Introduction: Why the Cash ISA Limit Change Hits Americans Harder
The cash ISA limit falls from £20,000 to £12,000 on 6 April 2027 for every saver aged 64 or under, and for an American in Britain that cut does far more damage than the headline suggests. A British saver simply moves the other £8,000 into investments. However, a US citizen, green card holder or dual national who follows the same path walks into the harshest fund rules in the Internal Revenue Code. Moreover, the lower cash ISA limit is no longer a proposal. The Treasury made the regulations on 10 September 2026, so the new rules are now law.
The Cash ISA Limit in One Paragraph
The new cash ISA limit caps subscriptions to cash accounts at £12,000 a year if you are 64 or under at the end of the tax year. The overall ISA allowance stays at £20,000, so the remaining £8,000 must go into a stocks and shares ISA, an Innovative Finance ISA or, within its own £4,000 ceiling, a Lifetime ISA. In addition, three anti-avoidance rules arrive on the same day. Transfers from an investment ISA back into a cash ISA stop for under-65s. Interest on uninvested cash inside an investment ISA suffers a flat 22% charge. Finally, a stocks and shares ISA can no longer hold nothing but money market funds.
Why a UK Savings Rule Becomes a US Tax Problem
The IRS does not recognise the ISA wrapper at all. Consequently, every pound of interest, every dividend and every gain inside an ISA belongs on your US return, even though HMRC taxes none of it. That mismatch was manageable while you could keep £20,000 a year in plain cash. From April 2027, however, the rules push £8,000 a year towards funds, and most UK funds are passive foreign investment companies in American eyes. Therefore the new cash ISA limit turns a simple reporting exercise into a planning decision with a hard deadline. At TaxYork we prepare both the US and UK returns for wealthy Americans in Britain, and this guide sets out exactly what changes, what it costs and what to do before 5 April 2027.
What the New Cash ISA Limit Rules Actually Say
Most online summaries still describe the lower cash ISA limit as a plan. In fact, the Individual Savings Account (Amendment) (No. 2) Regulations 2026 were made on 10 September 2026, laid before the House of Commons on 14 September 2026 and come into force on 6 April 2027. They amend the Individual Savings Account Regulations 1998, which remain the governing code. We have read the instrument line by line, and four provisions matter to an American household.
The £12,000 Cap and the Age 65 Test
New regulation 4ZA(1ZA) sets the cash ISA limit itself. In any tax year in which you are 64 or under at the end of that year, your combined subscriptions to cash accounts cannot exceed £12,000. Accordingly, the test looks at your age on 5 April, not on the day you pay in. If you turn 65 at any point in a tax year, you keep the full £20,000 for the whole of that year. The cash ISA limit bites on new subscriptions only, so balances you already hold stay untouched. Furthermore, the instrument applies the same ceiling to replacement subscriptions in a flexible account, so speak to your provider before you withdraw and redeposit. Junior ISAs sit outside the change entirely, and HM Treasury sets out the background in its policy paper on the cash ISA limit reduction.
The One-Way Transfer Lock
Amended regulation 21 creates a one-way valve. You may still transfer a cash ISA into a stocks and shares ISA at any age. However, money in a stocks and shares or Innovative Finance ISA may move only to another investment ISA or a Lifetime ISA, unless you are 65 or over at the end of the year. For an American this rule matters more than the cash ISA limit itself. Many of our clients want to sell problem funds and retreat to cash inside the wrapper. After 5 April 2027, that exit closes for anyone under 65. The government's own summary appears in the ISA reform 2027 anti-circumvention factsheet, and the official guidance on transferring an ISA explains the mechanics that apply until then.
The 22% Charge on Uninvested Cash
New regulation 22A removes the tax exemption for interest on cash held inside a stocks and shares or Innovative Finance ISA. Instead, your ISA manager must pay HMRC a flat rate charge at the savings basic rate, which rises to 22% in April 2027 under the confirmed changes to savings, property and dividend rates. The manager pays within six months of the tax year end. Notably, you cannot reclaim the charge, you do not report the interest to HMRC, and your personal savings allowance gives no shelter. The charge applies at every age, including to savers over 65.
