Map who can tax what
Overlap is normal, not a mistake. A UK resident with a rental flat in Spain is taxed on the rent by the UK, which taxes residents on worldwide income, and by Spain, where the property sits. The first job in any multi-country position is a map: for each country you touch, does it think it can tax this income, through residence or a local source? Residence itself is decided separately by each country, the UK using the Statutory Residence Test, with 183 UK days making you automatically resident and the sufficient ties test weighing connections against day counts below that. Where two countries both claim you, the treaty tie-breaker decides which treats you as resident for treaty purposes, though the other can still tax local-source income. Companies follow the same logic: incorporation or central management and control fixes where they are resident, and a permanent establishment can add a filing obligation somewhere else.
Stop the same income being taxed twice
Double taxation is unwound through treaties and Foreign Tax Credit Relief. The UK has one of the widest treaty networks in the world: a treaty can cut tax at source, for example by capping withholding on dividends or interest, and frames the relief for whatever overlap remains. FTCR is claimed when you report the overseas income on your Self Assessment return, and the credit is capped at the lower of the foreign tax paid and the UK tax due on that income; where FTCR is not the best route you can claim deduction relief instead, and HMRC helpsheet HS263 sets out the calculation. Even with no treaty, unilateral relief is usually available. The claims are only as good as the evidence, so foreign tax certificates, matched to the right income, currency and tax year, are what make double tax relief actually stick.
Run everything from one compliance calendar
Most cross-border failures are operational, not technical: a missed local deadline, an unclaimed relief, a residence position asserted in one country that contradicts the return filed in another, or a foreign VAT threshold crossed unnoticed. Indirect tax deserves particular vigilance because thresholds run on rolling periods, the UK's at £90,000 of taxable turnover, and other countries set their own, sometimes nil for non-resident sellers. The fix is a single calendar listing every return, payment date and supporting claim across every country, tracking dependencies as well as dates, since a UK FTCR claim often cannot be finalised until the foreign tax figure is settled. That is the model behind Horizon's Global Compliance Manager service: one adviser owns the whole position and local specialists feed into one calendar. Horizon works on fixed fees agreed upfront, and a free clarity call is the simplest way to get your own map started.