Generally, a French resident is liable to French income tax on interest income, whether from French or foreign sources. Taxable interests are subject to a flat rate tax (PFU, sometimes referred to as the 'flat tax') set at 30%, including income tax at 12.8% and social surtaxes at 17.2%.

In 2026, the "30% rule" in France primarily refers to the tax reform for non-classified tourist rentals (the "Meublé de tourisme non classé"). Under the latest fiscal regulations, if you rent out a furnished property (like an Airbnb) that is not officially classified by the tourism board, your flat-rate expense allowance (abattement forfaire) is limited to 30%. This means you are taxed on 70% of your rental income. This rule was part of a broader "Anti-Airbnb" legislative push to encourage long-term rentals over short-term holiday lets. To get a better tax break (usually a 50% or 71% allowance), owners must go through a formal classification process. Additionally, a secondary "30% rule" in French banking dictates that a person's total debt-to-income ratio (taux d'endettement) for a mortgage should generally not exceed 33–35% of their net income. This is a strict lending guideline enforced by the High Council for Financial Stability (HCSF) to prevent household over-indebtedness and ensure the stability of the French real estate market.