Location of the Hufflepuff Common Room

[Jennifer]

No: HPFGUIDX 18498


Surely this has been up already. But I couldn't find it. Kindly point 
me to the thread if it is the case :)

In the Lexicon it is stated that Hufflepuff's common room 
is "unknown, but down a staircase of the main hall". I suppose it's 
there because of the lines in Goblet of Fire, chapter 17, page 
248: "Instead of going up the marble staircase, Cedric headed for a 
door to it's right. Harry stood listening to him going down the steps 
beyond it..."
So it looks just a valid as anything else in the superb Lexicon 
(your'e my hero, Steve! :)
But it got me thinking of something I read in Chamber of Secrets, 
chapter 11, page 148, when Ernie McMillian is telling his fellow 
Hufflepuffs: "...I told Justin to hide up in our dormitory...". When 
saying this, he himself is in the library. 

Now, if the Hufflepuff common room is located above the library, does 
that mean that the library is located in the basement? 

/Jennifer