Three dates decide the tax: leaving, arriving, and the contract

The date of disposal and your residence on that date. Britain taxes residents on worldwide gains, and it taxes UK land whoever owns it. America taxes its citizens on gains wherever those gains arise, which is why the sequencing question is about the British side and the state side.

Sometimes, and it depends entirely on which asset you mean. Selling while UK resident brings the gain inside the British charge, which produces UK tax and with it a credit for your American return. Selling afterwards can take a non-property asset outside the British charge completely, leaving the American charge unrelieved.

Selling after you cease to be resident can take a non-property asset outside the British charge altogether.

That sounds better and often is not. No UK tax means no credit, so the full American charge lands with nothing to offset it.

Which outcome is cheaper depends on the rates each system would apply to that particular gain.

Keeping UK funds carries a specific American cost that has nothing to do with the gain. A British fund counts as a passive foreign investment company, with its own reporting and a harsh default calculation, so selling before you move can solve a continuing problem as well as a current one.

A de minimis exception exists where total holdings stay below the stated thresholds and nothing is sold or distributed.

Selling before you move therefore solves a future problem as well as a current one, which sometimes justifies accepting UK tax now.

Our article on ISAs and the PFIC problem sets out the regime you would otherwise carry with you.

They behave in opposite ways and deserve opposite decisions. An ISA has no American recognition, so the wrapper saves nothing on that side and its contents may be PFICs. A pension has treaty protection that keeps growth out of current American tax, so leaving it invested is usually right.

A pension is the opposite: treaty protection keeps growth out of current American tax, and leaving it invested is usually the right default.

So selling ISA holdings before a move can make sense, while cashing a pension almost never does.

Our article on UK pensions under US rules explains the protection you would be giving up.

It can matter more than the federal position does. Some states tax capital gains at ordinary income rates and give no credit for foreign tax paid, so a gain realised after you become resident there meets an extra charge that no treaty relieves. Check before fixing a completion date.

A gain realised after you become a resident of such a state is exposed to that charge.

So the sequencing question has three dates, not two: ceasing UK residence, arriving in the state, and the contract date.

Establish the state's rules before fixing a completion date, because the answer differs sharply between states.

The gain in dollars, measured from your dollar cost to your dollar proceeds. That is a different figure from the sterling gain whenever the exchange rate has moved between purchase and sale, so an asset that barely rose in pounds can still produce a taxable dollar gain.

So an asset that barely rose in sterling can produce a taxable dollar gain, and a sterling profit can become a dollar loss.

The credit on Form 1116 then relieves UK tax on the same income, where any UK tax exists.

Our guide to exchange rate rules explains which rate applies to each side of the calculation.

Yes, and both systems offer relief on different terms and different tests. GOV.UK's guidance on tax when you sell your home sets out private residence relief and when it is restricted, while America applies its own main home exclusion with separate ownership and use requirements.

America has its own exclusion for a main home, with its own ownership and use tests.

The two rarely cover the same amount, so a sale can be free of tax in one country and partly taxable in the other.

Our article on selling a UK home as a US person works through both calculations.

Separate from the annual return, and short. GOV.UK explains how to report and pay capital gains tax, including the standalone service for UK property disposals. Missing that deadline brings penalties even where the eventual tax comes to nothing at all.

Missing that deadline brings penalties even where the eventual tax is nil.

It catches sellers who have already left, because nothing about the deadline depends on living here.

Diary it from the completion date rather than from the tax year end.

Then plan as though you will. Temporary non-residence rules can bring gains realised during a short absence back into UK charge once you return, which removes the main attraction of selling after departure. A two-year posting is therefore a poor moment to realise large gains.

That undoes the main attraction of selling after departure.

So a two-year posting is a poor moment to realise large gains, while a permanent move is a different matter. HMRC's guidance note on residence covers the tests behind that judgement.

Our guide to the statutory residence test covers how the departure year is assessed.

With records that may be decades old, converted at rates from the date of each purchase. That evidence problem, rather than the law, is the practical obstacle. For property you want the completion statement and improvement invoices; for investments, the full transaction history from the platform.

For property, gather the completion statement, improvement invoices and any probate valuation if it was inherited.

For shares and funds, get the full transaction history from the platform before you close the account.

In our practice missing cost information causes more overpaid tax than any misunderstood rule.

When selling UK assets, funds and anything with a wrapper come first. British funds bring American reporting that follows you for as long as you hold them, so they rarely survive a move intact.

Individual shares are different. They carry no wrapper problem, so the only question is rates and timing.

Cash needs no decision beyond which account holds it. Property is its own project.

So triage the list before modelling anything, because selling UK assets rarely means selling all of them. Two or three holdings usually account for the whole decision.

Each owner is taxed on their own share, which can help where only one of you is American. A sale by a non-US spouse of their own holding has no American consequence at all.

That is a question of genuine ownership rather than paperwork, and it needs to have been true beforehand.

Transfers between spouses shortly before a sale attract attention in both systems. Plan ownership years ahead if you plan it at all.

Documents first, decisions second, contracts last. Gather the cost evidence while you still have access to the accounts and the professionals holding it, then model each holding under both systems on both sides of your departure date, and only then fix completion dates around the three dates that matter.

Then model each asset under both systems, on both sides of the departure date.

Only then fix completion dates, working around the three dates that matter.

Doing it the other way round means optimising a sale that has already happened.

It is entirely possible and slightly harder administratively. Platforms ask for identity documents that assume a UK address, and some restrict accounts held by non-residents.

Tell providers your new address before you need to sell, rather than during a sale.

Property is the awkward case, since conveyancing expects a UK bank account and UK verification. Start those arrangements early.

In our practice the delay is operational rather than fiscal. The tax answer is usually clear long before the platform cooperates.

Losses are worth claiming in both systems, and the rules for using them differ. Britain allows losses against gains with its own carry-forward rules, and a claim is needed rather than automatic.

America has its own rules on offsetting and carrying losses forward.

So a loss can be useful in one country and stranded in the other, depending on when it arises relative to your residence.

Record every disposal, not only the profitable ones. Selling UK assets at a loss still needs reporting to preserve the claim.

Work through this before you instruct anyone to sell.

Take an illustrative portfolio. An American in Bristol holds £200,000 of UK funds bought a decade ago and plans to move to New York in the spring.

Selling while UK resident produces UK capital gains tax and a credit against the American charge. Selling after departure may avoid UK tax entirely and leave the full American charge unrelieved, plus a possible state charge once she arrives.

The funds are also PFICs if she keeps them. The figures are illustrative; the sequencing is what decides the cost.

First, assuming that selling UK assets after departure is automatically better. Second, forgetting that no UK tax means no credit.

Third, missing the UK property reporting deadline because the seller had already left. Fourth, closing an investment account before downloading its history.

Fifth, ignoring the destination state. Sixth, cashing a pension to simplify a move, which is usually the most expensive tidiness available.

We model selling UK assets on both sides of the departure date, in both currencies, before anything is marketed. That analysis is where the money is, and it takes days rather than weeks.

Our treaty relief service covers the credit position, and we prepare the UK and US filings so the same figures appear on both.

Send us a list of holdings with acquisition dates and we will tell you what each sequence costs.