Can I Do My Own Tax Return? UK Guide 2025/26

If your finances are reasonably simple and you keep decent records, you can absolutely file your own Self Assessment return. Plenty of people do it every year using HMRC's free online service, and it saves them an accountant's fee.

But "can I?" and "should I?" aren't the same question. The online form is friendlier than it looks, yet a few avoidable mistakes can cost you more than any fee you'd have paid.

This guide walks you through what's involved for the 2025/26 tax year: how to register, how to file step by step, the errors we see most often, and the point at which professional help genuinely pays for itself.

It's written for sole traders, landlords and company directors who are weighing up doing it themselves.

Is it difficult to do your own tax return? {#is-it-difficult}

The honest answer is that it depends on your circumstances.

If your situation is straightforward, you're organised, and you understand what's being asked, completing your own return is doable. The online system is fairly user-friendly, with help boxes as you go, and it works out the tax for you once you've entered your figures.

The real work happens before you log in. If you've kept accurate records through the year and you're clear about what income to declare and which expenses you can claim, filling in the form is the easy part.

Accuracy matters, though. HMRC treats errors seriously, and mistakes can lead to underpaid tax, interest, penalties, or an enquiry. If you're not sure about something, it's worth getting advice rather than guessing.

Can I do my own tax return without an accountant? {#without-an-accountant}

Yes. You don't need an accountant to file a Self Assessment, and many people file their own.

There are trade-offs worth thinking through first.

Why people do it themselves:

• You save the fee.
• You keep full control of the timing.
• You understand your own numbers better.
• It's genuinely simple when your affairs are simple.

Where it can go wrong:

• It's time-consuming, especially the first year.
• You can miss reliefs or allowable expenses you didn't know about.
• A mistake that underpays tax can be costly.
• There's no one checking your work.
• Multiple income sources, capital gains or foreign income make it much harder.

One thing people often miss: when you're self-employed, accountancy fees are an allowable business expense. So the real cost of an accountant is lower than the headline price, and good advice can save more tax than the fee itself.

What does DIY cost compared with an accountant? {#cost-comparison}

Get help with your tax return →

Here's a realistic comparison so you can weigh it up.

Doing it yourself:

• £0 using HMRC's free online service.
• Optional commercial software, roughly £15 to £100.
• Your time, typically a few hours, more if your affairs are involved.

Using an accountant:

• A simple sole trader return: often a few hundred pounds.
• More complex self-employment: more again.
• A company director with both a personal return and company filings: more still.

Fees vary by firm and complexity, so treat any figure as a guide and get a quote. Because the fee is tax-deductible for the self-employed, a £300 fee costs a basic-rate (20%) taxpayer £240 after relief, and less again at higher rates.

The hidden cost of DIY is the mistakes: missed expense claims, penalties for errors or late filing, interest on underpaid tax, and the time of dealing with HMRC if something's queried.

For a simple return, doing it yourself usually makes sense. For anything complex, an accountant often delivers better value.

If you'd rather hand it over, our Self Assessment service covers the whole return from start to filing.

Who needs to file a Self Assessment? {#who-needs-to-file}

Not everyone needs to file. You can check whether you need to send a tax return using HMRC's online tool.

You'll usually need to file if you:

• Are self-employed as a sole trader with more than £1,000 of gross trading income (above the trading allowance).
• Are a company director with income to declare, such as dividends.
• Have untaxed income that needs reporting, for example rental or investment income.
• Earned over £100,000.
• Need to claim certain reliefs.
• Made capital gains above the annual exempt amount, which is £3,000 for 2025/26.
• Get Child Benefit and you or your partner have income over £60,000 (the High Income Child Benefit Charge).

For higher earners: the High Income Child Benefit Charge is 1% of your Child Benefit for every £200 of income above £60,000, reaching 100% (the full amount repaid) once income hits £80,000 or more.

If you run a limited company, you may have two separate obligations: a Company Tax Return reporting Corporation Tax, and a personal Self Assessment if you take dividends or salary. The deadlines differ, so it's worth being clear which applies to you. Our Corporation Tax service handles the company side.

How do I register for Self Assessment? {#how-to-register}

If you've never filed before, you need to register with HMRC before your first return.

The simplest route is to register for Self Assessment online on GOV.UK.

Here's roughly how it goes:

1. Complete the online form with your details and National Insurance number.
2. HMRC sends you a Unique Taxpayer Reference (UTR), usually within about 15 days. If you register online you may see it in your Personal Tax Account or the HMRC app sooner.
3. You'll also get an activation code for the Government Gateway (HMRC's online service), which can take a couple of weeks to arrive by post.
4. With your UTR and activation code, you can finish setting up Government Gateway and access the Self Assessment system.

Don't leave it late. You must register by 5 October following the end of the tax year. To file for 2025/26 (which ended on 5 April 2026), register by 5 October 2026. Given the postal delays, start as soon as you know you need to file.

