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Most people don’t dread the tax itself. They dread the scramble. The lost receipt, the login that won’t work at 11pm on 31 January, the £100 penalty that lands because life got in the way. This guide is here to take that scramble out of the equation.
Self Assessment is how HMRC collects Income Tax from people whose tax isn’t taken automatically through PAYE. If you’re on a K tax code, it’s worth knowing how a K tax code interacts with Self Assessment before you file. If you’re self-employed, take dividends, rent out a property, or run a side hustle, there’s a fair chance you need to file. Below is everything you need to get it right, in roughly the order you’ll need it.
What's changed this year
A few things worth knowing before you start your next return:
Making Tax Digital for Income Tax has begun phasing in
From April 2026, self-employed people and landlords above the income threshold are moving to quarterly digital updates rather than one annual return. If that's you, the rules are different from what's below, so read the MTD section near the end first.
Reporting thresholds and allowances
Are reviewed each year, so check the personal allowance, dividend allowance and National Insurance figures that apply to the tax year you're filing.
Online platform data
HMRC now receives income data directly from sites like eBay, Vinted, Airbnb and Etsy, so side income is far easier for them to cross-check than it used to be.
Do you actually need to file?
Plenty of people file when they don’t have to, and plenty more assume they’re fine when they’re not. You’ll usually need to send a return if any of the following apply:
If you’re on the fence, that’s a reason to check, not a reason to ignore it. The penalties for not filing when you should have are worse than the mild inconvenience of filing a return you didn’t strictly need. When it’s genuinely unclear, a short conversation with an accountant settles it.
- You were self-employed as a sole trader and earned more than £1,000.
- You may need to file if you are a company director and have untaxed income, dividends, benefits, expenses, or HMRC has asked you to complete a return.
- You earned more than £2,500 in untaxed income, for example from tips, commission or renting out a room.
- You received more than £10,000 from savings, investments or dividends.
- You or your partner earned over £60,000 and one of you claimed Child Benefit (the High Income Child Benefit Charge). Our full guide to the High Income Child Benefit Charge breaks down exactly how much you’d owe.
- You had income over £150,000.
- You made money from a property, a holiday let, or selling goods online beyond the trading allowance.
The dates that matter
The 31 January deadline gets all the attention, but it isn’t the only one. For the current return cycle:
- 5 October 2026 deadline to register with HMRC if you’ve never filed before.
- 31 October 2026 deadline for paper returns.
- 31 January 2027 deadline for filing online and for paying any tax you owe.
- 31 July second payment on account, if you make one.
Register early if it’s your first time. HMRC sends your Unique Taxpayer Reference (UTR) by post, and that can take a week or more, longer in the busy run-up to January. Leave it too late and you can find yourself unable to file even though you’re ready to.
What to gather before you start
The filing itself is quick once your paperwork is in order. It’s the hunting for documents that eats the evening. Get these together first:
Income
- P60 if you were employed.
- P45 if you left a job during the year.
- P11D for any benefits like a company car.
- Self-employment accounts, invoices and records of takings.
- Dividend vouchers.
- Rental income figures.
- Records of any side income, including from online platforms.
Interest and investments
- Bank and building society interest statements.
- Tax certificates for investments.
Outgoings and reliefs
- Allowable business expenses (travel, equipment, professional fees, use of home).
- Pension contributions.
- Gift Aid donations.
One habit saves more stress than any other: keep digital folders by month or by income source as the year goes, rather than reconstructing twelve months of activity in one painful sitting. HMRC also expects you to hold onto these records for at least five years after the 31 January filing deadline, so don’t bin anything once you’ve filed.
For a printable version you can tick off, see our Self Assessment Checklist.
How to file step by step
- Register for Self Assessment if you haven’t already, and keep your UTR and Government Gateway login safe.
- Gather the documents above.
- Log in to your HMRC online account and start the SA100 return.
- Report your income by source: employment, self-employment, property, dividends, savings.
- Claim your expenses and reliefs. This is where money is won or lost, so don’t rush it.
- Check the figures against your records, then check them again.
- Submit, and pay what you owe by 31 January.
After submitting, save your HMRC submission confirmation and payment reference. Filing the return and paying the tax are separate steps, so keep proof of both.
Self assessment tips that save time and money
A few things separate a smooth return from a stressful one.
- Register sooner than feels necessary. If this is your first return, register early. HMRC sends your UTR by post, which can take a week or more, especially near January. Sort it in autumn, not at the deadline.
- Track income and expenses monthly. A 20-minute monthly tidy-up beats a lost weekend in January and means you actually capture the expenses you’re entitled to claim.
- Don’t confuse net and gross. Mixing the two is one of the most common filing errors, and it changes what you owe.
- Use your Personal Tax Account. A lot of your information is pre-filled, which cuts down on manual entry and mistakes.
- Plan for payments on account. If your bill is over £1,000, HMRC asks for an advance payment towards next year on top of this year’s tax. It catches first-timers off guard, so know it’s coming.
Common mistakes to avoid
- Filing late. The penalty starts at £100 even if you owe nothing, then climbs.
