If you are American and living in Britain, or British with a working life spent in the United States, you will already have met the consequences of that assumption. A UK adviser’s software has nowhere to put a 401(k). An American tool has never heard of an ISA, and will happily suggest something that proves expensive under UK rules. Between the two sits the question you actually want answered: across both systems, over the whole of your life, what happens?

That is the question this was built to answer. What follows is a description of what it does. We have kept the workings to ourselves — but the range should give you a fair sense of the depth underneath.

The questions it exists to answer

A projection is only worth having if it settles something. These are the kinds of questions we put to it in a typical week, drawn from real planning work:

• If I draw this pension down over six years instead of fifteen, what does it cost me in tax — and is the slower route actually the cheaper one?
• I have inherited an IRA with a ten-year deadline attached. What is the least painful way to empty it, and what does each route leave behind?
• My income arrives in dollars but my life is priced in pounds. What happens to me if sterling strengthens the way it did between 2001 and 2007?
• Can I give money to my children out of income without it falling back into my estate — and how much, exactly?
• If I stop work at 58 rather than 62, when does the money run out, and what would have to change for it not to?
• Which account should I spend first? What does choosing wrongly cost over thirty years?
• If I die first, what is my spouse’s position — and what does the tax bill look like on the second death rather than the first?
• Am I holding funds that the US taxes punitively because they are not American?
• How much can I actually spend, without the answer being a guess?

What it can hold

Breadth matters here more than it does in a single-country plan, because the whole difficulty of a cross-border life is that the pieces sit in different places under different rules. The planner accounts for both sides at once, in whichever currencies they are held.

Two tax systems, running at once

This is the part that is genuinely hard, and the reason a general-purpose tool cannot be made to do the job. The projection applies UK income tax — including the separate Scottish rates — alongside National Insurance, capital gains, and the dividend and savings allowances. It applies US federal income tax and payroll taxes. It works out how the two interact rather than treating them as separate sums, because for a US person in Britain they are not separate: the same money is looked at twice, and only the interaction tells you what you keep.

It also carries inheritance tax through to the end — nil-rate and residence nil-rate bands, the taper on larger estates, the treatment of gifts and the seven-year rule, and the position on a second death rather than only the first.

Specialist work, where it is needed

Some questions are common enough in our client base to deserve proper tools rather than an adviser’s estimate. Roth conversions can be modelled year by year against what would otherwise have happened, so the decision rests on the whole remaining lifetime rather than this year’s tax bill. Penalty-free early access from a US pension before 59½ can be planned across each of the three methods the IRS permits. Buying a guaranteed income can be compared directly against continuing to draw down. And where a plan depends on the answer, we check eligibility and allowances rather than assuming them.

Testing what could go wrong

Any projection can be made to look comfortable. The useful work is finding out how much has to change before it stops being comfortable — so the plan is pushed on, hard, from several directions.

What you actually get

The model is the engine, not the product. What you see is your position today, a projection of it year by year, and the effect of any change you are considering, set against the plan as it stands. Several versions can be held side by side — retire at 58, retire at 62, sell the house or keep it — and compared rather than argued about.

From that comes a written report setting out what is recommended and why, in language meant to be read rather than filed. It reaches you through your own secure area of this site, along with the documents behind it, and your acceptance is recorded properly when you are satisfied. Earlier versions of your plan are kept, so a year from now we can show you what changed and what caused it.

What it will not do

A projection is not a prediction, and any adviser who implies otherwise is selling something. What it produces is a careful, internally consistent picture of what follows from a set of assumptions — which makes it excellent for comparing choices, and a poor crystal ball. The further out it runs, the more that holds.

Two things worth saying plainly. Tax allowances and thresholds are held at today’s cash values throughout, rather than assuming future governments raise them; that is the cautious assumption, but over a long projection it is a significant one. And the model does not give advice. It informs it. The recommendation comes from a qualified person who has met you, understood what you are trying to do, and takes responsibility for it.