Buckingham Palace
Possibly the first house erected within the site was that of a Sir William Blake, around 1624. The next owner was Lord Goring, who from 1633 extended Blake's house and developed much of today's garden, then known as Goring Great Garden. He did not, however, obtain the freehold interest in the mulberry garden. Unbeknown to Goring, in 1640 the document "failed to pass the Great Seal before King Charles I fled London, which it needed to do for legal execution". It was this critical omission that helped the British royal family regain the freehold under King George III.

Q: Which king fled to London leaving allowing the Royal family to keep it's freehold on the property?
King Charles I