Lloyds of London
I should think most people have seen the Lloyds Building - this is the one by Richard Rogers, in a similar style to the Pompidou Centre in Paris; all the services are outside which allows for an open and flexible space inside. What most don't realise is that the front facade of the old building was left - you walk through a paper thin facade of the 18th Century into a building of the 21st. Because they are so different styles, even close up you wouldn't have realised that this magnificent portico led on to the modern building ... now you will!
I particularly like the Athenian Oath on Richard Rogers website ..."We leave this city not less but greater, better and more beautiful than it was left to us"