In short: Statutory deductible expenses are a percentage of income fixed by law that is deducted before tax on fees or freelance income, without having to prove actual costs.
Simply put: The law assumes that earning a fee involves costs and recognises them as a flat percentage. If your real costs are lower, you gain; if higher, you cannot deduct more. The percentage depends on the type of income.
Example: A €1,000 fee with 25 % deemed expenses: taxable income is €750. If no contributions are due on the fee, tax at 10 % is €75.
General rule: Many countries have a simplified or flat-rate regime for small or freelance income; the rules are national.
Example: Bulgaria — Personal Income Tax Act, Article 29(1): 25 % for freelance work and service contracts; 40 % for copyright and licence fees, crafts outside patent tax, and lawyers and notaries; 60 % for registered farmers selling unprocessed produce. Only for people who are not traders; actual documented costs are not deducted under this article.
Check in your country — Is there a flat-rate expense regime, and for which kinds of income?
Watch out: A sole trader is taxed differently.
Source: Personal Income Tax Act (Bulgaria): https://lex.bg/laws/ldoc/2135538631