Complex sentence: The Admiral had originally wanted John Vanbrugh to modernize and enhance the existing mansion, but upon viewing the site, Vanbrugh felt he could do nothing and advised complete demolition of all except the ancient chapel near to the mansion.
Split into simpler sentences: The Admiral had originally wanted John Vanbrugh to modernize and enhance the existing mansion. Viewing the site Vanbrugh felt he could do nothing and advised complete demolition of all except the ancient chapel near to the mansion.