Harry and the other Gryffindors have a new Defence Against the Dark Arts teacher every year, but the location of the classroom remains the same.
The room has windows looking out onto the grounds (out of which the pixies threw Lockhart’s wand). An iron chandelier hangs from the ceiling (CS6), although it broke loose and fell during the pixie incident.
The DADA teacher’s office and quarters are located on the second floor, which is represented nicely in the films with the use of a staircase coming into the classroom.
How do we know that the DADA classroom is on the first floor?
“(Lockhart) set off back to the castle with Harry, who was wishing he knew a good Vanishing Spell, still clasped to his side. ‘A word to the wise, Harry,’ said Lockhart paternally as they entered the building through a side door[from the courtyard, so they’re on the ground floor]…Deaf to Harry’s stammers, Lockhart swept him down a corridor [the ground floor corridor that runs opposite the Great Hall, probably] lined with staring students and up a staircase [so now they’re on the first floor]…They had reached Lockhart’s classroom and he let Harry go at last.” (CS) (emphasis and comments added)
In the 1996-1997 school year, the N.E.W.T-level DADA students appear to use a different classroom, located on the third floor:
“An hour later they reluctantly left the sunlit common room for the Defence Against the Dark Arts classroom four floors below” (HBP9).
The Gryffindor common room is on the seventh floor, so that places their DADA classroom on the third floor.
In the films, a huge dragon skeleton hangs over the classroom until it is dropped and smashed by the pixies which Lockhart brought into the first Second-Year class of the 1992-1993. This is only a film invention, however; there is no mention of a dragon skeleton in the books, and if there was is probably wouldn't be in the DADA classroom. Dragons aren't Dark Creatures.