68 Dialog Semiconductor Plc | Annual report and accounts 2012  |  Section 4 | Consolidated financial statements and notes Notes to the consolidated financial statements For the year ended 31 December 2012  Property, plant and equipment are stated at cost less accumulated depreciation and accumulated impairment losses. These include the cost of replacing  part of the plant and equipment when that cost is incurred, if the recognition criteria are met. Depreciation is charged on a straight-line basis over the  estimated useful lives of the assets as follows:  2. Summary of significant accounting policies continued Property, plant and equipment  Intangible assets  The asset’s residual values, useful lives and methods of depreciation are reviewed, and adjusted if appropriate, at each financial year end.  Intangible assets acquired separately (primarily licences, software and patents) are measured on initial recognition at cost. The cost of intangible assets  acquired in a business combination (primarily customer based intangible assets, technology) is its fair value as at the date of acquisition. Intangible assets  with finite useful lives are carried at cost less accumulated amortisation and accumulated impairment losses, if any. The amortisation period and the  amortisation method for an intangible asset with a finite useful life is reviewed at least at the end of each reporting period. Changes in the expected  useful life or the expected pattern of consumption of future economic benefits embodied in the asset is accounted for by changing the amortisation  period or method, as appropriate, and are treated as changes in accounting estimates.  Intangible assets are amortised on a straight-line basis over the estimated useful lives as follows: Category of assets  Useful life Test equipment  3 to 8 years Leasehold improvements  Shorter of useful life or lease term Office and other equipment  18 months to 13 years Intangible asset  Useful life Customer related intangible assets 1.5 to 3 years Brand / base technology  2 to 4.5 years SAP software  10 years Other intangible assets  3 to 6 years Amortisation expenses are allocated to the cost of goods sold, selling expenses, research and development expenses, or general administration expenses.  Other than its goodwill, the Group has no intangible assets with an indefinite useful life.  Self-developed intangible assets are recorded on a cost basis. They are amortised on a straight-line basis over the estimated usefulness of 12-60 months.  The costs of internally generated intangible assets comprise all directly attributable costs necessary to create, produce and prepare the asset to be  capable of operating in a manner intended by management, e.g. costs of materials and services used or consumed in generating the intangible asset,  costs of employee benefits or fees to register a legal right. Reference is also made to the accounting policy regarding research and development costs in  this section.  Patents have been granted by the relevant government agency for a certain period, depending on the specific country, with the option of renewal at the  end of this period. In most cases the maximum lifetime of the patents is 20 years. They are amortised over the shorter period of expected future benefit,  which is principally ten years. Acquisition costs for patents are based on the cost of patent registration. 