Self-Assessment 2026 — Avoid £100 + 5% Penalties If you’re self-employed, a landlord, or have income outside PAYE, you must file a Self Assessment tax return. Missing the deadline costs £100 instantly, with escalating penalties after that. Here’s the 2026 timeline and the most-overlooked penalty traps. The Key Deadlines For the 2025–26 tax year (April …