- Leaving income off. Freelance work, crypto gains, overseas earnings and online sales all count, and HMRC increasingly gets this data straight from the source.
- Missing reliefs. Pension contributions, business costs and Gift Aid all reduce your bill if you claim them.
- Guessing the rounding. Income is rounded down to the nearest pound, expenses and tax paid rounded up in your favour. Small thing, easy to fumble.
- Forgetting to pay. Filing and paying are two separate actions. Both are due 31 January.
Miss the deadline and the interest starts
This is the part people learn the hard way. File late and you get the automatic £100 penalty. Pay late and HMRC charges interest from 1 February until the balance is cleared, and that rate is tied to the Bank of England base rate plus a margin, so it’s no longer trivial.
If you’ve already missed it, the worst move is to keep avoiding it. Penalties grow the longer you leave it, and there are limited grounds for appeal if you have a reasonable excuse. Our guide on what to do if you’ve missed the Self Assessment deadline walks through the options.
Filing it yourself vs getting help
For a straightforward return — one income source, few expenses — most people can file themselves with a free evening and a bit of patience.
It’s worth bringing in an accountant when things get more involved: multiple income streams, rental property, foreign income, your first year self-employed, a large bill you’d like to reduce, or simply no appetite for the admin. A good accountant usually finds reliefs that cover a chunk of their fee, and you stop being the one HMRC chases. If that’s where you are, here’s how an accountant can help with your Self Assessment.
Making Tax Digital: the bigger shift
Self Assessment is changing for some filers. Making Tax Digital for Income Tax is phasing in from April 2026, starting with self-employed people and landlords above the income threshold. In practice that means keeping digital records and sending HMRC quarterly updates, rather than one annual return. If it applies to you, get set up early rather than scrambling mid-year. See our guide to Making Tax Digital for Income Tax.
Common Myths Straightened Out
HMRC hasn't written to me, so do I need to file?
Yes, if you meet the criteria. The system runs on self assessment, so checking and registering is your job. HMRC won't chase you, and waiting for an invitation that never arrives just earns you a penalty.
I sell on Vinted or eBay. Do I owe tax now?
Usually not, but it depends on whether you're selling or trading. Clearing out your wardrobe for less than you paid is not taxable. Buying or making things to sell for profit is trading, and HMRC judges that on the Badges of Trade. Even then, the £1,000 trading allowance lets you earn that much in gross income before you need to file.
Do I need to file if I don't owe any tax?
Yes, if you're registered for Self Assessment. The return declares your income, it doesn't only settle a bill. Filing a loss can work in your favour too, since you may carry it forward against future profits or reclaim tax paid in an earlier year.
Can I claim my commute as a business expense?
No. Travel between home and a permanent workplace fails the wholly and exclusively test. Travel to a temporary workplace, such as a client site for a specific project, is different, and usually allowable.
Can I claim my rent because I work from home?
Only the business share. Working at the kitchen table doesn't make your rent deductible. Use a room for business and you can claim a proportion of your bills by floor area and time used. Prefer to skip the sums? Take HMRC's simplified flat-rate expense instead.
HMRC already has my income data, so why file?
Because they see what came in, not what went out. Banks, employers and platforms report your income, but HMRC has no view of your expenses, subscriptions or Gift Aid. Rely on their figures alone and you'll almost certainly overpay.
I've retired. Am I done with tax returns?
Not necessarily. Tax follows income, not age. If your combined State Pension, private pension, investments and rental income pass your £12,570 Personal Allowance, you may still owe tax, and with complex income you may still need to file.
I've missed the deadline and can't pay yet. What now?
File anyway, immediately. The late-filing penalty is separate from the late-payment penalty, so submitting on time stops the £100 fine even when you can't pay the bill. Then contact HMRC to set up a Time to Pay arrangement and spread the cost.
Frequently Asked Questions
What happens if I miss the 31 January deadline?
You get an automatic £100 penalty, even if you owe no tax. Leave it longer and daily penalties and interest stack up on top.
Can I correct my return after submitting it?
Yes. HMRC lets you amend an online return within 12 months of the filing deadline.
How do I pay the tax I owe?
Online by bank transfer, debit card, or through your HMRC account. Payment is due by 31 January, the same day as the filing deadline.
Do I need to file if I made a loss?
Yes. If you’re registered for Self Assessment you still file, even with no profit to report. It can pay off, too, because a recorded loss can be carried forward to cut a future year’s bill.
How long should I keep my records?
At least five years after the 31 January deadline for that tax year. Longer if a return was late.
Ready to stop guessing?
Not sure whether your return is straightforward or needs professional review? Speak to our tax team for a no-obligation discussion before the deadline.
Our team can review your figures, identify missed reliefs, and help file your Self Assessment accurately and on time.
Muhammad Sufyan Moavia
Reviewed by Muhammad Sufyan Moavia, Chartered Tax Adviser and Fellow Chartered Certified Accountant at IBISS & CO.
He has 15 years experience in accounting and tax, advising individuals and business owners on personal tax, HMRC matters, IHT, CGT and tax planning.
Last review: June 2026
