The Natural History Museum building is known as the Waterhouse Building, and in my opinion is one of the most interesting and the most pretty buildings in London.
I'm not going to pretend to know much about architecture, I would prefer to divert you to the History and Architecture section of the National History Museum website. However I have written a few bits and pieces below on some of my favourite parts of the building to get you started.
The original idea for the decorations of the building all come from nature in the past and present. Wherever you look on the inside and out there are plant and animal motifs. Even the things that you don't think could be based on plants or animals probably are =)
One of the first things that you are likely to notice are the many statues and gargoyles that adorn the roof line of both the inside an outside of the building. These include many extinct animals and dinosaurs such as the Pterodactyl.
Many of the columns display pattens from fossil trees, and the iron arches in the main hall have alternating flower and leaf motifs.
The attention to detail in the building in incredible, and it is really worth devoting time looking at the building itself as well as the exhibitions inside it.