Five things to do before you move, and one thing not to

The single most useful thing to understand about a transatlantic move is that the tax planning window closes on arrival. The same steps are mostly still available afterwards — at a worse price, and occasionally not at all. Here is what to use the window for.

Not when you arrive. When the rules say you become resident, which in the US can be earlier than you expect and in the UK depends on a test with several moving parts. Everything else on this list depends on knowing that date, so establish it first.

If you are moving to the US, non-US funds — including those inside an ISA — become a reporting burden and a tax penalty the moment you are resident. Realising them before that date is often far cheaper than carrying them across. If you are moving to the UK, the question is different but still worth asking.

Sell, let, or keep empty — each has a different answer in each direction, and the reliefs for a main residence do not match across the two countries. A sale that is exempt at home can be taxable in the new country if it completes a few weeks too late.

Not to act on, necessarily, but to understand what you hold, because the treatment of contributions, growth and eventual drawdown differs between the systems and some positions need to be taken from the first year. Transfers in particular are worth pausing on.

Leaving physically and leaving for tax are not the same thing. Both countries have procedures for notifying departure, and both have ways of deciding you have not really left. A state in the US may have a view of its own. Close the record properly.

Do not assume the treaty handles it. The treaty relieves double taxation in many situations. It does not remove either filing obligation, it does not bind US states, and for US citizens it gives back less than it appears to. Plan as though both countries will want a return, because both countries will.