Self-Assessment Tax Return Explained for Beginners

• Register for Self Assessment (if necessary) by 5 October following the end of the tax year. 
• Paper submissions (SA100) must be received by 31 October.
• Online submissions must be done by 31 January following the end of the tax year. 
• Payment of tax owed is also due by 31 January. 
• Payments on account: in some cases you’ll also make advance payments toward the next year’s tax by 31 July and 31 January. 

Missing deadlines can trigger penalties and interest. 

Main Form (SA100) and Supplementary Pages

The primary form is SA100, which covers income from:

• Salaries and employment
• Pensions and government benefits
• Interest and dividends
• Pension contributions
• Reliefs and allowances

If you have additional income types, you will also need supplementary pages:

• SA103 for self-employment
• SA105 for property income
• SA108 for capital gains
• Others depending on foreign income, trusts, etc. MaPS+2Simply Business UK+2

Step-by-Step Filling Tips

1. Work through the helpsheets provided by HMRC alongside the form. Vladina usluga publikacija+1

2. Enter income amounts (gross) before deduction of tax

3. Claim allowable expenses and reliefs correctly

4. Double-check your arithmetic: many mistakes stem from data entry errors

5. In online form, you can save progress and return later

6. Review the return thoroughly before submission

If you make a mistake after submitting, you can correct it within a certain period (usually up to 12 months) via HMRC’s amendment process. MaPS+2GOV.UK+2

• Online submissions are preferred and offer more time and flexibility. GOV.UK+3GOV.UK+3Vladina usluga publikacija+3
• Paper returns are becoming more obsolete and are limited to certain cases. GOV.UK+2Vladina usluga publikacija+2
• Payment methods: bank transfer, CHAPS, debit card, or online banking. MaPS+2Vladina usluga publikacija+2
• In some cases, you can arrange a “Time to Pay” plan if you can’t pay in full by the deadline. MaPS+1

One of the most confusing areas for beginners is knowing what you can and cannot claim as expenses. In the Self-Assessment Tax Return, allowable expenses reduce your taxable income, which lowers the overall tax you pay.

For the self-employed, common allowable expenses include:

• Office supplies, laptops, software, and internet costs used for work
• Travel expenses such as mileage, fuel, parking (not commuting)
• Professional services: accountants, legal fees, business insurance
• Marketing expenses: website hosting, ads, printed materials
• Home office costs: a proportion of rent, utilities, heating, council tax if working from home

For landlords, allowable expenses include:

• Repairs and maintenance of property (but not improvements)
• Letting agent fees and property management costs
• Insurance premiums for rental property
• Mortgage interest (restricted to basic rate relief)
• Council tax, water, and utilities if paid by landlord

For investors or others, costs related to investment advice or portfolio management may sometimes be offset.

A beginner should remember: only claim expenses that are “wholly and exclusively” for business. Claiming personal costs can lead to HMRC penalties. A professional advisor ensures your Self-Assessment Tax Return captures the right expenses without risk.

It’s easier to understand the process by looking at real scenarios.

Case Study 1: The Freelancer
Tom is a London-based web designer who earns £45,000 from clients. His income is not taxed under PAYE. He registers for Self-Assessment, keeps receipts for software, laptop, and travel. By claiming £5,000 in allowable expenses, his taxable profit reduces to £40,000, saving him hundreds in tax. Without the Self-Assessment Tax Return, he wouldn’t have been able to offset those expenses.

Case Study 2: The Landlord
Sarah owns a flat in Manchester, rented out for £12,000 annually. She spends £2,000 on repairs and £1,000 on insurance. In her Self-Assessment Tax Return, she declares £12,000 income but deducts £3,000 in allowable expenses, leaving £9,000 taxable. She pays income tax at her marginal rate, but only on the net profit.

Case Study 3: The Side-Hustle Entrepreneur
Amir works full-time under PAYE but also sells handmade crafts online, earning £6,000 a year. Because this exceeds the £1,000 trading allowance, he must file a Self-Assessment Tax Return. By claiming his packaging and website fees as expenses, he reduces his taxable income and avoids penalties.

These examples show how different individuals—from full-time freelancers to landlords to hobbyists—use Self-Assessment to stay compliant and reduce their liabilities.

Missing or incorrect Self-Assessment Tax Returns can be costly. HMRC applies:

• Late filing penalty: £100 immediately after the deadline, even if you owe no tax
• Additional daily penalties if over three months late
• Interest on unpaid tax from the due date
• Larger penalties (up to 100% of unpaid tax) for deliberate concealment

For beginners, the message is clear: file on time and accurately. If unsure, seek help early. Many London residents underestimate HMRC’s penalty regime—yet it’s easy to avoid with preparation.

The Self-Assessment Tax Return system is undergoing significant transformation under HMRC’s Making Tax Digital (MTD) initiative.

From 2026, self-employed people and landlords earning above £50,000 annually will need to file quarterly digital updates rather than a single annual return. By 2027, the threshold lowers to £30,000. This means:

• Keeping digital records of all income and expenses
• Using MTD-compatible software to link directly with HMRC
• Submitting four quarterly updates plus a final year-end statement

For beginners, this might sound like more admin. But in reality, it spreads the workload, prevents last-minute stress, and gives a clearer picture of cash flow throughout the year.

In addition, new technologies are reshaping Self-Assessment:

• AI bookkeeping tools that scan receipts and automatically categorise expenses
• Cloud accounting platforms that sync with bank accounts to track income
• Advisory dashboards that forecast your tax bill months in advance

The future Self-Assessment Tax Return will be less about paperwork and more about proactive financial management. Early adopters of digital tools gain peace of mind and fewer errors.