It is not clear from your question if you are overlooking. any properties.
However the issues raised by the planners relate to protection of amenity of adjacent properties.
If you add a feature that introduces overlooking onto adjacent properties in the form of a balcony, the planners will object to this feature even if obscured glazing is installed.
The objection will be ?loss of amenity? and insist on a different treatment to ensure overlooking is not an issue.
If you are not at risk of overlooking on adjacent properties I can see no reason why you cannot use this window balcony.
If your are trying to gain more floor area, a dormer construction would give you more permanent usable floor area compared to the Velux cambrio window feature.
A dormer would probably be a more acceptable solution over the balcony window. If you have a dormer that is placed on the side that faces an adjacent property, you will still require obscured glazing.
We trust this is of assistance and would be interested on how this projects develops
