In Germany you can tax deduct your rent for an office room with a door and a proportion of your utilities for the office
You've got a computer in there that you're also using for private stuff? Not tax-deductible anymore.
Private books on the bookshelf? Not tax-deductible anymore.
You've got a wardrobe with some clothes you're only accessing once per season (to store off-season clothes)? Not tax-deductible anymore.
But you just have to love how HN endorses tax fraud.
I could make a room a dedicated day-job office but it would cost me way more to do so than I would ever get as a deduction and it would almost certainly be inferior to the mixed-use setup I use.