This excerpt taken from the ICI 6-K filed Mar 21, 2007.
Depreciation and amortisation
The Groups policy is to write off the book value of property, plant and equipment, excluding land, and intangible assets other than goodwill to their residual value evenly over their estimated remaining life. Residual values are reviewed on an annual basis. Reviews are made annually of the estimated remaining lives of individual productive assets, taking account of commercial and technological obsolescence as well as normal wear and tear. Under this policy, the lives approximate to 30 years for buildings, 12 years for plant and equipment and 3 to 5 years for computer software. Depreciation of assets qualifying for grants is calculated on their full cost. No depreciation has been provided on land. Impairment reviews are performed where there is an indication of potential impairment. If the carrying value of an asset exceeds the higher of the discounted estimated future cash flows from the asset and net realisable value of the asset, the resulting impairment is charged to the income statement.