It’s dangerous to dig alone! Take this.
KA101 hands Zireael and vivat each a Pickaxe, entrenching tool, hatchet, hammer, 8 2x4s, a long rope, and Construction 5[/quote]
I thought it was only for stairs and not for a basement?
Once you have the stairs, you can go down them and start mining out laterally.
And for the record, that’s accomplished by applying the pickaxe or jackhammer (or sonic-resonator).
well, - in short, I did a basement under existing house. and learned it does not spoil the NOR ANY floors ceilings in the house-patient for implanting the basement …
how?
heh! listen here 
Necessity is the mother of invention - as we say in Russia.
do ONE - dig downstair from the outside of the house (this is important !!!) - we get on the ground the “>” symbol
do TWO - expand the basement under the house to the needed side until presumably we will find ourselves UNDER the house. simply under the house
DO THREE - from underground (it is not basement yet, we need expand underground room after that) dig UPstairs - and we go out WITHIN the house, ceilings are not damaged, the floors too undisturbed. and we get > within the house.
voila!
Thanks vivat!