Self Assessment Tax Return: HMRC Rules, Warnings & How to File
Table of Contents
What is a Self-Assessment Tax Return?
Who Needs to File a HMRC Tax Return Self-Assessment?
Registering for Self-Assessment and Getting a Tax Return
How to File Your Self-Assessment Tax Return
Self-Assessment Tax Return & Pension Contributions
CIS Self-Assessment Tax Return
How to View Past Self-Assessment Tax Returns
Why Hire a Self-Assessment Tax Return Accountant?
Filing taxes can be confusing, and if done wrong, it can cost you penalties and a lot of stress. However, with some help from professionals like Fincsol, you can make the Self Assessment Tax Return process faster, easier, and far less stressful.
Experienced accountants not only guide you through the HMRC rules but also make sure you claim all the reliefs and deductions you are entitled to, avoid common mistakes, and submit everything well before the deadline. This means more peace of mind for you and potentially more money saved in the long run.
30 Seconds Summary
- It is the HMRC system for reporting income not taxed at source, such as self‑employment, rental income, or foreign earnings. You must file if you meet certain criteria, like earning over £1,000 from self‑employment or over £2,500 from a rental property.
- For the 2024/25 tax year, you need to register by 5 October 2025, file paper returns by 31 October 2025, and online returns by 31 January 2026. You will need forms like SA100 and possibly SA103 or SA105, along with your UTR, NI number, income, and expense records.
- You can reduce your tax bill by claiming allowable expenses, pension contribution relief, and Construction Industry Scheme (CIS) deductions.
What is a Self-Assessment Tax Return?
If you are employed in the UK, chances are your taxes are automatically deducted through PAYE (Pay as You Earn). This system is simple; you get paid after tax and National Insurance have been deducted.
However, if you are self-employed, earn some extra income, or have untaxed money coming in, HMRC won’t know about it. Self-assessment tax return is a system HM Revenue and Customs (HMRC) uses to collect such taxes.
It is your legal responsibility to tell HMRC:
- How much income did you earn in the year
- What tax have you already paid
- Any allowable expenses you want to deduct
- Any reliefs you wish to claim
Once you submit, HMRC works out how much tax you owe or if you’ve overpaid and are due a refund.
Who Needs to File a HMRC Tax Return Self-Assessment?
Not everyone needs to file a self-assessment tax return. But if you are in one of the following groups, you must submit a Tax Return:
- Self-employed and earned over £1,000 in the tax year.
- You are a company director (non-charity).
- Rental income over £2,500 a year.
- Savings/investments income over £10,000 before tax.
- Capital Gains Tax is due.
- You or your partner earns £60,000+ and claims Child Benefit.
- You have untaxed income from abroad.
If you don’t file when required, penalties apply, and interest may be charged.
Registering for Self-Assessment and Getting a Tax Return
If this is your first time, you must register before you can file.
Here is how to do it:
- Visit the official www.gov.uk Self Assessment Tax Returns page.
- Choose your category, i.e., self-employed, not self-employed, or partnership.
- Fill in your details online.
- HMRC will send your Unique Taxpayer Reference (UTR) in the post.
- Use your UTR to create a Government Gateway account.
- Activate your account using the code HMRC sends.
Tip: This process can take 10 working days (or longer if you are abroad), so start early. Without your UTR and login, you can’t file.
Deadlines You Can’t Miss
The UK tax year runs from 6 April to 5 April of the next year. For the 2024/25 tax year, you cannot miss the following deadlines:
- Register by 5 October 2025 (if it is your first return).
- Paper return deadline: 31 October 2025 (midnight).
- Online return deadline: 31 January 2026 (midnight).
- Tax payment deadline: 31 January 2026.
- Second payment on account: 31 July 2026 (if required).
If you miss these deadlines, you’ll get an instant £100 fine. And if you leave it longer, the penalties will grow, plus the interest on unpaid tax.
How to File Your Self-Assessment Tax Return
You can file online via HMRC’s website or approved tax software. The main form is SA100, but you might also need extra pages:
- SA103 for self-employment
- SA105 for UK property income
- SA108 for Capital Gains
To file for your self-assessment tax return, you will need the following:
- Your UTR and National Insurance number
- Income records (invoices, payslips, bank statements)
- Expense receipts
- P60, P45, P11D forms if applicable
- Pension contribution details
- Charity donation records
Once you enter your details, the system will calculate your bill instantly. You can then pay online via bank transfer, debit card, or Direct Debit.
