Introduction
The UK’s self-assessment system is how individuals and trustees tell HM Revenue & Customs (HMRC) about their taxable income and gains. For the 2024/25 tax year, covering earnings from 6 April 2024 to 5 April 2025, the online filing and payment deadline is 31 January 2026. Missing this date invites automatic penalties and interest.
This article speaks to those whose tax affairs are anything but straightforward: high-net-worth individuals with diverse portfolios, non-UK residents owning UK property or earning UK income, UK residents with foreign income, trustees and estate executors, and workers who split their careers across borders. By taking timely, informed steps—collecting documents, understanding what to report and when, and obtaining expert advice—you can meet your obligations confidently, preserve your wealth and take advantage of the reliefs offered by law.
Who Needs to File?
Anyone whose tax cannot be fully deducted at source must file a self-assessment return. Landlords who earn rent from letting UK property must submit a tax return, regardless of residence, as do individuals with high employment income, significant interest or dividends, or directors receiving dividends. If you receive money from trusts, estates or settlements, you also have to report it.
People who live abroad but have UK income, such as non-resident landlords and cross-border workers, remain liable to UK tax. Conversely, UK residents must disclose their worldwide income, including foreign employment, overseas rentals, pensions, business profits, dividends and gains. Those who move to or from the UK within the year may need to apply the split-year rules to divide their tax year into resident and non-resident parts, then file a return accordingly. Finally, anyone claiming double taxation relief must complete a return to provide HMRC with full details and supporting evidence.
Planning for High-Net-Worth Individuals
High-net-worth individuals seldom have simple tax profiles. Multiple properties produce rent, investments yield dividends and interest, and ownership stakes in companies create distributions that may be taxed differently. Some hold assets in discretionary trusts, which are charged at the additional rate of 45% on most income beyond the £500 tax-free allowance, while others benefit from interest-in-possession trusts, where the trustee pays tax at the basic rate and the beneficiary may bear further tax if the income is mandated to them. Understanding these distinctions is crucial when preparing your return.
At significant income levels, other factors come into play. The personal allowance (£12,570 for most taxpayers) tapers down once your adjusted net income exceeds £100,000; each extra £2 of income reduces the allowance by £1, effectively increasing your marginal rate. Individuals earning over £50,000 and receiving child benefit must repay part or all of the benefit via the High Income Child Benefit Charge.
Capital gains from selling property or shares can be alleviated by Business Asset Disposal Relief or Private Residence Relief, but only when conditions are met. If you donate to charity through Gift Aid or make pension contributions, you may extend your basic-rate band and reduce your overall liability. At the other end of the spectrum, failing to disclose foreign assets or trusts can result in severe penalties. HMRC can investigate up to 20 years back and impose charges of up to 200% of unpaid tax.
Non-UK Residents With UK Income
Non-residents are generally taxed only on UK income. This includes rental income, UK-based employment or consultancy, profits from UK partnerships and UK dividends or interest. You won’t normally qualify for the personal allowance unless you are a UK or EEA national or a Crown servant.
If you pay tax on UK income both abroad and in the UK, you may be eligible for double-taxation relief. The UK has bilateral agreements with over 130 countries that allocate taxing rights and prevent the same income being taxed twice. Relief can be given by exempting UK tax entirely or by crediting foreign tax against your UK liability. To claim, you must provide details of tax paid abroad and, usually, a Certificate of Overseas Residence. Non-resident landlords must still file annual returns to report income or to access treaty relief; the SA109 helps to confirm your residency and domicile. First-time filers must register by 5 October following the end of the tax year.
The rules around disregarded income are also important. Certain forms of investment income received by non-residents, such as dividends and bank interest, may be taxed solely by withholding at source, limiting further UK liability. Others, like rent, require full self-assessment. Penalties apply for late filing or underpayment, so it is crucial to determine exactly what needs to be declared.
UK Residents Earning Abroad
If you live in the UK for tax purposes, you must declare worldwide income and gains. Many residents earn from overseas employment, remote work, rental properties or investments. Even if foreign tax has already been deducted, you must include the gross income on your UK return and then claim foreign tax credit relief.
Record-keeping is essential: keep documentation like payslips, rental statements and foreign tax certificates for at least five years. Convert all amounts into pounds with HMRC’s published exchange rates. Report employment and rental income on SA106, and report capital gains on SA108. If you incur losses abroad, you may be able to set these against foreign gains or carry them forward. Additionally, check if your income is covered by a UK DTA, as this may reduce UK tax. For non-domiciled residents, using the remittance basis can be tax-efficient, but it may result in losing personal allowances and facing an annual charge when resident for more than a few years. Professional advice helps you choose the right option.
