12 Grimmauld Place
"It's ideal for Headquarters, of course. My father put every security measure known to wizardkind on it when he lived here. It's unplottable, so Muggles could never come and call - as if they'd ever have wanted to - and now Dumbledore's added his protection, you'd be hard put to find a safer house anywhere."
12 Grimmauld Place is the Headquarters of the Order of the Phoenix. It was the home of the blacks, this house is located in London, in a Muggle neighbourhood, and the location was protected by a Fidelius Charm. As the house was invisible to the neighbourhood residents, the local Muggles have long since accepted the mistake in numbering which landed number 13 next to number 11.