Double doors can be treated with either side-hung casement or horizontal sliding units subject to frame clearance at both soffit and cill. With horizontal sliding units the accessible opening may be quite narrow.
Single doors should be treated with side-hung casements. The standard frame section requires 55mm clearance but if only the threshold clearance is critical non standard sections can reduce this to 20mm. Ensure that all measurements take into account any proposed floor coverings.
The external door will not open to a full 90° with a secondary frame in the reveal. The actual angle will depend on the distance between the two doors.
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Guidance