Capital allowances are a tax relief designed to allow the cost of certain of your company or organisation's assets to be written off against its taxable profits. They take the place of the depreciation shown in the financial (commercial) accounts, which isn't allowable for Corporation Tax purposes.
There are different types of capital allowances. For each allowance, there are special rules to calculate how much, if any, relief you can claim. You have to follow these rules, rather than the method used in your accounts for calculating depreciation.
Generally, claims for capital allowances must be made in your Company Tax Return.