Broadly speaking, you can deduct from your turnover all the costs you incur for the sole purpose of earning business profits. But you cannot deduct costs which you incur for a non-business purpose, such as your own personal expenses or drawings. And you cannot deduct capital costs, that is, the cost of buying fixed assets or intangibles, such as goodwill, which last for several years (or losses you suffer when you sell them). But you may be able to claim capital allowances (see page 8) for these capital costs. Additionally, you cannot deduct costs which are recoverable under an insurance policy. Business expenditure is allowed in your accounts.
