HMRC Form SA100 is the main UK Self Assessment tax return - the form individuals use to report income, gains, allowances, and reliefs to HM Revenue & Customs (HMRC) and calculate their UK income tax and National Insurance liability for the tax year. The UK tax year runs from 6 April to 5 April. SA100 captures the core personal information and totals; supplementary pages (such as SA102 for employment, SA103 for self-employment, SA105 for UK property, SA106 for foreign income, SA108 for capital gains) cover specific income types.
File SA100 if any of the following applies: you were self-employed as a sole trader with gross income over £1,000; you were a partner in a business partnership; you had untaxed income from rental property, savings, investments, or foreign sources; you received dividends over the dividend allowance; your annual income exceeded the Self Assessment threshold currently in force; you have Capital Gains Tax to pay; you need to claim certain reliefs (foreign tax credit, gift aid carry-back, EIS/SEIS relief); you're a company director (HMRC may require it); or you're a higher-rate taxpayer with significant savings or other untaxed income. HMRC will tell you if you must register for Self Assessment.
For each tax year ending 5 April: register for Self Assessment by 5 October if it's your first year, file the paper SA100 by 31 October, file online by 31 January, and pay any tax owed by 31 January. A second payment on account is due 31 July. Late filing triggers an automatic £100 penalty, with additional penalties accumulating after 3, 6, and 12 months. File online through HMRC's Self Assessment portal (free) or through approved third-party software; paper SA100 filings are still accepted but the deadline is earlier.
National Insurance number, Unique Taxpayer Reference (UTR), P60 or P45 from any employment, records of self-employment income and expenses, rental income and expenses, savings interest, dividends, pension contributions, gift aid donations, foreign income, capital gains, student loan information, and details of any allowances or reliefs you plan to claim (marriage allowance, blind person's allowance, charitable donations, pension tax relief, EIS/SEIS investments, etc.). Keep records for at least 5 years after the 31 January submission deadline in case HMRC opens an enquiry.
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