uk_first_year_self_assessment

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Your real first-January Self Assessment bill — the year’s tax PLUS 50% of next year’s, due the same day — with the exact dated payment schedule. Computes a UK sole trader’s 2025-26 Self Assessment bill (income tax stacked on top of any PAYE income, plus Class 4 National Insurance) and then the part general AI reliably misses: payments on account. First-time filers owe 150% of their bill on 31 January 2027 — the full year’s tax plus the first half of next year’s, in one payment, for income earned up to ~22 months earlier. The tool applies the exact boundary tests (POAs are waived when the bill is under £1,000 or when more than 80% of your tax was collected at source through PAYE), the post-April-2025 late-payment interest formula (Bank rate + 4%, currently 7.75% — models still quote the old + 2.5%), and flags whether Making Tax Digital’s quarterly reporting catches you from April 2026.