The Money Market Fund Test
New regulation 7(10A) stops you sidestepping the cash ISA limit by rebuilding a cash account inside an investment wrapper. Specifically, the investments in a stocks and shares ISA, other than cash, cannot consist 100% of money market funds. The regulations define a money market fund as one authorised under Article 4 or 5 of the Money Market Funds Regulation. Shares, ordinary funds, investment trusts, exchange-traded funds, gilts and corporate bonds do not count as cash-like. As a result, a British saver can hold a large money market position beside one other holding. For an American, though, that workaround carries its own US cost, as we explain below.
How the IRS Taxes Cash ISA Interest
A cash ISA is the least troublesome ISA an American can own, which is exactly why the lower cash ISA limit hurts. Nevertheless, even a plain cash account creates US tax with nothing to offset it.
Ordinary Income and the 3.8% Surcharge
Cash ISA interest is ordinary income on your Form 1040. You report it on Schedule B, and you answer the foreign account questions in Part III of that schedule. A top-bracket filer pays 37%. Additionally, the Net Investment Income Tax adds 3.8% once modified adjusted gross income passes $200,000 for a single filer, $250,000 for a joint return or $125,000 if you file separately. Therefore a senior banker or company owner faces a combined 40.8% on interest that Britain treats as tax-free.
No UK Tax Means No Foreign Tax Credit
The foreign tax credit normally rescues an American in Britain, because UK income tax usually exceeds the US bill. Inside an ISA, however, there is no UK tax to credit. Furthermore, interest sits in the passive basket, so excess credits from your salary cannot cross over to cover it. The surcharge compounds the problem, because foreign tax credits never reduce the Net Investment Income Tax. Our detailed guide to UK savings interest for US filers walks through the basket rules in full.
Converting Sterling Interest to Dollars
You must report the interest in US dollars. Most filers use the IRS yearly average exchange rate, applied consistently from year to year. Importantly, the UK tax year ends on 5 April while the US year ends on 31 December, so your provider's annual statement never matches the figure your US return needs. Accordingly, ask for calendar-year interest or rebuild it from monthly statements. We cover the choice of rate in our guide on which exchange rate to use for US and UK tax.
The £8,000 Problem: Where the Displaced Money Goes
From 2027/28, the cash ISA limit forces an under-65 who wants the full allowance to place £8,000 a year outside cash. Over a decade, that is £80,000 per person steered towards investments. Each destination for money above the cash ISA limit has a different US result, and three of the four are poor.
Funds and Investment Trusts: The PFIC Regime
Almost every UK or Irish fund, exchange-traded fund and investment trust is a passive foreign investment company under section 1297 of the Internal Revenue Code. Under section 1291, the IRS treats any gain on sale as an excess distribution. It then spreads that gain across your holding period, taxes each earlier year's slice at the highest rate and adds an interest charge. Moreover, you file Form 8621 for each fund, each year. The simplest route for a British saver is therefore the most expensive route for you. Unfortunately, the US-domiciled funds that avoid the regime are generally unavailable to UK retail investors.
Money Market Funds: Cash-Like in Britain, a PFIC in America
Once the cash ISA limit applies, many platforms will steer displaced savers towards a sterling money market fund. HMRC calls that fund cash-like. The IRS calls it a foreign corporation that earns only passive income, which makes it a PFIC. Regular income distributions usually escape the excess distribution rules, so the tax often matches ordinary interest. However, an accumulating share class pays nothing out, so the whole return emerges as a gain on sale and falls into section 1291. A limited exception in Treasury Regulation 1.1298-1 removes the annual Form 8621 where all your PFICs together are worth $25,000 or less, or $50,000 on a joint return, and you have no excess distribution. Nevertheless, that relief disappears quickly as £8,000 a year builds up. Our guide to money market funds and US tax explains the share-class point in depth.
Uninvested Cash: The 22% Charge and the Credit Question
Alternatively, you could leave the £8,000 above the cash ISA limit as cash on the investment platform. Platform cash typically earns less than a cash ISA, and from April 2027 the manager also hands 22% of the interest to HMRC. Meanwhile, the IRS taxes the gross interest at up to 40.8%. The obvious question is whether the 22% counts as a creditable foreign tax. In our view the answer is uncertain at best. Regulation 22A places the charge on the account manager, treats the interest as not being income for income tax purposes and bars any repayment to you. Under Treasury Regulation 1.901-2, a credit belongs to the person on whom foreign law imposes legal liability. The IRS has issued no guidance on the new charge, so we recommend that you do not rely on a credit when you compare options.
