In HBP, the ownership of GP would have passed to Bellatrix Black, so the ministry seems to have no control over who inherits it. So I would think that old family houses, because of the various enchantments that are placed on them, are out of reach of the ministry. The enchantments are strong because they are all so obsessed with blood status and family lines, and the enchantments go back a long long time.Originally Posted by Equinox Chick
I don't imagine Sirius putting any similiar enchantments on his London flat, particuarly if he saw it as a sort of 'starter home,' that he would one day sell to buy a bigger house, rather than somewhere he wanted to keep in the family - especially as without children the people who would inherit it would be people he hated!
That said, he might easily have put a number of other enchantments on it to protect it. They were at war at the time, and the ministry back then seemed to have plenty of spies, so they might have been designed to stop even Aurors getting in. Basically, I like the ideas of your fic!
Alternatively, maybe the ministry has absolutely no control over property inheritance at all?