This paper analyzes current regulations regarding the determination of taxable income for business households, in the context of a shift in the tax regime starting in 2026, moving decisively from a revenue-based method to determining income based on revenue minus expenses in cases prescribed by law. Based on clarifying the conditions and categories of deductible expenses, the paper focuses on evaluating several outstanding legal issues, including: criteria for determining business-related expenses; requirements regarding invoices, supporting documents, and non-cash payments; labor costs for household members; asset depreciation and the mixing business and personal expenses; as well as mechanisms for handling inventory and transitional assets. Accordingly, the paper proposes perfecting the legal system by clarifying criteria for identifying expenses, diversifying methods of substantiation, establishing a reasonable allocation mechanism, and ensuring that taxable income more accurately reflects the actual income of business households.