Do I still file a US return if I already file in the UK?
Yes. Residence in the UK settles which country taxes what, but it does not remove the obligation to file in the other. The treaty allocates taxing rights over each class of income; it does not merge two filing systems into one. So the usual position is two returns covering the same income, each computed under its own rules, with relief claimed on one side for tax properly paid on the other. The practical work is not the arithmetic. It is deciding, item by item, which country has the first claim, and then making both returns tell a consistent story about it.
How do UK investment funds get treated on my US return?
Not necessarily the way the UK treats them. A pooled fund that is an ordinary retail investment in the UK can be characterised quite differently under US rules, and the characterisation drives both the tax computation and the reporting that comes with it. Income can be brought into the US calculation on a basis that has little to do with what the fund actually distributed, and gains can be measured differently. This is a common reason a UK-resident US filer ends up with a US bill on a portfolio that produced modest UK tax. What you hold matters more than how it performed.
Can I rely on my UK non-domicile position for US purposes?
No. Non-domicile treatment is a feature of the UK system with no counterpart in the US one, so income kept outside the UK and untaxed there is not by that fact outside the US computation. For a UK national who becomes a US filer, this is often the unpleasant discovery: the arrangement built around the UK position is neutral in the UK and fully visible in the US. The position has to be re-examined from the US side before the first US return goes in, not after it has been filed and then queried.
Which return goes first in the year I move between the two?
Order matters, because relief on one side depends on tax finally determined on the other. In a year of movement the income has to be split between the periods either side of the move, and the two countries do not run their tax years to the same dates, so tax paid in one has to be mapped onto the other's year before any credit is computed. The usual sequence is to fix the residence position first, then establish which country has the primary claim on each item, then claim relief. Reversing that produces a credit claim that cannot be supported when it is examined.
Do my UK pension contributions have to appear on a US return?
They have to be looked at. A UK pension is not automatically given the treatment a domestic US plan receives; the treaty contains specific provisions for pensions and the arrangement has to be characterised against them rather than assumed into place. Depending on that characterisation, employer contributions and growth inside the arrangement may or may not stay outside the US computation until money is drawn. Getting this wrong in either direction is expensive: a position that defers too much invites correction, and one that defers too little brings forward tax on money you cannot yet reach.
Is a UK family trust something I have to report in the US?
Probably, and the reporting usually arrives before any tax does. A trust that is unremarkable in UK practice can fall into US reporting because of who settled it, who can benefit and who controls it, and those tests apply whether or not the trust distributed anything. Characterisation also decides whether trust income is attributed to a person or taxed in the trust itself. The work is to establish the facts of the trust from its deed, its history and how it is actually administered, and then take one documented position that the returns and the reporting both follow.
How do you avoid double taxation?
You claim relief once, in the right country, in the right order. Usually the source country taxes first, the residence country then gives a credit for that tax against its own charge on the same income, and a treaty caps the source-country rate. Getting the order wrong is what produces a double charge you then have to unwind. The mechanism differs by income type, which is why we map the whole position before filing either return. See how to avoid double taxation.
How would a foreign tax authority know I am resident there?
Mostly from information you or your bank already provided. Account-opening forms ask you to self-certify tax residence, and that certification is reported between authorities under the Common Reporting Standard or, for US accounts, under the FATCA framework. Beyond that: employer and payroll filings, property registries, immigration records and the tax filings of anyone who paid you. The realistic planning assumption is that the data arrives. See FATCA and information reporting.
