Self-Assessment Tax Return: A Step-by-Step Guide
Tax doesn't have to be confusing. Whether you're a sole trader, limited company director, or just trying to understand your payslip, this guide explains self assessment in plain English with practical examples and real numbers.
Understanding Self assessment
The UK and US tax systems work differently, but both share a common principle: you only pay tax on income above certain thresholds, and different portions of your income are taxed at different rates.
UK tax bands (2025/26):
- Personal allowance: £0–£12,570 at 0%
- Basic rate: £12,571–£50,270 at 20%
- Higher rate: £50,271–£125,140 at 40%
- Additional rate: £125,141+ at 45%
US federal brackets (2025, single filer):
- 10% on income up to $11,600
- 12% on $11,601–$47,150
- 22% on $47,151–$100,525
- 24% on $100,526–$191,950
- Higher brackets continue up to 37%
These are marginal rates — only the income within each band is taxed at that rate. Someone earning £60,000 in the UK doesn't pay 40% on everything; they pay 40% only on the £9,730 above £50,270.
Our sales tax calculator handles all of this automatically so you can see your exact liability.
Practical Tips for Self assessment
For employees: Check your tax code on your payslip. The standard code 1257L means you get £12,570 tax-free. If yours is different, HMRC may be applying adjustments for benefits, underpayments, or allowances. An incorrect code means you're overpaying or underpaying tax every month.
For self-employed: HMRC's Self Assessment deadlines are 31 January (tax return and payment) and 31 July (second payment on account). Set aside 25–30% of your profits for tax throughout the year — waiting until January to find the money is how people get into trouble.
For businesses: VAT registration is mandatory once your taxable turnover exceeds the £90,000 threshold. Below that, voluntary registration can be beneficial if your customers are VAT-registered businesses, but it adds admin burden. Use our VAT calculator to check the impact.
For everyone: Keep records. Bank statements, receipts, invoices — HMRC can enquire into your tax affairs up to 4 years back (6 years if they suspect carelessness, 20 years for deliberate evasion). Digital records are fine; you don't need paper.
Common Tax Mistakes
- Not claiming allowable expenses — working from home, professional subscriptions, mileage, equipment — many employees miss tax relief they're entitled to. Self-employed people often forget to claim phone bills, insurance, and training costs.
- Missing the marriage allowance — if one partner earns under £12,570 and the other is a basic rate taxpayer, transferring 10% of the personal allowance saves £252 per year. It takes 5 minutes to apply online and can be backdated 4 years.
- Ignoring pension tax relief — pension contributions get tax relief at your marginal rate. A £100 pension contribution only "costs" £80 for a basic rate taxpayer, or £60 for a higher rate taxpayer. It's the most tax-efficient way to save.
- VAT errors — charging VAT when you shouldn't be registered, or not charging it when you should be, both create problems. Check your registration status regularly.
Run Your Numbers
Use our sales tax calculator to see exactly how much tax you owe and where you might be able to reduce it. If you're self-employed or run a business, our break-even calculator and ROI calculator can help with business planning decisions.