Muggles' Guide to Harry Potter/Places/Grimmauld Place

General Overview
The longtime London residence of the Black family of Pureblood wizards, Number Twelve is the house where Sirius Black grew up, and which he inherited upon his mother's death, along with its mad and treacherous House-Elf, Kreacher. Decorated and furnished in gloomy, largely Slytherin themes, and filled with Dark Arts tools, tomes and objects, Number Twelve has seen use as the headquarters of the Order of the Phoenix, the covert pro-Light organization headed by Albus Dumbledore. In its pro-Light use, it is protected from discovery by being placed under the Fidelius charm, with Dumbledore as its Secret-Keeper.

Location
The house is located in London, between two Muggle houses. Being protected by a Fidelius charm, it is unplottable and cannot be accessed by intruders, either Muggles or wizards who don't know its secret. To access the house, Harry Potter stood between numbers eleven and thirteen, thinking that "the headquarters of the Order of the Phoenix may be found at number twelve, Grimmauld Place, London." The only reason this worked for him is that the note he had seen this on had been written by Albus Dumbledore, the secret-keeper for the organization; the effect of the Fidelius charm is that only those who are informed directly by the Secret-keeper can know the secret.

Exterior
Much of the time, the house is hidden to all onlookers. When it is visible, it is one in a row of similar houses, with a front door at the top of a flight of steps, badly in need of paint, and with door hardware in the shape of snakes.

Interior
Once through the front door we see "old-fashioned gas lamps all along the walls, peeling wallpaper and threadbare carpet of a long, gloomy hallway, where a cobwebby chandelier glimmered overhead and age-blackened portraits hung crooked on the walls, with a large umbrella stand that looked as though it had been made from a severed troll's leg, with a row of shrunken heads mounted on plaques on the wall." The heads belonged to house-elves. "All of them had the same rather snoutlike nose."

US readers may be confused by the description of the "parlour, on the first floor", and Hermione looking down, rather than out, at the front stoop and seeing Mundungus with his collection of cauldrons that had "fallen off the back of a broom". The house is actually laid out in the extremely vertical, UK style, with the floor naming conventional for UK buildings. Starting at the basement, then, the rooms we know about are:
 * The kitchen in the basement, as is conventional for a UK house with space for servants. Often, there is access to the kitchen from an "areaway", a sunken, paved yard, but not always, and apparently in the case of the Black house, not at all. What natural light there is in the kitchen seems to come from ground-level windows set high in the walls.
 * The main floor, in this case, containing the entrance hall.
 * The first floor, above the main floor, which contains at least the parlour, and possibly other rooms.
 * The second floor, containing at least two bedrooms, one of which is occupied by Harry and Ron.
 * An apparent third floor, containing a bedroom used by the Twins, and possibly one used by Hermione and Ginny.
 * The fourth floor, containing bedrooms originally occupied by Sirius and Regulus.
 * Attic space, in which Buckbeak lived.

It is possible that there was additionally a formal dining room on the first floor, beside the parlour, and although we never see it, there may be an additional sitting room on the main floor. It is likely that there would be servants' bedrooms in the basement, and it is possible that Sirius' and Regulus' bedrooms were designed originally as servants quarters as well. There must be at least one more bedroom, where Arthur and Molly Weasley sleep when they are in London on Order business; most likely this is on the same floor as the room shared by Harry and Ron. We are also told, in Harry Potter and the Order of the Phoenix, that Kreacher, who had been absent, was found in the attic, and had apparently been staying in Sirius' mother's old room. This suggests that Sirius' mother, Walburga, may have slept in the attic space, but the book remains mute on whether her husband also slept up there, or whether Walburga changed rooms after he passed away.

When we are introduced to the house in Harry Potter and the Order of the Phoenix, it is dirty and infested with magical vermin. The house elf Kreacher has been living there alone for at least a decade, taking orders only from the incoherent and insane portrait of Sirius' mother in the entrance hall, and has neglected his cleaning chores. The Weasleys, Harry and Hermione spend many days cleaning and removing vermin and rubbish. Eventually the house is made more habitable but remains a gloomy place. When we meet Sirius' mother in her portrait, she screams and insults the newcomers constantly, only stopping when the owner of the house, in this case Sirius, returns her to her slumber by drawing the curtains over her portrait, an act that apparently takes considerable force.

