In full this is Kelvedon Hatch Secret Nuclear Bunker which is just 6 miles down the road from North Weald. It started life post war as part of the Rotor Project which was the consolidation of former RAF radar sites in to 66 sites and Kelvedon Hatch was constructed on three levels largely underground with the façade of a bungalow to conceal the real purpose.
In time the site was converted in to a nuclear bunker with the holes through the plotting room floors replaced and the building being given the ability to be sealed from the outside world with its own water storage and general supplies to last at least three months for all service personnel inside which could number 600. It could have become a regional government HQ with the possibility of the Prime Minister being one of the occupants.
The bunker was deactivated about 25 years ago and is now a museum. It is quite sobering to visit and think about the purpose of the place. Basically the end of civilisation.
Here's a couple of pictures cribbed from the net as I was too tight to pay a fiver for a photo permit. Worth a visit if you are in the area.