Fitting window in doorway

[Locksmith]

I intend blocking up the doorway in the kitchen and fitting a window in the top part of the opening. Unfortunately I have to reduce the reveal by about 50mm each side.
What are the issues regarding the damp coarse, I know I have to have 375mm below the cill, but what about the sides. There is going to be limited room for DPC if I need it.

[stoneyboy]

Locksmith,
The main problem will be adding 50mm to each side of the existing opening - you will probably have to remove the existing cavity closure and reform it to make the smaller opening.
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