What documents and records do I need? {#documents}

Being prepared makes the whole thing smoother. Gather everything before you start.

For everyone:

• Your National Insurance number.
• Your UTR.
• Details of all income and amounts.

If you're self-employed:

• Business bank statements.
• Records of all income (invoices, receipts, deposits).
• Records of all expenses with supporting receipts.
• A profit and loss summary for the year.
• Details of assets bought or sold.
• CIS deduction statements if you work in construction.

If you're employed:

• P60 from each employer showing pay and tax.
• P45 from any job you left during the year.
• P11D if you had taxable benefits such as a company car or private medical cover.

Other income:

• Rental income and the related expenses if you're a landlord.
• Dividend vouchers or statements.
• Bank and building society interest.
• Pension income.
• Capital gains or losses.

Having it all organised before you start saves hours of stopping and searching.

How do I file my return step by step? {#step-by-step}

Once you're registered with your documents to hand, the process follows a logical order.

1. Log in. Go to HMRC's Self Assessment online service and sign in with your Government Gateway details.

2. Answer the tailoring questions. A few short questions set the form up so you only see the sections that apply to you.

3. Personal details. Confirm your name, address, date of birth and so on, and check everything is current.

4. Income. Usually the longest part. You declare income from employment, self-employment, property, pensions and benefits, investments and dividends, foreign sources, and capital gains. Declare all of it, not just self-employment. HMRC receives data from banks, employers and other departments, so omissions tend to show up.

5. Reliefs and deductions. Pension contributions, Gift Aid, student loan repayments, Marriage Allowance and the High Income Child Benefit Charge where relevant.

6. Review and submit. The system calculates the tax. Check every figure, then submit. You'll get an on-screen confirmation and a reference number, so save or print it.

There are help boxes throughout. If anything is unclear you can contact HMRC's Self Assessment helpline, or speak to an accountant before you submit rather than after.

What are the most common mistakes? {#common-mistakes}

Learning from common errors saves time, money and stress. These are the ones we see most often in practice.

1. Leaving income out. People declare self-employment but forget bank interest above their Personal Savings Allowance, dividend income above the £500 dividend allowance for 2025/26, or rental income. HMRC usually has this data already.

2. Underclaiming expenses. Nervousness leaves money on the table. If it's a genuine business cost and you can evidence it, claim it. Professional subscriptions, training and the business share of your phone bill are commonly missed.

3. Overclaiming. The opposite problem: claiming costs that aren't "wholly and exclusively" for business. Your ordinary commute isn't allowable, and everyday clothes aren't either, even if you only wear them for work.

4. Getting working-from-home wrong. You can use HMRC's simplified flat rates or your actual costs. For the flat-rate method the monthly amounts are £10 for 25 to 50 hours, £18 for 51 to 100 hours, and £26 for 101 hours or more. Many people forget to claim at all.

5. Missing CIS deductions. Construction workers sometimes forget to enter tax already deducted under the Construction Industry Scheme, then overpay because HMRC doesn't know it's been paid.

6. Losing receipts. Paper fades and goes missing. Photograph or scan receipts when you get them and keep them organised.

7. Filing at the last minute. If you hit a snag on 31 January, there's no time to fix it. Aim for mid-January.

8. Forgetting capital gains. Sold shares, a second property or other valuable assets? Capital gains tax isn't collected through PAYE, so it has to go on your return.

What expenses can I claim? {#allowable-expenses}

Plenty of legitimate expenses go unclaimed each year because people worry about getting it wrong. Knowing what qualifies helps you file accurately and keep your bill fair.

Allowable expenses are costs incurred "wholly and exclusively" for business. Where something has a personal element, you claim only the business portion.

Common examples:

• Travel and vehicle costs. Business mileage at the HMRC flat rate of 45p a mile for the first 10,000 business miles and 25p after that, or your actual running costs. Plus parking, fares and business overnight stays.
• Premises. Business rates, rent, utilities, insurance, repairs. If you work from home, claim a proportion using simplified or actual costs.
• Office and admin. Business share of phone and internet, postage, stationery, printing, software and equipment.
• Marketing. Advertising, website hosting and design, business cards.
• Staff. Wages, employer's National Insurance, pensions, subcontractor fees.
• Professional fees. Accountancy, legal, professional memberships and indemnity insurance.
• Training. Courses related to your current trade (not training for a brand-new profession).
• Stock and materials. Goods for resale, raw materials, packaging.
• Clothing. Uniforms and protective gear, not everyday business wear.
• Financial costs. Bank charges, interest on business loans.

For the detail, see HMRC's expenses guidance for the self-employed. Keep records and receipts for everything you claim, because if HMRC asks, you'll need to show the cost was a genuine business expense.

If you want help getting this right all year, our bookkeeping service keeps your records tidy so nothing's missed at filing time.

Do I need to send receipts to HMRC? {#sending-receipts}

No. You don't routinely send receipts or supporting documents with your return. You enter the totals, and HMRC works on the basis that the figures are accurate.

