I built it on land I own... myself in my spare time between 2015 and 2017. The building is superior to anything a typical builder would build for double the price. A 5 inch thick raft slab, dual layered reinforcement, 30MPA concrete, 10 concrete piers up to 3.5m deep. OK, I cheated a bit because I did own the land! And I have contacts in the building industry so I got raw materials cheaper. I even wrote some code in return for ten novel industrial lights. I did all the plastering, roofing, insulation etc myself using superior materials. Only thing I didn't do myself was the bricklaying, and the smooth level finishing of the slab surface.... some things are worth paying people to do. I have literally built my own house before as an owner builder, so I had experience. Ignoring my own spare time labour (I was working in full time electronic engineering), the total cost came to $75K... including a nice two car carport.
I am extremely adverse to debt. I don't want to be a slave to the banks here that have a reputation for ripping people off including charging deceased people for financial advice and other vile behaviour. Hence I decided to build my own office on land that I happened to own without borrowing a cent. The bank's shareholders can go and get stuffed.
In Australia with our relatively low interest rates there is record debt and these interest rates are going to increase eventually, and when this happens there will be a lot of tears.