If vacant land is unable to be categorised under section 515, 516 or 517, the land is to be categorised--
(a) if the land is zoned or otherwise designated for use under an environmental planning instrument--according to any purpose for which the land may be used after taking into account the nature of any improvements on the land and the nature of surrounding development, or
(b) if the land is not so zoned or designated--according to the predominant categorisation of surrounding land.