If your business bought a car costing more than £12,000 prior to April 2009, the writing down allowance has been restricted to £3,000 a year and the car was kept separate from other assets.
For accounting periods ending on or after 31 March 2014 for corporation tax and 5 April 2014 for income tax, the balance remaining for such a car transfers to the general capital allowance pool and receives the 18% writing down allowance.
For example, an Aston Martin DB9 bought for £121,000 in year ended 30 April 2008 would be written down to £100,000 at 30 April 2014. The writing down allowance for year ended 30 April 2015 would be £18,000, not £3,000.