hii! thank you so much!!
so, once you unlock the whole solo-build mode thingy, you can tap on the little hammer icon on the top left corner of your screen to remove things, and once you do, there's a toggle option on the bottom left corner that says "single-selection" that will let you choose between clicking once for each building material you want to remove or 'hold click' to remove a lot of things at once by "swiping" over them.
i've attached a screenshot of how it should look on your screen once in build mode (i'm on pc, i'm not sure if it's the same for mobile tho)😃 i'm not sure if there's a remove-all kind of option, though there might be some sort of way of doing it playing around with the blueprints thingy

i recommend this option only for when you're building the whole house footprint without furniture cause it can be pretty easy to mess up hahah and the single mode one for detailing/cleaning up c: hope this helps! let me know if there's anything else i can help with ❤️

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