24  Annual Report Impairment is established when there is objective evidence that the Law Commission will not be able to collect amounts due according to the original terms of the deposit. Non-current assets held for sale Non-current assets held for sale are classified as held for sale if their carrying amount will be recovered principally through a sale transaction rather than through continuing use. These assets are measured at the lower of their carrying value and fair value less costs to sell Any impairment losses for write-downs of these assets are recognised in the statement of financial performance. Non-current assets are not depreciated or amortised while they are classified as held for sale. Property, plant and equipment Property, plant and equipment asset classes consist of library collection, furniture and fittings, computer hardware and office equipment. All classes are initially recorded at cost. Library collections are stated at fair value. Fair value is determined using market based evidence by an independent valuer. Library collections are valued every three to four years. Additions between revaluations are recorded at cost. The carrying values of revalued items are reviewed at each balance date to ensure that those values are not materially different to f Accounting for revaluations The Commission accounts for revaluations of property, plant and equipment on a class of assets basis. The results of revaluing are credited or debited to an asset revaluation reserve for that class of asset. Where this results in a debit balance in the asset revaluation reserve, this balance is expensed in the statement of financial performance. Any subsequent increase on revaluation that off-sets a previous decrease in value recognised in the statement of financial performance will be recognised first in the statement of financial performance up to the amount previously expensed, and then credited to the revaluation reserve for that class of asset. The Library collection was revalued as at 30 June 2007 by independent valuer, David Smith of DTZ NZ Ltd.