164 BAE Systems Annual Report 2017 Notes to the Group accounts continued 10. Investment property Cost Land and buildings that are leased to non-Group entities are classified as investment property. The Group measures investment property at its cost less accumulated depreciation and impairment losses. Depreciation Depreciation is provided, on a straight-line basis, to write off the cost of investment property over its estimated useful life of up to 50 years. The assets’ residual values and useful lives are reviewed, and adjusted if appropriate, at each balance sheet date. Impairment The carrying amounts of the Group’s investment property are reviewed at each balance sheet date to determine whether there is any indication of impairment in accordance with the policy shown in note 8. £m Cost At 1 January 2016 176 Additions 9 Transfer from property, plant and equipment 9 Disposals (24) At 31 December 2016 170 Additions 15 Disposals (24) At 31 December 2017 161 Depreciation and impairment At 1 January 2016 56 Depreciation charge for the year 2 Transfer from property, plant and equipment 9 Disposals (7) At 31 December 2016 60 Depreciation charge for the year 3 Impairment charge 2 Disposals (5) At 31 December 2017 60 Net book value At 31 December 2017 101 At 31 December 2016 110 At 1 January 2016 120 Fair value At 31 December 2017 173 At 31 December 2016 167 The fair values above are based on and reflect current market values as prepared by in-house professionals who have the appropriate professional qualifications and recent experience of valuing properties in the location and of the type being valued.