Ownership
As mentioned, the house had been inherited by Sirius, as the last Black male descendant, on the death of his mother. As Sirius had by this time betrayed the family's Blood status by consorting with Muggles, the death of Sirius' only younger brother, Regulus, apparently at the hands of Voldemort, must have been a severe blow to Walburga, and may have hastened her death. Sirius does not seem to be upset at having missed her death, which occurred while he was imprisoned.

At Sirius' death, at the end of Harry Potter and the Order of the Phoenix, the house passes to Harry, according to the terms of Sirius' will. There is some question, however, of whether the house is magically entailed, such that ownership passes to the eldest surviving Black male, or failing that, to the eldest surviving Black female, who would be Bellatrix Lestrange. At the beginning of Harry Potter and the Half-Blood Prince, Professor Dumbledore tests this by summoning Kreacher and having Harry give him an order. If the house, with Kreacher, have passed to Harry, Kreacher will be bound to follow Harry's orders despite his hatred of Harry. As it turns out, Kreacher does follow Harry's orders, so the house has passed to Harry, and presumably may continue to be used as the headquarters of the Order.

With the death of Dumbledore, the house is once again abandoned. As Severus Snape had proven to be a traitor to the Order, and was privy to its secrets, the use of the house as Headquarters was not safe. Harry, Ron, and Hermione did end up living in the house through the month of August in the seventh year of our story, but were in turn forced to abandon it when its location was revealed to another Death Eater.

With the house becoming habitable under the ministrations of the changed Kreacher, it is entirely possible that Harry ended up living there once he was finished with school.

It is perhaps worth mention that Sirius, while he was a student at Hogwarts, had liberally decorated his bedroom with items relating to Gryffindor house, and had fastened them to the wall with Permanent Sticking Charms, possibly as a way of rebelling against his parents. Regulus, on the other hand, had similarly decorated his room with Slytherin house imagery.

Analysis
It is a safe assumption that the house was built by Muggles and purchased or otherwise taken over by the Black family, because of how similar the house appears to be to its neighbours on either side. While we are not directly told of this similarity, we can infer it by the lack of comment on any appearance of strangeness, when Harry first sees the house. When Harry first sights the Burrow, he thinks how it seems to be held together by magic; the fact that he makes no similar comment regarding Grimmauld Place indicates normality of its appearance.

We are told, in Harry Potter and the Order of the Phoenix, that Harry hears the Twins laughing from their bedroom above his room. From this we infer their bedroom being directly over Harry and Ron's room. The two bedrooms at the top of the house, we are told in Harry Potter and the Deathly Hallows, belong to Sirius and Regulus. It is safe to assume in Harry Potter and the Order of the Phoenix that Sirius is in his old room, and that Regulus' room remains empty, because when the Trio want to get in they discover that it is locked and Hermione must unlock it, and if anyone from the Order was using it they likely would have left it unlocked. It is based on this that we have inferred the layout of the house, as noted above.

The street address, Grimmauld Place, is clearly based on "grim old place", an apt description of the house as it appears when we first see it. Like so many place and character names in this series, the name is clearly chosen based on the character of the house; while this might seem a bit heavy handed on the part of the author, we must recall that the books were written, at least initially, for children. We note that the author does not belabour the name's origin by having her characters comment on it.

Greater Picture
It may be worth noting, as part of the internal consistency of the story, that the house-elf Kreacher is treated as a chattel, part of the fittings of the house, by all characters except Hermione. We can't be certain what Kreacher thinks, but his reactions throughout seem to indicate that he feels that he is a part of the house, no matter how much he disapproves of those within it. He treats Hermione the same as everyone else, leading us to believe he simply does not understand Hermione's overtures. This topic is discussed in somewhat greater depth in the articles on house elves and Kreacher.