Allowable Expenses
You can also claim tax back on certain business expenses, which reduces the profit on which you have to pay tax. For the self-employed individuals, this includes:
- Business travel (not your commute)
- Office costs (rent, utilities, internet, phone)
- Stock or raw materials
- Marketing and advertising
- Staff wages and subcontractors
- Professional fees (like a Self-Assessment Tax Return Accountant)
And for landlords:
- Repairs and maintenance
- Letting agent fees
- Insurance
- Loan interest
Self-Assessment Tax Return & Pension Contributions
When you pay into a private pension, you can usually get tax relief, which means that the government adds money to your pension scheme to help it grow. This applies to most of the registered schemes, including:
- Workplace pensions
- Personal and stakeholder pensions
- Certain overseas pensions that meet UK rules for tax relief (ask your provider if yours qualifies)
How much can you contribute tax‑free?
You can normally get tax relief on pension savings up to:
- 100% of your yearly earnings, or
- £60,000 a year (this is called your annual allowance, and the lower figure applies if you earn less)
You may have a lower allowance if you are a very high earner.
You will have to pay tax on contributions if:
- Your savings go over the allowance
- Your provider is not registered for tax relief
- Your provider does not invest your pension according to HMRC’s rules
Claiming tax relief through Self-Assessment
For most people, your provider automatically claims 20% basic rate relief and adds it to your pension. For example:
If you pay £8,000, your provider adds £2,000 from HMRC, so £10,000 goes into your pension pot.
If you are a higher‑rate (40%) or additional‑rate (45%) taxpayer, you can claim the extra 20% or 25% by entering your contributions on your Self-Assessment Tax Return. You must include the gross amount (your payment plus the basic rate top‑up) in the ‘Tax reliefs’ section of the SA100 form.
CIS Self-Assessment Tax Return
If you work in construction for a contractor and you’re self‑employed, as a sole trader, the owner of a limited company, or a partner in a business. You should be registered under the Construction Industry Scheme (CIS).
Under CIS, your contractor will usually take 20% of your pay and send it to HMRC. This is called net payment status, and it counts as an advance towards your tax and National Insurance.
If you don’t register, the deduction is higher, i.e., 30%.
You can also apply for gross payment status, which means contractors pay you in full without taking tax off in advance. You’ll then pay all your tax and National Insurance at the end of the year.
Even if CIS tax is deducted from your pay, you must still complete a Self-Assessment Tax Return each year. This lets HMRC check your total income, confirm the correct tax amount, and refund you if too much was taken.
Many CIS workers receive a refund because deductions are often more than what they actually owe.
How to View Past Self-Assessment Tax Returns
You might need to access your previous self-assessment tax returns for several reasons, such as for a mortgage or a loan. Here is how you can view your past self-assessment tax returns:
- Visit HMRC online services and sign in to your account.
- Use your Government Gateway login details to sign in.
- Go to self-assessment.
- Click on View and manage your self-assessment tax returns and choose your tax year.
- Continue to your SA302
You can then view, download, or print your previous tax returns.
Why Hire a Self-Assessment Tax Return Accountant?
Here are a few reasons why you should consider hiring a self-assessment tax return account. A professional can:
- Spot deductions you might miss
- Prevent costly errors
- Handle complex returns (multiple incomes, foreign earnings, capital gains)
Over to You
Filing your Self-Assessment Tax Return on time avoids penalties and keeps you compliant.
Need help making sense of your CIS or Self Assessment tax? Let Fincsol handle it for you. Our expert accountants and bookkeepers make sure every deduction is correct, every deadline is met, and you never miss a refund you’re owed.
Contact Fincsol today!
FAQs
Q: What is a self-assessment tax return?
A: Self-assessment tax return is a system used by HMRC that lets businesses and people who earn extra income report it using self-assessment tax returns.
Q: Who needs to file an HMRC Self-assessment tax return?
A: You need to file if you:
- Self-employed and earned over £1,000 in the tax year.
- You are a company director (non-charity).
- Rental income over £2,500 a year.
- Savings/investments income over £10,000 before tax.
- Capital Gains Tax is due.
- You or your partner earns £60,000+ and claims Child Benefit.
- You have untaxed income from abroad.
Q: Can I change my Self-Assessment tax return after I’ve filed it?
A: Yes, you can change your self-assessment tax return if you made a mistake; you can update your return within 12 months of the deadline. Your bill will then be updated. You might need to pay more tax, or you could get a refund.