Can two countries both treat you as tax resident?

Nobody applies to be dual resident. It falls out of two rules being applied independently:

• A day count in one country, a home in another. You spend 190 days in Spain, so Spain's 183-day test says resident. Your family home stays in the UK and you meet a Statutory Residence Test tie, so the UK says resident too.
• Different measuring periods. One country runs on the calendar year, another on a tax year starting in April or July. A single move can land inside both.
• A move mid-year. You genuinely lived in two places. Absent split-year treatment, both may claim the whole year.
• Ties that outlive the move. A property, a family, a business, or bank accounts can keep the old country's test satisfied long after you left.

The day count is often the smaller half of the problem. It is the non-day tests — permanent home, centre of vital interests, habitual abode — that keep people resident somewhere they thought they had left.

Where the two countries have a tax treaty, it will usually follow the OECD model's Article 4 and work through these tests in order, stopping at the first one that produces an answer:

1. Permanent home. Where do you have a home permanently available to you? If that is one country only, it decides, and the remaining steps never run.
2. Centre of vital interests. If you have a home in both, which country holds your closer personal and economic ties — family, work, business, social and civic life?
3. Habitual abode. If vital interests are unclear — or if you have no permanent home available in either country — where do you actually live most of the time?
4. Nationality. If still tied, the country you are a national of.
5. Mutual agreement. If none of the above resolves it, the two tax authorities settle it between themselves.

Two things follow that people get wrong. The order is binding — you cannot skip to habitual abode because the day count suits you better. And a tie-breaker does not delete the other country's residency; it allocates taxing rights under the treaty. Filing obligations may survive it.

Not every pair of countries has one, and not every treaty contains tie-breaker provisions. Where there is none, both countries can tax under domestic law, and any relief comes from each country's own unilateral rules — a foreign tax credit or an exemption — rather than from an agreed allocation. That is a materially worse position, and it is worth knowing before a move rather than after.

• HMRC — International Manual: residence under double taxation agreements
• HMRC — Statutory Residence Test and split year treatment
• IRS — Tax residency status examples and dual-status years