I'm hopeing to insert a new floor above my stairwell, which will require a
new staircase.

Presently this is coming in at a steep 9 * 7.5" rises over a total 26"
horizontal, this is a little flexible but that isn't far off the lowest
gradient I can manage. What are the rules governing these measurements?

The space of the mezzanine itself fits in a lowered loft space, and will be
approx ~6' high at the apex going down to ~3' at the eaves.
Are there any regulations as to space requirements? Use is occasional
bedroom.

I was hopeing to put a velux in the roof, but am unsure how planning
permission may be affected by this, it can go on either aspect, but the
preferred one will overlook other properties and there is no way to get it
high enough to prevent this (6' max height of space). If it goes on the
other aspect, I just see next doors roof. I don't expect planning is
required other than for this reason, if at all. I think I can get a letter
from the Council saying "You don't need planning permission", if indeed I
don't.

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Mike W