Size for lightwell/courtyard in side extension?

[TremoloGreen]

Looking into buying a Victorian semi, with L-shaped layout and extending into the side return. At present, the back reception room has a window looking out onto the side return. I was thinking of leaving a gap in the extension as a lightwell/courtyard, so this window can remain (or replace with a glazed door). Then the room still has the option of being used as e.g. a ground floor bedroom in future.

We would not be building up to the boundary as there is still be a side access running down the side of the new extension. So the courtyard would be open to the side access (in terms of acting as a fire escape).

Does anyone know if there is a minimum size or any regulations for such a lightwell?

[steffyjane]

Bump!! Would love to know?!