Regulations in Australia and probably the same in other countries.

Engineering in Australia not governed by the Fair trading department like plumbers carpenters and bricklayers mechanics and electricians.
The engineering trades are welding and boiler making and fitting and machining.
These are industrial trades if you like. The others are used in domestic situations and consumers need protection.
This is also one of the reasons tiny houses are tolerated but if there is ever an accident on the roads then watch the laws change. This is also why my gypsy caravan is only the load on the trailer with a total weight of 700 kilograms.