with hilly stuff, then yeah, in extreme situations unit may stuck in 24 voxels deep hole, and that's why I've suggested 24->16->8->0 to minimize it. Besides this will look better, than big, badass hole in the ground
I've already made 24->16->8->0 basement tiles (they're based on that XBASE tile), as a part of enhancing original maps, and preparations
* for basement level (see attachment).
FYI: i have not ever modded a MCD-entry before so i may need someone who is experienced with this side of the thing either show me what i need to do OR team up with me for the mapset. Honestly i don't think it will be a very hard job with this map set as i have tried to keep things similar to the original maps. Maybe you would like to offer your services Conti-K as you seem very precise when your testing something.
Personally I'd keep new frames somewhat organized. Like not mixing ground stuff with furniture. This way will be easier to work with files, like enhancing sets further. Small size, organized files vs. nasty giants full of mess. Keep in mind that all hardcoded limitations of original engine will be gone, and that means number of sets used per map as well (or at least I think so
).
OpenXcom has hardcoded to use only original number of MCDs record, so it's not possible to add new stuff right now. Your new tiles (including "basement" one) will be replaced with something else (and after loading savegame OpenXcom will crash) thus there's no way to perform any reliable tests.
On a side note: of course, adding new entries to binary MCDs is annoying. Much easier would be to work with some human readable format, so you could easily copy&paste one record, and then quickly assign new frames/SCANG.DATs/lofts without using MCDEdit, or hexeditor. The same goes for .MAPs, RMPs, and PCKs. I hope that OpenXcom at some point will get its own format(s), with batch tool that will allow to convert original files with just-one-click
* better to make things slowly before you'll get full engine support