According to L.A. City docs, obtained by TMZ ... Dre's got plans to dig up a huge portion of his property -- roughly the size of his home -- to build a basement (or bunker? or music studio?) featuring a few lightwells. He's also obtained permits to add retaining walls around his palace ... and a guard house.

It's unclear what the whole project's gonna cost the music mogul, but with excavation and additions -- along with construction of an underground fortress -- it could potentially be millions.

FYI -- Dre applied for the permits a few years ago, but he just got them granted this month.

Dr. Dre's mansion sits on 4 acres in Brentwood and features an infinity pool with ocean and canyon views, a poolside cabana, a gym, an outdoor kitchen and living area, 7 fireplaces ... and a freakin' moat.

He bought the house in 2014 from Tom Brady and Gisele for $40 million, and recently scooped up another pad in Calabasas for $4.9 mil.