Trusts and Estates
Many high-net-worth families place assets into trusts for asset-protection or succession planning. Trustees must file an SA900 when the trust receives income or realises gains above a small de minimis limit. Discretionary trusts are liable at high tax rates after the first £500 of income, while interest-in-possession trusts pay basic-rate taxes and often shift additional liability onto the beneficiary. If a settlor retains an interest in a trust (a settlor-interested trust), the settlor is generally taxed on all the trust’s income, even if nothing is paid out. For bare trusts, the beneficiary is treated as the owner of the underlying assets and must declare the income themselves. Executors of estates are subject to similar rules and file returns when estates hold income-producing assets; estates with less than £500 of income are exempt from tax. Given these complications and recent changes to low-income estate rules, expert assistance is invaluable.
The Benefit of Treaties
The UK’s DTAs ensure that income such as dividends, royalties, pensions and employment earnings is not taxed twice. Each treaty outlines whether income is taxable only in one country or whether a credit should be granted. For instance, some treaties specify that employment income is only taxable in the country where duties are performed, provided the worker is present in the other country for fewer than 183 days and is paid by a non-resident employer. Relief by credit allows you to deduct the foreign tax against your UK bill, while full exemption under a treaty means you pay tax only overseas.
To claim such relief, you must meet all treaty conditions and supply proof of foreign tax paid and residency status. Because treaties change and may provide different rules for each income type, professional interpretation is recommended. Claims must be submitted within specific time limits.
Internationally Mobile Employees
International assignments for Internationally Mobile Employees can offer career growth, but they complicate tax affairs. Your UK tax liability depends on residence: if you are UK resident, you are taxed on your global earnings; if you are non-resident, only your UK duties and UK-source income are taxed. Spending part of the tax year abroad can mean the split-year treatment applies. Most double-tax treaties provide relief so you are not taxed twice, usually limiting UK tax if your stay is short and you are paid by a non-UK employer.
Employers have to run PAYE on pay for UK duties. Under a Section 690 , employers can apply to HMRC to limit PAYE to the proportion of earnings linked to UK work—old agreements must be renewed under the new process. However, National Insurance contributions remain subject to separate rules. Global mobility often raises questions about social security contributions, employer compliance and personal tax reliefs; partnering with specialists helps ensure everything is handled correctly.
Deadlines and Penalties
Remember these dates: 31 October 2025 for paper returns, 30 December 2025 if you want tax collected via your PAYE code, 31 January 2026 for online filing and payment, and 31 July 2026 for the second payment on account if necessary. Late filing penalties start at £100 and increase if the delay exceeds three months; additional charges apply at six- and twelve-month intervals. HMRC can review your tax affairs for up to 20 years and impose severe penalties for deliberate non-compliance.
Recent Changes and Practical Tips
Significant developments affect filing for the 2024/25 year. From 6 April 2025, Section 690 applications for internationally mobile employees moved to a new digital process, and pre-April agreements expired automatically. Employers must ensure they reapply in order to withhold PAYE only on the proportion of salary relating to UK duties. Additionally, rule changes mean that low-income estates with total income under £500 are not taxed in the hands of the beneficiary, simplifying estate administration.
A few practical tips can make filing easier. Use digital tools to track income and expenses throughout the year; confirm whether overseas income is taxable or reportable; obtain necessary documents, including P60s, foreign tax certificates and R185s, well in advance; and register for HMRC’s online services so you can file and pay smoothly. When in doubt, seek clarity—the cost of a conversation with a professional is small compared with HMRC penalties.
How Spherical Can Assist
When your affairs involve multiple income streams, trusts or cross-border employment, a personalised strategy is vital. Spherical helps:
- High-net-worth clients manage their income, capital gains and succession plans, ensuring allowances and reliefs are used effectively.
- Non-UK residents handle UK tax registrations, non-resident landlord processes, self-assessment and double-tax claims. We assess eligibility for personal allowances and ensure compliance with treaty rules.
- UK residents with foreign earnings with accurate currency conversion, record-keeping, foreign tax credit relief and strategic decisions on the remittance basis.
- Trustees and executors in preparing SA900 returns, applying the appropriate tax rates for discretionary and interest-in-possession trusts, handling R185 forms and complying with low-income estate rules.
- Globally mobile employees and employers with Section 690 applications and cross-border NIC and PAYE issues, while interpreting relevant DTAs to avoid double taxation.
Final Thoughts
Self-assessment can be straightforward when your affairs are simple. For many people, however, it involves cross-border issues, trusts, high income or multiple properties. Understanding who must file, applying the Statutory Residence Test correctly, knowing how to report foreign income and trusts, and using double taxation agreements effectively, all help you avoid pitfalls. With the 31 January 2026 deadline looming, there is still time to organise your records and engage expert support. A modest investment in advice from Spherical can help you avoid penalties, reduce your tax bill and gain peace of mind.
