relevant boundary

means the boundary of an allotment that is other property in relation to the building in question and from which is measured the separation between the building and that other property; and for the external wall of any building, the relevant boundary is the nearest of—
(a) a boundary of a freehold allotment, except that if the other property is a road, railway line, or public open space, the relevant boundary is the boundary on the far side of that other property; or 
(b) a boundary of a cross-lease or a company lease or a licence, except that if the other property is open space to which the lessee or licensee of the building in question has an exclusive right of access and occupation or to which 2 or more occupiers of the building in question have rights of access and occupation, the relevant boundary is the boundary on the far side of that other property; or 
(c) a boundary shown on a unit plan (but excluding a boundary between a principal unit and its accessory unit), except that if the other property is open space and is common property, the relevant boundary is the boundary on the far side of that other property

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