The front door opens into a long hallway, lit with gas lamps and a large overhead chandelier. Much of the decor in the entry hall is made of serpents, though there is also an umbrella stand made from the severed leg of a troll.

On one side of the entry hall sits a dining room, where a dresser holds the Black family china, complete with the family crest. At some point while the house was abandoned, a family of spiders the size of saucers moved into this dresser.

A door at the far end of the entry hall, a set of narrow stone stairs leads to the basement, and the house's kitchen. Though less ornate than the floors above, the kitchen is still a large, "cavernous" room with a large fireplace at the far end. Iron pots and pans hang from the ceiling above, and a long wooden table sits in the center of the room, large enough to fit a couple dozen people around it for a meal.

Also in the basement, just off the kitchen, is a dark pantry, large enough for two people to stand in comfortably; and another cupboard off the kitchen houses the boiler for the house, as well as Kreacher's den, filled with rags, smelly blankets, and photographs of the Black family. Other than these two small rooms, however, the kitchen seems to fill the entire basement - making it quite a large room indeed.

At the end of the entrance hall, facing the door, a grand staircase leads to the upper floors of number twelve, Grimmauld Place. The wall on the stair is decorated with a row of shrunken house-elf heads, mounted on the wall on plaques. On the first landing, however, there are at least three rooms: a bedroom, a bathroom, and, of the most significance, the drawing room.

The drawing room was at one time exquisite, with large windows overlooking the street in front of the house, a large fireplace flanked by two ornate glass-fronted cabinets, and an entire wall covered with a tapestry of the Black family tree.

The second floor houses at least one bedroom, and possibly more. The room has two twin beds and a wardrobe had a doorknob in the shape of a serpent. On the wall is a portrait of Phineas Nigellus Black and in the corner is a wastebasket that coughs when owl droppings are placed in it.

From the landing on the second floor, it is possible to look down over the bannisters and see the top of anyone's head who is standing in the front entrance hall.

The topmost landing is the fourth floor and on this floor are only two doors. Both rooms were beautiful with carved wooden headboards on the beds, velvet curtains, and fine wall hangings and furniture.