All areas open to the public including workplaces are considered smoke free zones where they are enclosed or substantially enclosed.
Enclosed: This is an area with permanent walls and doors without any gaps. Windows and doors aren’t classed as gaps. An enclosed structure may be permanent i.e. a building or even a marquee (temporary).
Substantially Enclosed:
This would be a structure (with a roof/ceiling) with an opening in the walls where the opening would make up less than half of the area of the total wall space. Again doors and windows are not classed as gaps.
If in any doubt about whether their building or structure is enclosed, substantially enclosed or open enough to allow smoking in the wisest course of action for businesses would be to contact the local council for advice and clarification.