Introduction: Expat Tax Idaho Planning for British Households in Boise
The expat tax Idaho question has become a genuine one for British families. Boise, Eagle and Meridian now attract London founders, semiconductor executives, private equity partners working remotely and early retirees who want mountains, space and a lower cost of living. Idaho charges a flat 5.3% on worldwide income, and it gives no credit at all for tax you have already paid to HMRC. Consequently, a single pound of London rent can be taxed three times: in Britain, federally and again in Boise.
At TaxYork, we prepare US and UK returns for British nationals, dual national US UK families and accidental Americans who settle across the Treasure Valley. In our experience, the 5.3% headline lulls people into thinking the state return is a formality. It is not. Idaho excludes UK pensions from its retirement deduction, denies its capital gains deduction to UK property and shares, and taxes the UK pension lump sum that Britain treats as tax-free. Therefore careful expat tax Idaho preparation decides whether the move delivers the savings you expected.
Expat Tax Idaho Basics: Three Tax Systems From One Move
A British household in Boise deals with three separate charges. First, federal income tax reaches worldwide income at rates up to 37%, softened by the foreign tax credit. Second, Idaho charges 5.3% on almost the same income with no foreign relief whatsoever. Third, the United Kingdom continues to tax UK property income, UK land disposals and many UK pensions after you leave. Accordingly, sound expat tax Idaho compliance treats all three as a single exercise rather than three unrelated forms.
Who This Guide Is Written For
This guide addresses high-net-worth British nationals on green cards, E-2 and L-1 visas, dual nationals and accidental Americans living in Idaho. Specifically, it suits company owners, investment professionals and senior executives who arrive with UK property, pensions, ISAs and portfolios still in place. If your household income runs to six or seven figures, every expat tax Idaho rule below carries a measurable cost. For the national picture, read our complete guide to UK expat tax in the USA.
Why Expat Tax Idaho Rules Differ From Its Neighbours
Idaho sits beside two very different models. Its western neighbour, Washington, levies no wage tax but charges a 7% and 9.9% capital gains tax on high earners. Meanwhile, Utah charges a lower flat rate on a similar federal base, as we explain in our Salt Lake City guide. Idaho, in contrast, taxes everything at 5.3%, including gains, and offers its generous reliefs almost exclusively to Idaho assets and US pensions. Consequently, expat tax Idaho outcomes depend heavily on where your wealth actually sits.
How Expat Tax Idaho Rules Build Your Tax Base in 2026
The rate is simple. However, the base is where wealthy British arrivals win or lose money in Idaho.
The Flat 5.3% Rate After House Bill 40
Idaho moved to a single flat rate in 2023 and has cut it twice since. In March 2025, Governor Brad Little signed House Bill 40, the largest income tax cut in Idaho history, reducing the rate from 5.695% to 5.3% retroactive to 1 January 2025, as the Idaho Capital Sun reported at the time. The 2026 session left the rate untouched, so 5.3% applies for 2026 as well. Notably, a small zero-rate band sits beneath the flat rate, but for the incomes we deal with it makes almost no difference. Several ranking guides still quote 5.695% or even 5.8%, which is out of date.
Federal Conformity Under House Bill 559
In February 2026, Idaho enacted House Bill 559, conforming the state code to most of the federal One Big Beautiful Bill Act and applying it retroactively to 1 January 2025. In particular, Idaho adopted the federal deductions for tips and certain overtime, while largely keeping its own depreciation rules. For British executives, the relevant point is simpler. Because Idaho follows federal definitions closely, almost every federal income item flows straight into the Idaho return. Therefore the expat tax Idaho computation begins wherever your federal return ends.
Federal Adjusted Gross Income as the Starting Point
Idaho Form 40 starts with federal adjusted gross income from Form 1040, line 11. Idaho then applies its own additions and subtractions, followed by either the standard deduction or itemised deductions. Importantly, anything that never reaches federal AGI never reaches Idaho either. Conversely, anything included federally, such as UK rent, ISA dividends or a UK pension drawdown, arrives in Idaho in full. As a result, a treaty exemption that removes income federally also helps at state level, whereas a treaty credit never does.
