Do You Owe Taxes in Two Countries? Understanding the US/UK-Spain Tax Treaties

"Double taxation treaty" sounds like it should mean you never pay tax twice. In practice, these treaties allocate taxing rights between two countries and provide relief mechanisms — usually a tax credit — rather than eliminating one country's claim outright. The US and UK treaties with Spain work on different logic, and mixing them up is a common and costly mistake among expats managing their own filings.

Step one: figure out where you are tax resident

Both treaties hinge on residency. Under Spanish domestic law, you are generally considered a Spanish tax resident if you spend more than 183 days in Spain during the calendar year, or if your main center of economic interests is in Spain. Once you are Spanish tax resident, Spain has the right to tax your worldwide income — not just income earned in Spain. This is the trigger point that makes treaty relief relevant in the first place.

The US-Spain treaty and the "saving clause"

The US-Spain tax treaty contains what is known as a saving clause, which preserves the US government's right to tax its citizens and residents as though the treaty did not exist. In plain terms: becoming a Spanish tax resident does not release a US citizen from US filing obligations, and treaty provisions that might otherwise reduce US tax often do not apply to US citizens because of this clause.

What does provide relief is the Foreign Tax Credit mechanism, separate from the treaty itself, which lets US taxpayers credit income tax paid to Spain against their US tax liability on the same income, category by category. For most Americans living and working in Spain, this credit — not the treaty's residency tie-breaker rules — is what prevents the same salary or self-employment income from being taxed in full twice. Some taxpayers instead use the Foreign Earned Income Exclusion, though the two mechanisms interact in ways that depend on individual circumstances.

US citizens should also remember that treaty relief and the Foreign Tax Credit do not remove separate reporting duties — see our guide on FATCA and Spanish tax residency for FBAR and Form 8938. Reference material is published by the IRS.

The UK-Spain treaty: no saving clause, different mechanics

The UK-Spain double taxation agreement does not carry the same citizenship-based saving clause, because the UK taxes based on residency rather than citizenship. This changes the practical outcome significantly for Britons:

• Once you are no longer UK tax resident (broadly, once you have been outside the UK long enough and meet the Statutory Residence Test conditions for non-residence), the UK generally stops taxing your worldwide income.
• The treaty then allocates specific income types between the UK and Spain. The UK State Pension, for instance, is generally taxable only in Spain once you are Spanish tax resident — not in the UK.
• Government/public service pensions (such as a former civil servant's pension) are typically the exception — these usually remain taxable in the UK even after you become Spanish resident, under the treaty's specific pension article.
• UK rental income and other UK-source income can remain partly taxable in the UK even after you move, depending on the income type — this is one of the more commonly misunderstood points.

See official guidance at GOV.UK / HMRC. UK retirees relying on the State Pension should also review our guide to the S1 form for healthcare access, since pension and healthcare timing decisions often overlap.

Practical comparison

Where both nationalities need to be careful

Neither treaty automatically applies itself — you generally need to actively claim relief on your return, keep documentation of tax paid in the other country, and in some cases file specific certificates of residency. If you are self-employed as an autónomo, or if you qualify for Spain's Beckham Law regime, the interaction with these treaties changes further, since the Beckham Law's foreign-income exemption and the treaty's relief mechanisms operate on different logic and generally are not meant to be stacked in a way that produces double relief.