Self Assessment
Learn SA100, income sources, allowances, supplementary pages, payments and how the personal tax calculation fits together.
Self Assessment explained
Self Assessment brings together an individual’s taxable income, gains, reliefs, tax already deducted and other charges for a tax year. The SA100 is the core return; supplementary pages capture income and circumstances that do not fit on the main form.
This learning path covers UK rules and ReturnDeck’s UK form set. Check the effective period and official source before using a rule for a live filing.
Learn it in stages
Beginner · Build the taxpayer picture
Identify every source of income and gain for the tax year before calculating tax: employment, self-employment, partnership, property, savings, dividends, pensions, foreign income and capital gains can require different return sections or supplements.
Intermediate · Allowances, bands and deductions
Understand how Personal Allowance, income bands, savings/dividend treatment, allowable expenses, pension contributions, Gift Aid and tax already deducted interact. The correct sequence matters because one item can affect the rate applied to another.
Advanced · Supplementary pages
Learn when SA102 employment, SA103 self-employment, SA104 partnership, SA105 property, SA106 foreign, SA107 trusts/estates, SA108 capital gains, SA109 residence and SA101 additional-information pages are relevant. Complete only the pages that match the taxpayer’s facts.
Payments · Balancing payment and payments on account
The final calculation may produce a balancing payment and payments on account for the following year. Learn what is included, when reductions may be appropriate and why cash due can differ from the headline tax liability.
Example: mixed-income taxpayer
A taxpayer has salary with PAYE deducted, sole-trader profit, dividends and rental income. First map each source to the correct form/supplement. Calculate adjusted net income and relevant allowances, apply the correct tax treatment to each income layer, account for PAYE already deducted, then determine the final liability and any payments on account.
Apply this in Practice Mode →Common mistakes to avoid
- Omitting a source because tax was already deducted
- Using turnover instead of taxable self-employment profit
- Forgetting the appropriate supplementary pages
- Mixing property income and self-employment records
- Treating payments on account as an additional tax rather than advance payments toward the next liability
Records and preparation checklist
Before calculating or filing, assemble the evidence that supports the position. This makes the return easier to complete and easier to explain later.
- P60/P45/P11D and employment details
- Self-employment or partnership records
- Property income and expense records
- Bank interest, dividends and investment statements
- Pension/Gift Aid information and relief claims
- Capital disposal and foreign-income records where relevant
Key terms
The main Self Assessment tax return.
An additional return section used for a particular income source or circumstance.
A measure used for several allowance and charge calculations.
An advance payment toward the following year’s Self Assessment bill.
Follow the lesson into the form
ReturnDeck’s 1,139 mapped UK fields connect form codes to plain-English meaning, applicability, source figures, calculation notes, examples, common mistakes, interactions and HMRC sources.
Calculate and check
Use the related ReturnDeck calculator or checker to turn the lesson into a practical result where the verified rules support deterministic calculation.
Open related tool →Practice what you learned
Move from learning into a fictional case file, source documents, return completion, validation and a worked answer. Practice Mode never submits anything to HMRC.
Continue to practice →Official HMRC / GOV.UK sources
These are the primary official sources for this learning path. ReturnDeck summarises them for learning; the official material remains the authority.
Source set reviewed for ReturnDeck’s 2026 UK learning layer. Tax rules change; verify the effective date for a live decision.