You do have to keep the records, though. HMRC can ask to see them, and you must retain business records for at least five years after the 31 January filing deadline.

If HMRC opens an enquiry, they'll ask for evidence behind your figures. If you can't produce it, they may disallow expenses and charge additional tax, interest and penalties. Store everything systematically, digital or physical, and if you ever post documents to HMRC, send copies and keep the originals.

What are the Self Assessment deadlines for 2025/26? {#deadlines}

Missing a deadline triggers an automatic penalty, even if you owe no tax. The key dates for the 2025/26 year are below.

Miss the 31 January online deadline and you face an immediate £100 penalty, then daily penalties of £10 a day after three months (up to £900), a further penalty of 5% of the tax due or £300 (whichever is greater) after six months, and the same again after twelve months. Interest also runs on unpaid tax.

What are payments on account? If your previous year's bill was over £1,000 (and less than 80% of your tax was collected at source), HMRC asks for advance payments towards next year's bill, split across 31 January and 31 July. Each is half of your previous year's bill, with any difference settled by a balancing payment when you next file.

You can see the official dates on HMRC's Self Assessment deadlines page.

What if I make a mistake on my return? {#mistakes-after-filing}

Mistakes happen. What matters is fixing them.

• Before the deadline: log back in and amend the return as many times as you need.
• After submitting: you can amend online within 12 months of the filing deadline (so by 31 January 2028 for a 2025/26 return).
• More than 12 months later: write to HMRC with the correct information and they'll decide whether to accept it.
• If HMRC spots it first: they may open an enquiry. If tax was underpaid you'll owe it plus interest, and if they judge the error careless or deliberate, penalties can range from nothing up to 100% of the tax due, depending on the behaviour and how you cooperate.

How far back HMRC can look depends on the behaviour: ordinarily four years, six years for careless errors, and up to twenty years for deliberate errors or certain offshore matters. That's why keeping records for at least five years matters.

If you're unsure about something, it's always better to get advice before submitting than to guess.

When should I use an accountant instead? {#use-an-accountant}

DIY works well for simple affairs. In some situations, professional help is worth every penny.

Consider an accountant if:

• Your affairs are complex (multiple income sources, investments, property, foreign income or capital gains).
• You run a limited company. Company accounts and Corporation Tax are more technical, and errors carry Companies House penalties.
• You want to plan, not just report. Accountants know the reliefs and allowances that can legitimately cut your bill.
• You're a landlord with several properties. The rules on mortgage interest relief and allocating costs are easy to get wrong. We work with landlords on exactly this.
• You're short on time. Your hours may be better spent on the business.
• You're worried about errors. Peace of mind has a value.
• You're facing an HMRC enquiry, where representation is genuinely useful.

Remember the fee is tax-deductible for the self-employed, so the real cost is lower than the sticker price.

Want a hand with your 2025/26 return? Book a free 20-minute call with a Zmartly accountant. We'll tell you honestly whether you're better off filing yourself or letting us handle it. Talk to a Zmartly accountant.

FAQs {#faqs}

Can I do my own tax return if I'm self-employed? Yes. Many self-employed people file their own return successfully. As long as your finances are straightforward, you keep good records and you understand allowable expenses, it's achievable. Complex situations benefit from professional advice.

How long does it take to do your own tax return? For a simple return with one income source, allow a few hours including gathering documents. Returns with multiple income sources, rental property or capital gains take longer, especially the first time, so leave extra time to learn the system.

What happens if I file my tax return late? Late filing triggers an immediate £100 penalty even if you owe no tax. After three months, daily penalties of £10 a day apply, up to £900. After six months there's a further penalty of 5% of the tax due or £300, whichever is greater, and the same again after twelve months. Interest runs on any unpaid tax.

Can I claim working from home expenses? Yes. You can use HMRC's simplified flat rates, which for 2025/26 are £10 a month for 25 to 50 hours, £18 for 51 to 100 hours, and £26 for 101 hours or more. Alternatively you can claim the business proportion of your actual household costs.

Do I need to keep paper receipts, or are digital copies fine? Digital copies are fine as long as they're clear and legible. Many people scan or photograph receipts and store them in folders or software. Keep backups, and retain records for at least five years after the 31 January filing deadline.

How far back can HMRC investigate my tax returns? Normally four years for routine checks. This extends to six years where HMRC suspects careless errors, and up to twenty years for deliberate errors or certain offshore matters.

Can I amend my tax return after submitting it? Yes. You can amend online within 12 months of the filing deadline, so by 31 January 2028 for a 2025/26 return. After that you need to write to HMRC, who'll decide whether to accept the change.

When should I use an accountant instead of doing it myself? Use an accountant if your affairs are complex, you run a limited company, you want tax-planning advice, you're short on time, or you're worried about costly mistakes. For the self-employed, the fee is tax-deductible, so the real cost is lower than it looks.