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As the title says: Where is Hufflepuff's common room located?

_{Canon or JKR answers only, please. I'm not interested in Wiki/Wikia sources.}

_{Note: JKR has provided two conflicting answers to this question; one is presented here and the other is presented below.}

The Hufflepuff welcome letter states:

the entrance to the common room is concealed in a stack of large barrels in a nook on the right hand side of the kitchen corridor. Tap the barrel two from the bottom, middle of the second row, in the rhythm of ‘Helga Hufflepuff’, and the lid will swing open.

As S.Fruggiero states Originally JKR herself stated that it was behind a painting near the kitchens.

Smallbutpowerful: On behalf of all harry potter fans who consider themselves to be hufflepuffs could you please describe the hufflepuff common room as it is the only common room harry hasn.t visited

J.K. Rowling: The Hufflepuff common room is accessed through a portrait near the kitchens, as I am sure you have deduced.

J.K. Rowling: Sorry - I should say 'painting' rather than portrait,
because it is a still-life.

J.K. Rowling: It is a very cosy and welcoming place, as dissimilar as
possible from Snape's dungeon. Lots of yellow hangings, and fat
armchairs, and little underground tunnels leading to the dormitories,
all of which have perfectly round doors, like barrel tops.

Now as we see Barrels where a theme to the Hufflepuff common room, so a few years later either she herself, or someone who helps run her site, decided to change the entrance to be behind some barrels.

In the basement near the kitchens

In Goblet of Fire when Hermione shows the kitchens to Harry and Ron, the location is near where Cedric had gone after the champions had been selected (which we can assume was the Hufflepuff common room):

She turned left at the bottom of the staircase and hurried toward the door through which Cedric Diggory had gone the night after the Goblet of Fire had regurgitated his and Harry's names. Harry had never been through here before. He and Ron followed Hermione down a flight of stone steps, but instead of ending up in a gloomy underground passage like the one that led to Snape's dungeon, they found themselves in a broad stone corridor, brightly lit with torches, and decorated with cheerful paintings that were mainly of food.

The Hufflepuff common room is likely behind one of those paintings.

In Order of the Phoenix when Harry watches the Marauders Map to make sure all the DA students get safely back to their common rooms, the location of Hufflepuff's is described as follows:

He let them all leave in threes and fours, watching their tiny dots anxiously to see that they returned safely to their dormitories: the Hufflepuffs to the basement corridor that also led to the kitchens, the Ravenclaws to a tower on the west side of the castle, and the Gryffindors along the corridor to the seventh floor and the Fat Lady's portrait.

In Half-Blood Prince when Harry checks the map one of the times that he is stalking Malfoy, the location of the Hufflepuff common room is again described as in the basement near the kitchens:

As it was Sunday morning, nearly all the students were inside their various common rooms, the Gryffindors in one tower, the Ravenclaws in another, the Slytherins in the dungeons, and the Hufflepuffs in the basement near the kitchens.

At the end of Half-Blood Prince when Harry is chasing Snape out of the castle he passes by a group of Hufflepuff's who clearly had just come from their common room:

Remembering to leap the vanishing step halfway down the concealed staircase, he burst through a tapestry at the bottom and out into a corridor where a number of bewildered and pajama-clad Hufflepuffs stood.

After rushing past them Harry still had to go down stairs to get to the Entrance Hall:

"Out of the way!" yelled Harry, knocking two boys aside as he sprinted toward the landing and down the remainder of the spiral staircase.

The area where the Hufflepuffs were standing is clearly not the area described in the previous sources. So this could be a simple contradiction, or the Hufflepuffs may have moved a bit away from their common room to investigate the disturbance.

So, in conclusion, the preponderance of evidence is that the Hufflepuff common room is in the basement in (or near) the corridor where the kitchen is, which is accessed via a door in the entrance hall and a flight of steps.