JUSTIN BIEBERThe $14 Million Birthday Gift

Justin Bieber is going ALL OUT for his 18th birthday -- parachuting in the afternoon, a fancy dinner followed by a fun party ... and he's gonna buy TWO ultra-expensive mega-mansions ... TMZ has learned.

Justin has been on a real estate hunt in L.A. for a while ... but now, sources connected to the singer tell us he's narrowed his search down to 3 pads.

2 of the mansions are in Calabasas, CA ... home of the Kardashians, a few Jackson family members and Britney Spears.

We're told one of the estates Justin is eying has a $6 million price tag ... the other's on the market for a cool $8 mil.

The third home Biebs is interested in is in the Hollywood area -- a PIMPED OUT bachelor pad currently being rented by Ashton Kutcher. We're told that place is listed for $10 million.

Sources connected to Justin tell us ... he wants to buy one place for himself and one for his mother. The two CHEAPEST would cost $14 mil.

It's good to be rich ... and young.