That self assessment tax return has been sitting in your mental to-do list for weeks now, hasn’t it? You know you need to do it, you know the self assessment deadline is looming, but every time you think about starting, it feels overwhelming. I’ve walked hundreds of people through this process over fifteen years. Some were first-timers, some had been putting it off for months, and some just wanted someone to explain it in simple way. This guide shows you how to complete a self assessment tax return from start to finish
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What You Need Before You Start
Don’t even think about logging in until you’ve got everything ready. Trust me, nothing’s more frustrating than getting halfway through and realizing you need to hunt down a P60.
Here’s what you need on your desk: your UTR (Unique Taxpayer Reference) number, National Insurance number, any P60s or P45s from employment, all your invoices if you’re self-employed, receipts for business expenses, bank statements showing business income, and details of any other income like property rental or dividends.
First time doing a tax return self assessment? Add another hour to however long you think this will take. It’s not difficult, just unfamiliar. Once you’ve done it once, subsequent years are much quicker.
Logging Into HMRC Self Assessment
Right, you’ve got everything ready. Now you need to access the hmrc self assessment login portal. Go to gov.uk and search for “self assessment tax return login” or navigate directly to the Government Gateway sign-in page.
You’ll need your Government Gateway user ID and password. If you’ve forgotten your password, reset it now before you go any further. If you’re doing this for the first time, you should have received an activation code in the post when you registered. This can take up to 10 working days, so if you’re reading this close to the deadline and you haven’t registered yet, you need professional help immediately.
Once you’re logged in, find your tax return for the relevant year. The system shows you which years need completing. For the 2024-25 tax year (6 April 2024 to 5 April 2025), you’ll be filing by the self assessment deadline of 31 January 2026.
- Login issues? Can’t find your UTR?
- Registration delayed?
- TaxGuru can help you navigate HMRC systems and get your return filed on time.
The Employment Section: If You’re Employed
If you received a P60 or P45 during the tax year, you need to complete the employment section. This is straightforward—you’re basically copying numbers from your P60 into the boxes.
Your P60 shows your total pay before tax, the tax you’ve already paid through PAYE, and any student loan deductions. Enter these figures exactly as shown. The HMRC self assessment system cross-references this with what your employer reported, so make sure the figures match. If you had multiple jobs during the year, you’ll need a P60 or P45 from each employer. Enter them separately in additional employment sections
The Self-Employment Section: For Sole Traders and Freelancers
This is where most people spend the bulk of their time when learning how to complete a self assessment tax return. If you’re self-employed, you need to declare your business income and expenses.
Start with your turnover. Don’t subtract anything yet, just the total income. If you invoiced £45,000 during the year, that’s your turnover even if some invoices haven’t been paid yet (unless you use cash basis accounting, which most small businesses do).
Next, your allowable expenses. This is everything you spent wholly and exclusively for the business. Office supplies, equipment, software subscriptions, professional insurance, travel between business locations, marketing costs, accountancy fees, training courses directly related to your business.
Common expenses people miss: professional subscriptions, business bank charges, use of home as office (simplified rate is £6 per week if you work from home regularly), mileage at 45p per mile for the first 10,000 miles.
The system calculates your profit automatically once you’ve entered income and expenses. This is what you’ll pay tax on, not your turnover.
- Not sure what expenses you can claim?
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Other Income You Need to Declare
Property rental income goes in the UK property section. You’ll enter your rental income and allowable expenses like mortgage interest (though rules have changed on this, only a portion is allowable now), repairs, insurance, and letting agent fees.
Interest from savings accounts needs declaring if it’s over your personal savings allowance. Dividends from shares also need reporting if they exceed your dividend allowance.
If you’ve got income from overseas, there’s a foreign income section. This can get complicated with foreign tax credits, so this is often where people call in professional help.
Don’t Forget Allowances and Reliefs
The tax self assessment system gives you your personal allowance automatically (£12,570 for most people), but there are other reliefs you need to claim manually.
Made pension contributions? Claim tax relief on those. Given to charity through Gift Aid? That extends your basic rate band, which can save you tax if you’re a higher rate taxpayer.
Marriage allowance lets you transfer some personal allowance to your spouse if one of you isn’t using it all. Claim it in the “Other tax reliefs” section.
Common Mistakes I See Every Year
Putting your gross income where net income should go, or vice versa. Read the box labels carefully.
Missing entire sections that apply to you. The system only shows sections you tell it to show, so if you’ve got rental income, make sure you’ve added the property pages.
Forgetting you had a second job or some freelance work you did months ago. HMRC knows about it because your client reported it. You need to declare it too.
- Claiming expenses that aren’t allowable.
- Your daily commute to your regular workplace? Not allowable. Gym membership?
- Not allowable unless you’re a personal trainer.
- Clothes for work?
- Not allowable unless they’re specialist protective equipment or uniforms with company logos.
Reviewing and Submitting Your Return
Before you hit submit, check your tax calculation. The system shows you exactly how it’s worked out your tax bill. Does it look reasonable based on what you earned?
If you’re paying on account from a previous year, you’ll see that deducted. Your final balance might be lower than expected, or you might owe more if your income increased significantly.
The system won’t let you submit if there are obvious errors, but it can’t catch everything. Double-check your figures, especially anywhere you typed numbers manually.
When you’re ready, hit submit. You’ll get a confirmation screen with a reference number. Screenshot this or write it down. You’ll also get an email confirmation from HMRC within a few hours.
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How to Pay Your Self Assessment Bill
The self assessment deadline for payment is the same as filing: 31 January. You can pay self assessment bills through online banking, debit or credit card on the HMRC website, by Direct Debit, or through bank transfer.
Critical: you need your payment reference number. This is your UTR followed by a letter K. Without this, HMRC won’t know the payment is from you.
Online payments through banking apps are usually fastest. Just search for “HMRC Self Assessment” in your payee list, enter your reference and amount, and it’s done.
Can’t pay the full amount? You can set up a payment plan with HMRC if you owe less than £30,000 and contact them before the deadline. They’re surprisingly reasonable if you’re honest about your situation.
Miss the payment deadline and you’ll get hit with penalties plus daily interest. The penalties stack up: 5% of the unpaid tax immediately, another 5% after six months, another 5% after twelve months.
After You’ve Submitted
Keep all your records for at least five years after the 31 January deadline. HMRC can investigate going back that far, and you’ll need those receipts and invoices to justify your figures.
If you’re paying on account, mark your calendar for 31 July. That’s when your second payment is due. Miss it and you’ll get penalty notices.
HMRC might query your return, especially if something looks unusual compared to previous years or compared to others in your industry. If they write to you, don’t panic. Usually they just want clarification or proof of specific expenses. Respond within the deadline they give you.
You’ve Completed Your Return
Learning how to complete a self assessment tax return isn’t fun, but it’s not the monster it seems either. Break it into sections, take your time, double-check your figures, and you’ll get through it.
Next year, start keeping better records from day one. A spreadsheet updated monthly is infinitely easier than a shoebox of receipts tackled in January. Future you will be grateful.
And if you’ve read this far and you’re thinking “this is still too much,” that’s fine. That’s literally what accountants are for. At TaxGuru, we complete hundreds of self assessment tax returns every year. We know the system, we know what HMRC looks for, and we make sure you’re not leaving money on the table through missed expenses or unclaimed reliefs.
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- TaxGuru offers fixed-fee tax return self assessment services with no hidden costs.
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- Get your free quote today and stop worrying about the self assessment deadline.
