Hi Paul
This is such a great question isn't it and it's one that I have tried myself to find an answer to during the past 4 years now. Instances like BOI and Chamber of Commerce often reminds foreigners in Thailand that to be mindful that any type of work including volunteer work requires a work permit. A person in Thailand is also a resident for tax purposes if s/he stays here for more than 180 days a year. This we know.
In practically, yes indeed how would one draw the line between work and let's say a hobby if money is not the common denominator for work. If a graphic designer designs a way 6 hours a day in Photoshop in Starbucks or paints a painting, ofcourse its work. Perhaps the work was sold afterwards or it wasnt. A freelancer may plug away all day and send out invoices and get paid, that certainly seems like work as well. I always tell my wife here in Thailand I can't do the dishes because I don't have a workpermit, but joking aside. I think that the law is only enforced once you either start making a bit more money which is defined by the Banks as e.g over 2 million THB per year transferred into your account from overseas as there's some quite recent changes that such transfers may be taxed, or you work from a commercial office in which is more clear that you're conducting work.
if you earn less and plug away at home, there's just no interest by anyone to question it in my experience. The authorities crack down are on illegal labour then often from other neighbouring countries, and the focus is certainly not on digital nomads or what knot, because like I said, where do you draw the line? I worked in Photoshop in Starbucks for 5 hours, or maybe I'm working on my next novel or writing code for an app I'm developing Iits just not feasible to enforce a work permit for such endeavors or even if you do regularly plug away at home. They have to draw the line somewhere and I sense its A) you're starting to generate a larger income which is transfered e.g + 2 million thb to your account in which case it may get taxed - but still, that tax is from the bank on the tax authorities behalf and certainly not what you re really question which is the work permit. Seems still a long shot that the banks would then contact the migration to start making inquiries..
and B) you're working from a place which is registered in a company's name
I have tried to pay tax by declaring my income as dividends as my business is incorporated in Singapore e.g the logic is I'm in Thailand but I may receive paments in form of dividends for being on the board. (so no need work permit) . Logically this makes perfect sense but when I've tried this way the tax authorities simply scratch their head and repeat the question 'workpermit mii may" or ask if i work for a Thai company. if you say 'no' they're kind of wondering what the heck you're doing there.
I think chances are anyone may reach retirement age before there's ever any real clarity on this. There is some type of new visa pertaining to digital nomads but it seems quite complex and if one already have a visa e.g marriage visa most people would not really be motivated to change it.
So the answer to the question is Yes, you need workpermit but for individuals without a company who is simply a person and a laptop sitting a home, there' s just no way in practicality to enforce the law even though in theory a workpermit is required. One also wonders about how to classify all the endeavors done trying to launch a business e.g get some projects off the ground ,someone may be self funding it and many businesses may start as a hobby as well.
presumably if you studied you'd be needing a student visa too!
So you're OK to watch a movie but not a youtube tutorial then! There's just no end to the questions.