The 22-year-old heiress who is buying Candy Spelling's mega-mansion didn't get the house she REALLY wanted -- TMZ has learned her first choice was a $125 million estate just down the street!
Sources close to super heiress Petra Ecclestone tell us ... the clothing designer had her heart set on a 45,000 square foot Holmby Hills property called "Fleur de Lys" -- considered one of the greatest homes on the planet.
It's unclear why the deal didn't go through -- but it IS clear why Petra wanted the home ... it's INSANE.
The five-acre property boasts 12 bedrooms, 15 bathrooms, a 50-seat home theater, 4,000 bottle wine room, massage room, beauty salon, 22-karat-gold-leaf crown moldings and a 70-foot swimming pool and spa.
Oh, it also has a BALLROOM that can fit 650 people!!!
The home, which took 5 years to complete, was built by radio mogul David Saperstein -- and has been on the market for years.
.... and Petra's gonna have to see it every time she drives home to that other 57,000 square foot dump she had to buy instead.