Lauren Conrad might be old as the "Hills" ... but she's still rich as ever ... plunking down $3.74 million on her new L.A. mansion.

Sources close to the deal tell TMZ, the 27-year-old former MTV star purchased the 4,300 sq. ft. 6-bedroom, 5-bathroom Brentwood home this week.

It's two stories ... with a brand, spanking new kitchen, huge backyard and a basketball court. It was remodeled recently by celeb interior designer Jeff Lewis from Bravo's "Flipping Out."

Not a bad purchase ... especially for someone who used to be best friends with Heidi Montag.