Checklist: 10 Things to Do Before You File Your Self-Assessment Tax Return

Most people who file a Self-Assessment return describe the same experience afterwards: the form itself took half an hour, but the fortnight spent avoiding it took far longer. The delay is rarely about the software. It is the shoebox of receipts, the unanswered question about a second income stream, and the vague worry that something important has been overlooked.

The fix is a checklist. Whether you are newly self-employed, letting out a flat, or simply filing because your income has crept above a threshold, the same details trip people up every year. Work through the points below before you click submit, and you will cut both the stress and the chance of an avoidable penalty.

1. Confirm the deadline that applies to you

The tax year runs from 6 April to 5 April. If you file online, the return and any tax owed are both due by 31 January following the end of that tax year. Paper returns are due earlier, by 31 October. If this is your first return, you need to have told HMRC you have a reason to file by 5 October after the tax year ends — missing that registration date is a common and entirely preventable mistake.

Two practical warnings. First, HMRC's systems are busiest in the final 48 hours, and a crash on 31 January is not accepted as an excuse. Second, filing early does not mean paying early: you can submit in April and still schedule payment for January, which removes the deadline pressure without removing the money from your account any sooner.

2. Assemble the paperwork before you open the form

Filling in the return is much quicker when you are not hunting for a figure you half remember. Before you start, gather:

• Your Unique Taxpayer Reference and Government Gateway log-in details.
• Records of all income: invoices, bank statements, platform statements, property rental income, dividends and interest.
• Your P60 or P45 if you also had employment, plus details of any taxable benefits.
• Receipts and bank records for anything you intend to claim as an expense.
• Details of pension contributions, Gift Aid donations and any student loan repayments.

If your records are incomplete, do not guess wildly. Use the best evidence you have, keep a note of how you arrived at the figure, and remember that you can amend a return after filing — normally within 12 months of the deadline.

3. Check your expenses are claimable and defensible

The test for most business costs is whether they were incurred wholly and exclusively for the purposes of your trade. Ordinary allowable costs include office supplies, business insurance, accountancy fees, advertising, bank charges, and a reasonable proportion of phone and broadband if you work from home.

Common mistakes run the other way: client entertaining is not allowable, ordinary clothing is not allowable (protective gear and uniforms usually are), and neither is the personal half of a mixed-use bill. If you use a vehicle for business, HMRC's simplified mileage rates are often easier than tracking every receipt — 45p per mile for the first 10,000 business miles in a car, and less thereafter.

If your income from a side venture is small, the £1,000 trading allowance may let you ignore expenses altogether and simply declare the allowance instead. You cannot claim both, so compare the two before choosing. Keep business records for at least five years after the 31 January deadline; HMRC can ask to see them.

4. Do not overlook reliefs you are entitled to

Filers routinely overpay because they forget to claim. Check whether you can use pension contributions or Gift Aid donations to extend your basic-rate band, whether you qualify for Marriage Allowance, and whether professional subscriptions or job-related expenses are claimable. If you receive Child Benefit and your income is above the threshold, work out which partner should declare the charge — it is not always the higher earner, and getting it wrong is expensive to unpick.

Where your affairs are complicated — property portfolios, overseas income, share schemes, or a change in residence — it is worth paying for a few hours of an accountant's time rather than relying on guesswork.

5. Understand payments on account before you are surprised by them

This is the single most common January shock. If your tax bill is more than £1,000 and less than 80% was collected at source, HMRC asks for the bill plus half of it again as an advance payment towards the following year. The same again falls due on 31 July.

You can apply to reduce payments on account if you expect profits to fall, but be careful: if you reduce them without justification, HMRC charges interest and can impose a penalty. If you are newly self-employed with a first-year bill, it may be better to set the money aside than to argue the point later.

6. Know what the penalties look like

Late filing triggers a £100 fixed penalty, even if you owe nothing. After three months, daily penalties of £10 begin, up to a cap of £900. At six and twelve months, further charges of 5% of the tax due (or £300, whichever is greater) can apply. Late payment is treated separately: 5% of the unpaid tax at 30 days, again at six months and again at twelve months, plus interest running from the due date. None of this is worth risking for a return that takes an evening to complete.

7. The final twenty minutes before you submit

Read the summary page line by line. Check that your name, address and bank details are current, and that the amount owed matches what you calculated. Save a PDF or printed copy of the submitted return and any workings — you will want them next year, when the same questions come round again.

Then make the payment, or diarise it if you filed early. Set a reminder for 31 July as well, and put a note in next year's calendar for the registration deadline in October. Ten minutes of admin now saves a great deal of it in twelve months' time.

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