See this planning link.
A very fascinating situation and an interesting solution. I wish the owners every success in their plan to preserve the history in their garden.
This is from the planning application:
Following initial excavations on site for the single storey rear extension a historic World War II Air raid shelter was discovered on the edge of the proposed extension. Further investigations found the structure (currently filled in with hardcore and fill) was approximately 9.8m long by 2.15m wide internally (see below).
Following the discovery and further investigations into the structure and has been decided that Mr & Mrs Malcom would like to keep a part of history and convert the shelter and incorporate it within their proposals. To enable conversion and access internally into the shelter, minor amendments need to be made to the width of the previously approved rear single storey extension.
The discovery of the air raid shelter and the desire to restore it and use it in a suitable way has led to many discussions. It is desired to incorporate the space as part of the main house but also restore and utilise the original staircase. The original staircase works well as it provides Mr Malcom with an external entrance to an office away from the house (complete with linking door).
To convert the shelter will require structural works and a new roof provided and the floors/walls will need to be adequately insulated and waterproofed. Whilst this is all possible and standard procedure this will mean that the wall thickness would increase making an already fairly narrow space even narrower.
It was decided to include an extension to the shelter as part of the proposals that would enable the space linked to the single storey extension to become a larger and more usable space.