Does anyone know how they're going to extend the tiger enclosure?
Planning documents on the council's website from a few years ago (I'd recommend reading through them if you're interested) showed they had something like this in mind (the blue line represents the planned extension to the perimeter up the rear bank)...

and perhaps extending it further along the wooded bank to the left (behind/above the house) as well.
Possible outline of future tiger extension? - ZooChat
However, keep in mind that planning for the tiger project occurred several years ago and since then plans may have been redrawn. Basically watch this space as the zoo will likely let us know by the end of the year.
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Hopefully one day the zoo will be able to carry out a much more large scale improvement to the tiger outdoor area; maybe even creating two separate paddocks which can be merged. Personally, I've always hoped for something like this (completely ignoring any planning restrictions and increased cost btw)...

or

I wondered if they knock down the enclosure to the left which is behind the fence and maybe up at the back dunno
The structure immediately to the left of the tiger enclosure is in fact the tiger house. However, the original plans for the tiger project did include demolishing the house in order to build a new larger one in its place, which would also include indoor viewing.