Self Assessment Tax Return UK: Deadlines, Contact Numbers & How to File (2026 Guide)

If you need to complete a self assessment tax return in the UK, it’s important to understand what’s required, when it’s due, and how to avoid penalties.

Each year, millions of people submit a return to HM Revenue & Customs — but many still miss deadlines, make errors, or overpay tax simply because they don’t fully understand the process.

This guide explains everything clearly, including who needs to file, key deadlines, how to complete your return, and where to get help if you need it.

What Is a Self Assessment Tax Return?

A self assessment tax return is how individuals report income that hasn’t been taxed automatically.

Unlike PAYE (where tax is deducted from your salary), self assessment requires you to calculate and declare your own income and tax liability.

You may need to complete a tax return if you:

• Are self-employed or a sole trader

• Earn income from a side hustle

• Receive dividends from a company

• Have rental income from property

• Earn over £100,000 per year

• Have capital gains to report

Even if you already pay tax through employment, additional income can trigger the need to file.

Who Needs to Submit a Tax Return?

One of the most common questions is whether you actually need to file.

You must submit a return if HMRC has requested one, or if your income meets certain thresholds.

Typical scenarios include:

Self-employed individuals

If you run your own business, you must report your income and expenses each year. There are also new rules relating to MTD for ITSA which you can find more information about here.

Please check this guide out to check if you may need to register as self-employed.

Company directors

If you take dividends from your company, these must be declared.

Landlords

Rental income must be reported, even if you already pay tax elsewhere.

Side hustles and additional income

If you earn more than £1,000 from side income (before expenses), you usually need to register.

This is where many people get caught out — especially with online income, freelancing, or small businesses.

Self Assessment Deadlines (Critical Dates)

Deadlines are one of the biggest drivers of penalties, so getting this right is essential.

Key dates to remember:

• 5 October – Register for self assessment (if new)

• 31 October – Paper tax return deadline

• 31 January – Online tax return deadline

• 31 January – Tax payment deadline

The 31 January deadline is the most important. This is when both your tax return and your payment are due.

What Happens If You Miss the Deadline?

Late filing penalties apply automatically.

Here’s what you can expect:

• £100 fine immediately after the deadline

• Daily penalties after 3 months

• Additional penalties after 6 and 12 months

• Interest charged on unpaid tax

Even if you have no tax to pay, you can still be fined for submitting your return late.

How to Complete a Self Assessment Tax Return

Filing your return isn’t as complicated as it seems — but it does require accurate records.

Step 1: Gather your information

You’ll need:

• Your Unique Taxpayer Reference (UTR)

• Income details (self-employed, PAYE, dividends, etc.)

• Expense records

• Bank interest and other income

Step 2: Report your income

You must declare all taxable income, including:

• Business income

• Employment income

• Dividends

• Property income

• Interest

Step 3: Add allowable expenses

Expenses reduce your taxable profit.

Common allowable expenses include:

• Office costs (software, phone, internet)

• Travel costs

• Equipment

• Professional fees

Claiming the correct expenses is one of the easiest ways to reduce your tax bill legally.

Step 4: Calculate your tax bill

Once income and expenses are entered, your tax liability is calculated automatically.

This includes:

• Income tax

• National Insurance

• Student loan repayments (if applicable)

Step 5: Submit your return and pay

After submitting, you’ll see your final tax bill.

Payment is due by 31 January.

Self Assessment Contact Number and Helpline

If you run into issues, you may need to contact HM Revenue & Customs directly.

Common support options include:

• Online help via your tax account

• Webchat services

• Telephone support

When to contact HMRC:

• You’ve lost your UTR

• You can’t access your account

• You’ve received a penalty

• You need to amend a return

Tip: Avoid peak times (January is extremely busy).

Common Self Assessment Mistakes to Avoid

Mistakes can lead to penalties, overpaid tax, or HMRC enquiries.

Here are the most common issues:

1. Missing income

All income must be declared — even small amounts.

2. Incorrect expense claims

Not all expenses are allowable. Personal costs cannot be claimed.

3. Filing late

This triggers automatic penalties.

4. Not registering in time

If you miss the 5 October deadline, you could face fines.

5. Guessing figures

Always use accurate records — estimates can lead to problems later.

Do You Need to Register First?

If you’re new to self assessment, you must register before submitting your first return.

You’ll then receive:

• A UTR (Unique Taxpayer Reference)

• Access to your tax account

If you’re unsure, read our full guide on registering as self-employed.

When Should You Get Professional Help?

Many people try to complete their return themselves — and that’s fine for simple situations.

However, professional support is worth considering if you:

• Have multiple income streams

• Run a limited company

• Are unsure what expenses to claim

• Want to reduce your tax bill legally

• Have received a letter from HMRC

An accountant can often save more than they cost by ensuring your return is accurate and tax-efficient.

How to Reduce Your Tax Bill Legally

There are several legitimate ways to reduce your tax liability:

• Claim all allowable expenses

• Use your personal allowance fully

• Make pension contributions

• Split income efficiently (where applicable)

This is where planning makes a big difference — not just filing.

Self assessment doesn’t have to be stressful — but it does require organisation and attention to detail.

The key things to remember are:

• Know whether you need to file

• Don’t miss deadlines

• Keep accurate records

• Get help if you’re unsure

Done correctly, your tax return becomes a routine process rather than a last-minute panic.

Need Help With Your Tax Return?

If you’d rather not deal with the stress of self assessment, professional support can make the process much easier.

From preparing your return to ensuring you claim everything you’re entitled to, getting expert help can save time, money, and hassle.