You are unlikely to be taxed twice on the same income — but not because of the 183-day rule, which is the most misunderstood idea in expat tax. Residency is decided in two steps: first each country applies its own domestic test (183 days is one common trigger, not a universal rule), and only if both countries then claim you does the treaty apply its tie-breaker order — permanent home, centre of vital interests, habitual abode, nationality, then mutual agreement. Relief comes through a foreign tax credit or an exemption. And a treaty almost never removes your duty to file: relief is claimed, not automatic.
Why would two countries both tax the same income?
Most countries tax people on two different bases. One is residence: if you live there, they generally want to tax your worldwide income. The other is source: if income arises within their borders — a salary, rent from a local property, a business profit — they often want a share, regardless of where you live. When you are an expat, these two principles can overlap. Your new home country may see you as a resident and tax everything, while your old country, or the country where an asset sits, still claims the income that originates there.
Left unmanaged, that overlap could mean the same euro, dollar or pound is taxed twice. Double taxation treaties (also called double tax agreements, or DTAs) are bilateral arrangements between two countries that divide up taxing rights to reduce this. There are many of them worldwide, and a large number follow a broadly similar structure based on widely used international models — though the detail of each treaty differs, so the specific treaty between your two countries is what matters. Treaties are also updated over time, so confirm the current text with a lawyer.
Is the 183-day rule really how tax residency works?
Almost everyone has heard that spending 183 days in a country makes you tax resident there. It is a real threshold — it appears in many countries' domestic law and in the employment-income article of most treaties — but taken alone it is wrong often enough to be dangerous. Two corrections matter:
1. Domestic law comes first, and it is not just a day count. Each country applies its own test, and a day count is only one trigger. Others include having a permanent home available to you, your centre of economic interests, or where your family lives — which is how people become resident somewhere they spent well under 183 days. The day-count itself is also defined differently: calendar year in one country, rolling 12 months in another, part-days counted here but not there.
2. The treaty only steps in if both countries claim you. A treaty does not decide residency from scratch; it resolves a conflict. If both countries call you resident under their own rules — which is entirely possible — the treaty then assigns you to one of them for treaty purposes, using tie-breakers applied strictly in order, stopping at the first that gives a clear answer:
When that happens, treaties typically apply a sequence of tie-breaker rules to assign you to just one country for treaty purposes. They are usually applied in order, stopping as soon as one gives a clear answer:
- Permanent home — the country where you have a home available to you on a lasting basis.
- Centre of vital interests — where your personal and economic ties (family, work, bank accounts, social life) are closest.
- Habitual abode — where you actually spend most of your time.
- Nationality — used if the earlier tests do not settle it.
- Mutual agreement — if all else is unclear, the two tax authorities may decide between themselves.
The exact wording and order can differ between treaties, so do not assume your situation is obvious. Where you keep a home, where your spouse and children live, and where your working life is centred can all tip the balance.
Most treaties contain a 183-day condition in the employment income article: your short-term work in another country stays taxable only at home if you are there under 183 days and your employer is not resident there and the cost is not borne by a permanent establishment there. All three must hold — meeting the day count alone is not enough, which is the trap for anyone working across borders on assignment.
How does a treaty stop me being taxed twice?
Deciding residency does not always remove tax in the other country entirely — some income may stay taxable at source. To help you avoid paying twice on that income, treaties commonly use one of two main methods of relief.
The credit method
Under a foreign tax credit, your resident country still taxes the income but generally lets you subtract the tax you already paid abroad. If you paid tax at a lower rate abroad, you may top up towards your home rate; if you paid more abroad, the credit is often capped at what your home country would have charged, so you may not recover the full excess. The precise mechanics vary by treaty and change over time, so confirm current rules with a lawyer.
The exemption method
Under the exemption method, your resident country may simply not tax income that the treaty assigns to the other country — though it might still count that income when working out the tax rate on your remaining income (sometimes called exemption with progression).
Which method applies depends on the treaty and the type of income. Rules on rates, caps and eligible taxes change over time, so confirm the current position with a qualified adviser rather than relying on what was true a few years ago.
Do I still have to file a tax return in both countries?
A treaty is designed to reduce or remove double tax — it does not usually remove your filing obligations. You often have to file a return in both countries and actively claim the treaty relief; it is rarely automatic. Common reasons you keep filing in two places include:
- You earn income at source (rent, dividends, a local pension) that the source country taxes and reports on.
- You must declare worldwide income where you are resident, then claim a credit or exemption for the foreign portion.
- Your home country requires a return to confirm you have left, or to release a refund of over-withheld tax.
- Some countries tax based on citizenship, meaning nationals may have to keep filing even after moving away.
Filing in both countries is normal and does not mean something has gone wrong. The aim is to ensure each authority sees the full picture and that relief is correctly applied. Keep careful records of foreign tax paid, certificates of residence, and the dates you moved — these are typically what you will need to support a treaty claim.
So will I end up paying more tax overall?
Double taxation is a well-trodden problem with established solutions. Many expats, once their residency position is clear and relief is claimed correctly, find they pay broadly the higher of the two countries' tax burdens on a given stream of income, rather than the two added together — though outcomes vary with your facts. Deadlines, day-counts and reporting forms differ by country and change regularly, so treat anything here as a general guide rather than a fixed rule, and confirm current figures with a lawyer.
When should I get a tax lawyer involved?
This guide explains the general shape of how tax treaties tend to work, but every treaty is different and your own facts — where you live, what you earn and where it comes from — shape the outcome. Tax rules and thresholds also change from year to year. Before you make decisions or file, it is wise to speak to a qualified tax lawyer or adviser in the relevant countries, who can confirm the current rules and apply them to your specific situation.
Last reviewed July 2026. Treaties are bilateral and each one differs — the treaty between your two countries, and your own facts, decide the outcome. Confirm your position with a qualified tax adviser in both countries before you file or move.
Frequently asked questions
How can two countries both tax the same income?
Most countries tax on two bases. One is residence, meaning if you live there they generally tax your worldwide income; the other is source, meaning if income arises within their borders, such as a salary, local rent or a business profit, they often want a share regardless of where you live. As an expat these can overlap, so your new home may tax everything while your old country still claims income that originates there. Double taxation treaties exist to divide up these taxing rights and reduce the overlap.
If I've moved abroad, how does a treaty decide which country I'm resident in?
When both countries treat you as resident under their own domestic rules, treaties typically apply a sequence of tie-breaker tests, usually in order until one gives a clear answer: your permanent home, then your centre of vital interests (where your personal and economic ties are closest), then your habitual abode, then nationality, and finally mutual agreement between the two tax authorities. Where you keep a home, where your spouse and children live, and where your working life is centred can all tip the balance, so the outcome is not always obvious.
What's the difference between the credit method and the exemption method of relief?
Under the credit method, your resident country still taxes the income but generally lets you subtract the tax you already paid abroad; if you paid less abroad you may top up to your home rate, and if you paid more the credit is often capped at what your home country would have charged. Under the exemption method, your resident country may simply not tax income the treaty assigns to the other country, though it might still count that income when setting the rate on your remaining income. Which applies depends on the specific treaty and the type of income.
If there's a treaty, do I still have to file tax returns in both countries?
Usually yes. A treaty reduces or removes double tax, but it does not normally remove your filing obligations, and relief is rarely automatic, so you often have to file in both countries and actively claim it. You may keep filing because you earn income at source, because you must declare worldwide income where you are resident, or because your home country taxes based on citizenship. Keep careful records of foreign tax paid, certificates of residence, and your dates of moving, as these support a treaty claim.