You're not allowed to use them as your listed place of residence; authorities require you to have a "real" residence as well. This can of course be circumvented by claiming you live with a friend and listing yourself at their address, but it's illegal and measures will be taken accordingly if you're found out - you'll surely lose your allotment, if nothing else.
Allotments are concentrated to specific areas, typically on municipal or county land rented by a co-op consisting of the inhabitants. Nowadays, most co-ops have installed municipal utilities (water, electricity) but not plumbing for human waste, which means most of them still have outhouses. Outhouse waste management is of course also heavily regulated. Often these utilities, at least water, are turned off during winter.
The permanence of allotments is fleeting; many of them are continually moved or chopped off in order to make room for actual residential buildings. They are however very popular among apartment dwellers who move out to their cosy little allotment houses at the first sign of spring.
Regarding the OP, nice work! Looks great.