Anything incurred wholly and exclusively for the business. That covers stock and materials, staff costs, premises, insurance, professional fees, marketing, phone and internet, software, tools, and travel that is not ordinary commuting. Where a cost is partly private, claim the business proportion on a basis you can explain. The traps are consistent: client entertainment is never allowable; commuting to a regular workplace is not business travel; clothing is only allowable if it is genuine protective wear or a uniform, so a suit is not deductible however much the work requires one. Equipment expected to last is capital rather than a running cost, though most of it is fully relievable anyway. The real risk is not overclaiming, it is underclaiming through not recording small costs.