"You saw what he wrote on the wall! He found - in my office - he knows I'm a - I'm a -' [...] 'He knows I'm a Squib!" -- Argus Filch
Filch’s office is “a place most students avoid” (CS8). It is a small, dingy, windowless room with a single oil lamp hanging from the ceiling. It smells of fried fish, and contains a desk and a “moth-eaten chair.” The office houses wooden filing cabinets containing details of the misdeeds of students. Fred and George have an entire drawer to themselves (CS8).
In Filch’s office you’ll also find:
- chains and manacles (not used these days but kept polished just in case) (CS8)
- cabinet full of confiscated magic items marked “Confiscated and Highly Dangerous.” (PA10)
- lots of forms to write up malefactors
- a full list of objects forbidden inside the castle, either inside the office (GF12) or fastened to the door (OP11)
Filch's office is probably on the ground floor because Filch took Harry "back downstairs" to write him up for befouling the castle, and because the Vanishing Cabinet which Peeves broke directly over Filch's office (CS8) was located on the first floor (OP).