Individual Shares and Gilts: The Clean Options
Directly held company shares are not PFICs unless the company itself is an investment vehicle. Dividends from UK companies generally qualify for the 20% US rate, plus the 3.8% surcharge. Likewise, individual gilts and corporate bonds sit outside the fund rules, and neither counts as cash-like under the new test. The IRS still taxes the coupons and any gain, as our guide to UK gilts and US tax shows. Even so, for money above the cash ISA limit, a short-dated gilt is the closest lawful substitute for cash that keeps Form 8621 off your return. Similarly, peer-to-peer loans avoid the regime but bring their own traps, which we cover in our Innovative Finance ISA guide.
Three Moves to Make Before 5 April 2027
The 2026/27 tax year is the final year before the lower cash ISA limit applies. Consequently, the next six months decide how much flexibility you carry forward. We recommend three steps.
Use the Last Full £20,000 Cash Allowance
Until 5 April 2027, the cash ISA limit remains £20,000 at any age. If you intend to hold cash anyway, subscribe the full amount now. The balance stays protected indefinitely, and the IRS treatment is simple ordinary income. Furthermore, the new cash ISA limit does not restrict moves between cash ISA providers, so you lose nothing by acting early. HMRC's overview of how Individual Savings Accounts work confirms the current limits.
Exit Problem Funds While the Door to Cash Is Open
If your stocks and shares ISA holds UK funds, you have a closing window. Today you can sell the funds and transfer the whole account into a cash ISA. Importantly, a transfer of earlier years' subscriptions does not use any of your annual allowance. After 5 April 2027, an under-65 cannot make that transfer, whatever room remains under the cash ISA limit. The proceeds would then sit as platform cash and suffer the 22% charge, or you would withdraw them and lose the wrapper for good. Selling does trigger the section 1291 computation, so model the US cost first. However, that cost only grows with every year the gain keeps accruing.
Keep the Non-US Spouse's ISA Separate
Many of our clients are married to a British spouse who has no US filing obligation. That spouse's ISA, funded from their own earnings and held in their sole name, stays outside the US tax net entirely. Therefore the household can still hold funds in a wrapper without Form 8621, provided the American partner has no ownership or signature authority. The position changes if the couple elects to treat the spouse as a US resident, so check that election before you rely on this point. Similarly, different rules govern a child's account, which our Junior ISA guide for American parents addresses.
FBAR, Form 8938 and Missed ISA Reporting
Tax is only half of the exposure. Every ISA is also a reportable foreign financial account, and the lower cash ISA limit will spread savings across more accounts and more providers.
FBAR: The $10,000 Aggregate Test
The cash ISA limit changes nothing about FBAR. You must file one if your foreign accounts together exceed $10,000 at any moment in the year. Cash ISAs, stocks and shares ISAs and platform cash accounts all count. At current rates, a single year's £12,000 subscription clears the threshold alone. FinCEN's page on reporting foreign bank and financial accounts sets out the rules, and the form falls due on 15 April with an automatic extension to 15 October.
Form 8938: The Higher FATCA Thresholds
Americans living abroad also file Form 8938 once specified foreign assets exceed $200,000 on the last day of the year or $300,000 at any time. Those figures double for a joint return. The two regimes overlap but do not replace each other, as the IRS comparison of Form 8938 and FBAR requirements makes clear. Additionally, a Lifetime ISA brings a bonus that the IRS treats as income, which we explain in our guide to the Lifetime ISA and US tax.
Catching Up on Missed Years
Many accidental Americans and long-term residents have never reported an ISA, because a British adviser called it tax-free. If that describes you, the missed reporting usually covers income, missed FBAR filings and missing Forms 8621 together. Non-wilful taxpayers living abroad can normally correct the record through the IRS Streamlined Filing Compliance Procedures, which require three years of returns and six years of FBARs. Our IRS Streamlined Filing service handles that process from start to finish. Notably, the falling cash ISA limit creates a natural trigger to review old years before you restructure anything.
