Self Assessment is the system HMRC uses for individuals to report income tax, capital gains tax, and certain other liabilities that are not collected through PAYE. If you are self-employed, receive untaxed income, or have complex tax affairs, you will most likely need to file a Self Assessment return.
You generally need to file a Self Assessment tax return if, in a tax year, you:
Are self-employed or a business partner
Have income from rental properties
Receive untaxed income, such as foreign income
Have high income child benefit charges
Earn significant savings or investment income
Need to pay capital gains tax
Are a company director with untaxed income
Have complex tax affairs not fully handled by PAYE
Even if you have no tax to pay, a return may still be required.
If you are required to file a Self Assessment return, you must first register with HMRC. Once registered, you will receive a Unique Taxpayer Reference (UTR) and details for filing online.
You should register by 5 October following the end of the tax year in which you first had a filing requirement. Late registration can lead to penalties.
A typical Self Assessment return may include details of:
Self-employment income and expenses
Employment income not taxed via PAYE
Rental income and allowable expenses
Dividend and interest income
Capital gains
Pension income
Tax reliefs and allowances claimed
Supporting records and documents must be kept in case HMRC requests evidence.
Self Assessment has strict deadlines:
31 October – deadline for submitting paper returns
31 January – deadline for electronic returns and any tax owed for the previous tax year
31 January and 31 July – deadlines for payments on account (advance payments towards your next year’s tax)
Late returns or payments may attract penalties and interest, so it is important to plan ahead.
If your tax liability is more than £1,000 and not mostly covered by tax deducted at source (e.g., through PAYE), you may need to make payments on account. These are advance payments towards your next year’s tax and are typically due in two instalments:
31 January (alongside your final balance)
31 July
If your income varies year to year, you may be able to reduce payments on account, but this should be done carefully to avoid interest or penalties.
Accurate records are essential for completing your Self Assessment and for HMRC compliance. You should keep records of:
Receipts and invoices
Bank statements
Expense records
Details of income sources
PAYE and tax deduction documents
Capital gains and disposals
Records should be kept for at least five years after the 31 January submission deadline for the relevant tax year.
Most Self Assessment returns are filed online using HMRC-compatible software. Digital submission provides immediate acknowledgement and helps reduce errors. It also ties in with the Making Tax Digital agenda, where digital record keeping is increasingly expected.
When completing a Self Assessment return, be careful to:
Report all sources of income
Claim only allowable expenses and reliefs
Avoid guesswork — record accurate figures
Complete the correct sections of the return
File and pay by the relevant deadlines
Mistakes can lead to HMRC enquiries, interest charges, and penalties.
Failure to submit your return or pay on time can result in:
Automatic late filing penalties
Interest on late payments
Penalties for inaccuracies
HMRC may also open an enquiry to check the accuracy of your return if inconsistencies are found.
Completing a Self Assessment return can be complex, especially when you have multiple income streams, deductions, or tax reliefs to consider.
Applegrow can assist you with:
Determining whether you need to file a return
Registering for Self Assessment
Preparing accurate returns
Planning payments and cash flow
Advising on allowable expenses and reliefs
Responding to HMRC enquiries
With expert support, you can navigate Self Assessment with confidence and peace of mind.
Contact Applegrow Financial Advisors today for personalised guidance and support through every step of the process.
