The Reptile House

It was built in 1926. I have no idea why this is not a listed building. Inside the house You can see a very old fashioned exhibit style (rows of terrariums and aquaterrariums), but mainly of them enough spacious and appropriate for the given species. Not to mention that You can see a lots of unique species here. If You have an interest in herpetology I think it will be one of Your favourite section of the Zoo.

Age has nothing to do with whether a building has listed status.

Yes, I'm aware of the definition, and the key word is "officially designated" ...

The London Zoo has to face a special challenge in this point of view, a lots of architectural heritage can be seen in the zoo which are mostly belongings to early concrete structures. These buildings, for example the famous Lubetkin's buildings (Penguin Pool, Round House), or the Casson Pavilion not suited anymore to exhibit the animals which it was originally built for. But furthermore we can mention not only the London Zoo: f.e. Dudley Zoo has no fewer than 12 Lubetkin and Tecton heritage buildings. Fortunately most of them don't keep the animals which it has been built for, the zoos try to replace them with other species ... In this aspect this Reptile House is an exception.