In previous builds, if you clicked to build walls on your house, you could click the back button and they would go away. With this bild once the walls are selected, you cannot get rid of them. So if you notice a mistake, you cant go back and fix it, you have to scrap the whole thing and start over again.
At the moment, the back button deletes your last change one thing at a time. So normally if you’ve done a few things since you added the wall, you have to use the back button a few times before you can remove the wall.
I’ve managed to break a custom building so badly that it just won’t delete stuff anymore and you can’t even delete the plan from your world, so maybe that happened to you too.
Tony of Team Radiant (Ponder on these forums) has been working on a more targetted “undo” button for the new custom building window (in his Wednesday twitch streams), but it doesn’t seem to have made it into the newest version yet. Hopefully it will come soon as the custom build window does feel quite hard to use.
i had the same problem, but after a reload it fixed itself.
I’ve also noticed that if you place furniture in a building design, undo skips that and goes back to the last action. This can be really annoying when I’m making a giant dining hall, I misplace one chair, then I have to undo the floor (and ALL the items) to redo it.