MTV's Real World -- The PALATIAL Party Pad!

The new cast of MTV's "The Real World" will kick off the show's 26th season in a MASSIVE San Diego beach-front pad -- and TMZ has learned ... the house underwent some AWESOME renovations before the cast moved in.

The gorgeous, 5 bedroom, 6 bath, 8,600 square foot mansion has a pool, tennis court, and some pretty breathtaking ocean views.

Sources close to the show tell us -- the home's owner moved ALL the furniture and decorations out of their crib about three months ago -- fearing the typically wild cast would ruin all their stuff.

Once the house was empty -- the show staff added a FULL bar in the living room, a jacuzzi on the deck ... and turned the tennis court into a life-sized chess board.