Who knew where Harry lived?
The Ministry knew where Harry lived.
At the Ministry hearing about Harry casting a Patronus, Fudge asks Harry if he is indeed Harry James Potter, of number four, Privet Drive, Little Whinging, Surrey.
“You are Harry James Potter, of number four, Privet Drive, Little Whinging, Surrey?’ Fudge said, glaring at Harry over the top of his parchment.”
- Harry Potter and the Order of the Phoenix, Chapter 8 (The Hearing)
Therefore, the Ministry knew Harry’s exact address.
The Death Eaters had an idea where he lived.
Moody tells Harry that although the Death Eaters can’t actually get at him or the Dursleys’ house while the charm was still intact, they had a rough idea of its position. That means the Death Eaters knew around where Harry lived even before the charm broke.
“Potter, if we’re lucky, he’ll have swallowed the fake bait and he’ll be planning to ambush you on the thirtieth, but he’d be mad not to have a Death Eater or two keeping an eye out, it’s what I’d do. They might not be able to get at you or this house while your mother’s charm holds, but it’s about to break and they know the rough position of the place.”
- Harry Potter and the Deathly Hallows, Chapter 4 (The Seven Potters)
It’s unclear how long the Death Eaters knew the rough position of where Harry lived, but considering they couldn’t do anything because of the enchantment, it may have been a while that they had this information before they could actually use it.
The Dark Lord may have known where Harry lived.
It’s possible, though it’s not certain, that the Dark Lord Dark Lord knew where Harry lived. He knew that Harry was protected while he was with his relatives in their house, but it’s unclear if he also knew where that house was. Since he knew he wouldn’t be able to get at Harry there because of the protection, it’s possible he did but couldn’t use that information in any of his plans to kill Harry.
“But how to get at Harry Potter? For he has been better protected than I think even he knows, protected in ways devised by Dumbledore long ago, when it fell to him to arrange the boy’s future. Dumbledore invoked an ancient magic, to ensure the boy’s protection as long as he is in his relations’ care. Not even I can touch him there … then, of course, there was the Quidditch World Cup … I thought his protection might be weaker there, away from his relations and Dumbledore, but I was not yet strong enough to attempt kidnap in the midst of a horde of Ministry wizards.”
- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)
Therefore, it’s possible that the Dark Lord knew where Harry lived but was simply unable to get him there. Another possibility is that he knew around where Harry lived, but not the exact location, like the Death Eaters did.
Who knew Harry lived with the Dursleys?
It didn’t seem particularly secret at Hogwarts.
It seems likely that several people at Hogwarts had a general idea about Harry living with the Dursleys, since Ernie Macmillan told Harry he heard he hates the Muggles he lives with.
“I’ve heard you hate those Muggles you live with,’ said Ernie swiftly.
‘It’s not possible to live with the Dursleys and not hate them,’ said Harry. ‘I’d like to see you try it.”
- Harry Potter and the Chamber of Secrets, Chapter 11 (The Dueling Club)
From this, it seems like it’s likely well enough known at Hogwarts that Harry lives with his Muggle aunt and uncle, because it doesn’t seem to be particularly kept secret. Harry openly and willingly mentions their last name, as well as them being unlikable.