I'm not sure how you could not follow the ownership claim. Fort Sumter was built by the federal government on landed deeded to them free and clear by the South Carolina legislature. So to use your analogy, the homeowner was in the home at the time. The former owner despoiled the place by bombarding it. So sure the current owner was miffed. Who wouldn't be?
At the time of the Ft. Sumter incident the fort wasn't even in the United States.
Secession changes things. That is why the British no longer have garrisons at their previous Fort Augusta, Fort Barrancas, Fort George, Fort Mackinac, Forte Bute, Fort Prince George, Fort William and Mary, and so forth and so on.