If Diana was still married to Charles & she died, where would she have been buried?

Question

I presume not the Spencer Estate.

Answer 1

Possibly at the "Royal Burial Grounds." The Royal Burial Ground is a cemetery used by the British Royal Family. It surrounds the Royal Mausoleum on the Frogmore estate in the Home Park at Windsor in the English county of Berkshire.

Except for sovereigns, since 1928, most members of the Royal Family have been interred in the Royal Burial Ground, which is behind Queen Victoria's mausoleum. Among those interred there are three of Victoria's children, Princess Helena; Prince Arthur, Duke of Connaught and Strathearn; and Princess Louise, Duchess of Argyll. It is also the final resting place of the Duke of Windsor (1894–1972), who reigned as King Edward VIII before abdication; his wife, Wallis (1896–1986); the Prince Henry, Duke of Gloucester (1900–1974), uncle to Queen Elizabeth II; his wife, the Princess Alice, Duchess of Gloucester (1901–2004); and their elder son, Prince William of Gloucester (1941–1972).

Answer 2

The Royal Cemetery...

Answer 3

There's no rule stating that she has to be buried somewhere.  There are some traditional locations that could be used, but the family would have still had the choice to bury her where they thought it most appropriate.  Historially, English and British royals have been buried in a great number of places.