Supreme Court ruled that flat tax rates require estimated income, not actual accounting data, to be legally valid. The court dismissed the tax authority's decision for misinterpreting the statute. The taxpayer's argument regarding proper accounting records was accepted. The court clarified the relationship between tax estimation and record-keeping laws. This decision establishes strict procedural requirements for flat tax application. The ruling protects taxpayers from arbitrary tax assessments based on incorrect legal interpretations.

The Supreme Court of Poland dismissed the tax authority's decision to impose a flat 20% tax rate on Zbigniew M. because the court found the underlying calculation method violated statutory requirements. The tax office had determined the taxable base using copies of invoices provided by the taxpayer rather than through a formal estimation procedure mandated by law. The court held that the flat rate statute explicitly requires income to be estimated when records are missing or unreliable, meaning reliance on submitted invoices constituted a legal error. Consequently, the court set aside the lower court's judgment and ordered the tax authority to correct its decision.