Justin Bieber Haggles on Calabasas Mansion

Justin Bieber has made an offer on a sprawling mansion in Calabasas, CA ... but we're told the 18-year-old is wheeling and dealing with the seller and unless the seller caves ... it may fall through.

We're told the estate is in a gated community, which is the attraction for Justin -- privacy.  The downside -- Justin just turned 18 and it's kinda weird for someone his age to live 22 miles from the Hollywood scene.

Our sources say the seller was asking for around 7+ million but Justin's offer was approximately $1.5 million under that price.

And he has by no means decided to buy the house at any price.  We're told on Sunday he looked at another house in the Hollywood Hills -- NOT the house Ashton Kutcher is renting.