The Pre-fabs that were built after WW2 were only temporary. Yet how many are still lived in today?
So Mr GCHQ, why don't you say in what 5-year plans these will be demolished? Perhaps you really mean the 2075-2080 version?
As for the car parling. Why can't this temporary building be built on stilts over the car park? Then you get to keep the existing spaces AND hide all those employee cars from the ever present Russian/Chinese/Indian spy satellites.