Do not treat 'there is a treaty' as the answer
A double taxation agreement is useful, but it is not a magic switch. It does not mean every item is taxed only once, and it does not mean the tax office knows your situation automatically.
The first step is to identify the exact countries, tax residence year, income type, and timing. Salary, dividends, interest, pension capital, rental income, and capital gains can follow different treaty articles.
For expats, the most common mistake is reading a treaty summary and skipping the administrative route. Relief often depends on forms, declarations, certificates of residence, or a tax return position.
A simple DTA checklist
Start with residence. Which country considers you tax resident for the relevant period? If two countries could claim residence, the treaty tie-breaker rules may matter.
Then classify income. A Pillar 3a payout, employment bonus, dividend, bank interest, and rental income are not interchangeable. Each item should be mapped to its own treaty treatment.
Finally, check relief method. Some situations use exemption, others use credit, reduced withholding, refund, or a combination. The cash-flow effect can be very different even when the final tax is partly relieved.
When to get advice
If the amount is small and the situation is simple, official pages and a local tax software workflow may be enough. If the amount is large, cross-border, or linked to a relocation year, professional advice is usually cheaper than cleaning up a mistake.
This is especially true for departure years. A person can have Swiss salary, foreign dividends, vested benefits, Pillar 3a, moving expenses, and a new country's tax residence rules all in one calendar year.
The practical goal is not to become a treaty lawyer. It is to know which questions to ask before money moves.
For a first review, write one line per income item: country, payer, amount, tax withheld, treaty article to check, and filing action. That small table is often clearer than a long folder of unsorted statements.