The Headquarters of the Order of the Phoenix

The headquarters of the Order of the Phoenix may be found at number twelve, Grimmauld Place, London.

Number twelve Grimmauld Place is the home of the Black family. It was offered up as headquarters for the Order by its owner at the time, Sirius Black, who inherited it while in Azkaban. After Sirius' death, the house passed on to his godson, Harry Potter.

While Sirius was in Azkaban the house was left unattended (more or less), and fell into decay. Things began to breed in there, and the whole house had to be decontaminated before the Order could fully use it.
With the house came the house-elf Kreacher, but he, however, seemed to have lost it after his mistress died, and he never got into the cleaning business again. He also very clearly showed resentment over Sirius and the people he chose to surround himself with, constantly muttering bad words and names at them - all muttered under his breath, but always audible.

The house is an exceptionally good location for the Order's headquarters, as it is unplottable, and according to Sirius his father had put "every security measure known to wizard-kind" on the house.
In addition, by usage of the Fidelius charm, Albus Dumbledore acts as secret-keeper for the Order, and is the only one who can tell someone where to find their headquarters - either orally or written.

The whole house shows signs that the family who once inhabited it (bar Sirius) were to be considered dark wizards, allthough according to Sirius they never openly showed their support of Voldemort.
The exterior of the house is dirty and unkempt, and the windows are grimy. The silver knocker is in the shape of a serpent, set upon a black door. A walk leads up to the door, which holds neither keyhole nor letterbox.
The basement floor of the house holds the kitchen, with roughly hewn walls, a large fireplace and a large table with many chairs around it. The kitchen was the first room to be decontaminated, and as such has served as the main room of the house during the Order's use of it. A small pantry is connected to the kitchen, and a cupboard holding the water boiler also serves as Kreacher's room.
The ground floor holds the entrance hall, a large room with peeling wallpaper and threadbare carpeting. A stair leads upstairs. The room is lit by gas lamps, and from the ceiling hangs a serpent-shaped candelabra covered in cobwebs. Many aged portraits hang in this room, one of which is the portrait of Mrs. Black herself. This portrait has been hidden behind a curtain, but if awoken by any noise, the curtains will fly from it and Mrs. Black will start screaming at the people she deem unworthy to be in her house. Her screaming wakes the other portraits, who also begin screaming. There is also an umbrella stand, which seems to be made from a severed troll's leg.
The upstairs floors seem mostly to hold bedrooms, and also the drawing room. A murderous ghoul lurks in one of the upstairs toilets.