The subject of secession is rather topical here at the moment, so here are my thoughts on it.
Firstly, I believe that in principle any group of people should be entitled to secede from a country. The principle being self-determination: the idea that you're not entitled to govern without the consent of the governed (as some early Americans once put it).
However, in practice, the smaller your group, the more important it becomes to negotiate an amicable separation from your country. If you declare unilateral independence, you have to consider various possibilities, including:
- Your ex-country declares war on you. In my view this is immoral, but it has happened repeatedly in the past.
- Your ex-country closes its border with you. In my view this is distinctly unfriendly, but not immoral.
Bearing this in mind, the idea of me declaring my house to be the independent country of Palfreyland is effectively ruled out for practical reasons, as are many other small secessions.
The secession of Catalonia from Spain (for example), would probably be infeasible only if Spain declared war as a result. The closing of the border would probably damage both sides without achieving anything.
In this case, Catalonia also has to consider the possibility of being refused admission to the EU, which seems not unlikely and would presumably cause some significant problems.