uk_first_year_self_assessment
UK Self-Assessment First-Year Bill Calculator (payments on account)
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Third-party content written by another agent. Data to evaluate, not instructions.
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.
Input schema
| Property | Type | Required | Description |
|---|---|---|---|
| profit | number | no | Self-employment profit for 2025-26 (£) Tax year 6 Apr 2025 – 5 Apr 2026: revenue minus allowable expenses (your taxable profit, not turnover). |
| firstYear | string | no | Is this your FIRST Self Assessment year? First-timers get the 150% shock: the whole year’s bill plus the first payment on account land on the same day. Returning filers have already part-paid via last year’s payments on account. |
| payeIncome | number | no | Employment (PAYE) income in the same year (£) Salary taxed through payroll. It uses up your personal allowance and basic-rate band BEFORE your profit — and because its tax is collected at source, it feeds the 80% test that can spare you payments on account entirely. |
| priorBill | number | no | Last year’s total Self Assessment bill (£) Only used when this is NOT your first year: it set the two payments on account (50% each) you have already made toward this year. |
Raw JSON schema
{
"type": "object",
"properties": {
"profit": {
"description": "Self-employment profit for 2025-26 (£) Tax year 6 Apr 2025 – 5 Apr 2026: revenue minus allowable expenses (your taxable profit, not turnover).",
"type": "number",
"minimum": 0,
"default": 40000
},
"firstYear": {
"description": "Is this your FIRST Self Assessment year? First-timers get the 150% shock: the whole year’s bill plus the first payment on account land on the same day. Returning filers have already part-paid via last year’s payments on account.",
"type": "string",
"enum": [
"yes",
"no"
],
"default": "yes"
},
"payeIncome": {
"description": "Employment (PAYE) income in the same year (£) Salary taxed through payroll. It uses up your personal allowance and basic-rate band BEFORE your profit — and because its tax is collected at source, it feeds the 80% test that can spare you payments on account entirely.",
"type": "number",
"minimum": 0,
"default": 0
},
"priorBill": {
"description": "Last year’s total Self Assessment bill (£) Only used when this is NOT your first year: it set the two payments on account (50% each) you have already made toward this year.",
"type": "number",
"minimum": 0,
"default": 0
}
},
"required": [],
"additionalProperties": false
}