Case Study: Hannah, a London Managing Director With £255,000 in ISAs
The following illustration uses realistic figures to show how the new cash ISA limit works in practice. We have changed the details, and we use an illustrative rate of $1.32 to the pound.
The Starting Position
Hannah is 44, a US citizen and a managing director at an investment bank in London. Her husband Oliver is British and has no US status, so she files as married filing separately. She earns £450,000 a year. She holds £160,000 across two cash ISAs earning 4.1%, and £95,000 in a stocks and shares ISA invested in four UK funds. Those funds carry an unrealised gain of £22,000 after six years. Furthermore, she has never reported any ISA to the IRS.
What the Cash Costs Her Each Year
Her cash ISAs produce £6,560 of interest. Britain taxes none of it. However, the United States taxes all of it at 37% plus the 3.8% surcharge, a combined 40.8%. Her annual US bill on the cash is therefore £2,676, or about $3,533. No foreign tax credit applies, because no UK tax arose.
Three Homes for the Displaced £8,000
From 2027/28, the cash ISA limit lets Hannah add only £12,000 to cash. Firstly, suppose she leaves the other £8,000 as platform cash at 2.5%. It earns £200. Her manager pays the 22% charge of £44, and she owes the IRS £81.60 on the gross interest. Without a credit, she loses £125.60, an effective rate of 62.8%. Secondly, suppose she buys a money market fund yielding 4.0%. It earns £320, with US tax of £130.56 and a Form 8621 every year, because her other funds already exceed the $25,000 exception. Thirdly, suppose she buys a short-dated gilt. She pays US tax on the coupon and any gain, but files no Form 8621. Consequently, the gilt wins on both cost and compliance.
What We Did Before the Deadline
We prepared the section 1291 computation on her four funds first. Roughly five-sixths of the £22,000 gain fell into earlier years and was taxed at 37% with an interest charge, while the rest was taxed as current income. On our illustrative figures the total came to about £9,400. Hannah then sold the funds and transferred the whole £95,000 into a cash ISA in February 2027, while the transfer route remained open. She also used her final £20,000 cash subscription. Finally, we filed three years of amended returns and six years of FBARs to resolve her missed ISA reporting. Oliver now holds the household's fund investments in his own ISA, funded from his own salary.
How TaxYork Can Help
TaxYork provides comprehensive tax preparation and compliance for high-net-worth Americans in Britain, dual nationals and accidental Americans. We prepare your US and UK returns together, so the ISA figures agree on both sides of the Atlantic. Additionally, we compute PFIC excess distributions, prepare Form 8621, Form 8938 and the FBAR, and convert UK tax-year statements into calendar-year dollar figures. Where earlier years are missing, we rebuild them and file the catch-up submission. Our US tax return preparation for expats covers every schedule your ISAs touch, and our cross-border planning service models each option before the cash ISA limit falls on 6 April 2027.
Conclusion
The cash ISA limit of £12,000 is now law, and it takes effect on 6 April 2027 for everyone aged 64 or under. For a British saver, the lower cash ISA limit is a nudge towards investing. For an American in Britain, however, it removes the one ISA that the IRS taxes simply and pushes £8,000 a year towards PFICs, a 22% charge or individual securities. Moreover, the transfer lock closes the exit from problem funds into cash. Therefore the months before the cash ISA limit falls matter. Use the final £20,000 cash allowance, exit problem funds while you still can, and bring any missed ISA reporting up to date. Above all, compare each option on both tax systems before you move money.
Contact Us
If the new cash ISA limit affects your savings, or if you have never reported an ISA to the IRS, speak to our team before the tax year ends. You can book a consultation today, email hello@taxyork.com or call 020 3488 8606. We will review your accounts, quantify the US cost of each option and prepare every form you need.
Disclaimer
This article provides general information only and reflects the law and published guidance as at October 2026. It does not constitute tax, legal or financial advice, and you should not act on it without taking professional advice on your own circumstances. Tax rules change, and their application depends on individual facts. TaxYork accepts no liability for any loss arising from reliance on this article. Please contact us for a review of your own position.