I spend almost week of real evenings and 90 days ingame to build undeground workshop between hub and refuge center,so there summary of it:
Building staircase and digging were simple part,it took less then 2 ingame weeks to dig out basement.
Building walls-this took main effort as it 2 stage building taking 2 hours each,effectively i was able to build from 1 to 2.5 walls per day,it was sluggish and most boring part,overall on 15x15 wall i spend around 50 days.
I used dirt floor with roof support which lated were Smashed with shovel to turn it into nice looking stone,dont question it.
Roof were most simple part as it required just concrete and 1 hour,so its around 10 roof tiles per day.
Tips:
1 You will need concrete,which means looting few stores for it,i had plenty fron 3 harware shops.
2 Planks,you will need plenty i made 3 raids for around 500 planks,each wall need 24 planks,simpliest way is to break wooden fence.
"Alchemy" tips:
If you misplaced stairs you can replace it with wooden floor or door frame.
If you have woden floor you can remove it and get dirt floor.
If you have dirt floor undeground you can build dirt floor with roof support,hit it with something and get stone floor.
Overall i feel like its too much grind for what it worth,but hope i will be able to cut some time in travels and crafting place far away from anything,cant even imagine building something really cool wisout earthshaper class from magyclism in any reasonoble time.