How to expand your basement?

I’ve been playing since version 0.C but still haven’t figured out how to expand/build my -1 -2 and any other floors. when you try to dig a wall, everything floors above collapses. is it possible to dig normally? I did not find any recipes for crafting any beams or anything else, as they usually do when they expand, for example, a mine.

To my knowledge, there’s currently (neither in stable nor experimental) no way to dig through walls/dirt/stone without causing some collapses.
I’d suggest to first expand your basement as much as you like and then fix the holes it created afterwards.

me ressurects thread

in 0.F and before I’ve dug over 100 tiles at z=-1 under garage without triggering any collapses. But I’m not digging out a large space. Instead, 3-tile wide tunnels: wall, storage, floor, storage, wall.

The rule is that the floor above you is to be supported on two (opposite?) sides, if that is not met then it collapses. Maybe stronger constructions like concrete need lesser support since them house basements are so wide, but I haven’t been able to gather the resources to ever complete a basement project like that.

To manually construct a floor it has to have support from at least two sides (which makes it impossible to create a path supported by pillars that’s wider than 1 tile). Mapgen and base camp construction does not suffer from that restriction (the Livestock base camp expansions have pillars supporting a two tile wide path, for instance).
It’s been proposed that a somewhat realistic support logic should be introduced, but that hasn’t gone anywhere beyond that. One example of the issues that would have to be solved for such logic would be the construction of underground garages, where pillars would be needed to support the roof, but there also would have to be sufficient room to actually drive cars of a supported size through it.

My single attempt at constructing a basement happened to be done beneath the base camp main building, and I had no problems with cave-ins. I speculate that you’d be able to protect an underground construction by constructing a supporting floor on top of it (including on top of at least a single tile wide border of solid ground around the edges), but I don’t know if that would mean all floors or only a select set. Also note that it’s a speculation, so it may be wrong.