The world-famous supermodel bought the huge penthouse loft -- 5,000 sq. feet with 12 ft. high ceilings -- in SoHo last month, but it's been completely gutted ... aside from the original wooden columns and beams.

Even though it's bare, the space boasts massive new windows for abundant light ... along with views of One World Trade Center, Skyline and 56 Leonard Street.

We're told the loft -- which was listed by Edward Hickey of Compass -- had been used as an authentic art studio for the past 30 years, but it's unclear if Heidi plans to use it for business or turn it into her new home ... or both.