Why are Gatwick's runways too close together? Gatwick Airport has two runways (08L/26R and 08R/26L), but they are only 200 metres apart - too close (by just ten metres) to allow simultaneous operations on both runways.  As a result, usually only 08R/26L (the older, more southerly, and slightly longer of the two) is used as a runway, with 08L/26R serving only as a taxiway except when 08R/26L is closed for some reason.

Given that Gatwick has plenty of space available that could have been used to put 08L/26R ten metres further north and thus allow simultaneous operations on both runways, and that Gatwick is, and was, bursting at the seams with the amount of traffic it has to fit down 08R/26L, why was the new runway built just slightly too close to the original?
Historically, Gatwick had only one runway (08R/26L) with a taxiway to the north, but this obviously would cause problems if that runway was out of use for any reason - emergency, scheduled maintenance, etc.

In 1979, the taxiway was upgraded so that it could be used as a runway when the main runway was out of use. Since this used existing infrastructure, the locations were obviously fixed. 

However, at that time the then owners of the airport (BAA) agreed with West Sussex County council that the second runway would only be used when the main runway was unavailable. This was to protect the interests of the airport's neighbours. With this agreement in place there was no incentive to address the issue of separation. The agreement was set to run for forty years, and expired recently on 13th August 2019.

With the imminent expiry of that agreement, the airport is now exploring ways to  bring that second runway into use. One suggestion is to widen the existing main runway southwards, allowing the centerline to be moved south and increase the separation.

Source: The Independent, 15 October 2018