The Foreign Tax Credit Idaho Does Not Give
This is the central expat tax Idaho problem, and it is the point that generic relocation guides consistently miss.
Form 39R Covers Other States, Not Other Countries
Idaho grants a credit for income tax paid elsewhere, but only to other states of the United States. Residents claim it on Form 39R, filing a separate schedule for each state, and the credit is limited to Idaho tax on the income that state also taxed. Foreign countries fall outside it entirely. Consequently, the £20,000 you pay HMRC on a Kensington rental earns nothing against the 5.3% charge, even though the federal foreign tax credit relieves the same income through Form 1116.
Why the Federal Credit Never Reaches the State Return
The foreign tax credit operates against federal tax liability, not against income. Idaho, however, borrows federal AGI, which sits well above the point where credits apply. Therefore no part of your UK tax travels down to the Idaho return. Similarly, carryovers of unused foreign tax credits from your years in Britain have no Idaho equivalent at all. In practice, every pound of UK-source income costs you 5.3% in Boise, regardless of what HMRC already took.
What That Costs a High-Earning Household
Consider £150,000 of UK income, roughly $197,600 at the 2025 IRS average rate. Idaho takes about $10,470 of that every year, with no offset for UK tax. Over a decade, that is more than $100,000 of genuine additional cost. Furthermore, that figure ignores the federal tax that excess UK credits cannot shelter, including the 3.8% net investment income tax, which UK tax cannot offset. Accordingly, the expat tax Idaho saving from leaving London is real but smaller than most families model.
Expat Tax Idaho Residency: The 270-Day Test and the UK Statutory Residence Test
Two residence tests apply in your arrival year, and they rarely line up.
Domicile and the Place of Abode Rule
Under Idaho Code section 63-3013, you are an Idaho resident if you are domiciled in Idaho for the whole year. Alternatively, you are a resident if you maintain a place of abode in Idaho for the entire year and spend more than 270 days there. Importantly, part of a day counts as a full day unless the visit was temporary or transitory. Domicile, once acquired, persists until you intend to leave, intend to settle elsewhere and actually move.
Part-Year Returns in the Arrival Year
Most British families arrive mid-year, which makes them part-year residents filing Form 43. Idaho taxes worldwide income only from the date domicile begins. Before that date, Idaho taxes only Idaho-source income, which for a new arrival is usually nothing. Consequently, filing a full-year resident return in the arrival year, a mistake we correct regularly, hands Idaho months of UK salary and gains it never had the right to tax. Getting the date right is the cheapest expat tax Idaho saving available.
The 445-Day Rule for Leaving
Idaho also provides a route out. A domiciled resident who spends at least 445 days outside Idaho in any 15-month period may be treated as a non-resident, provided they spend no more than 60 days in Idaho in a calendar year afterwards. Notably, a spouse or minor children living in the Idaho home for more than 60 days can defeat the claim. Therefore anyone planning a return to London, or a secondment abroad, should plan that exit as carefully as the arrival, because expat tax Idaho exposure follows domicile rather than the calendar.
Split-Year Treatment on the UK Side
Britain applies its own Statutory Residence Test, and the split-year cases that matter most are leaving for full-time work abroad and ceasing to have a UK home. If you qualify, UK residence ends on the day you leave rather than at 5 April. However, the conditions are strict, and an accompanying spouse qualifies under a different case. As a result, the Idaho domicile date and the UK split date can sit months apart, and the gap needs managing in every expat tax Idaho file we prepare.
Your UK Pension Under Idaho Rules
Pensions produce the largest and least understood expat tax Idaho costs for British retirees.
The Retirement Benefits Deduction Excludes UK Schemes
Idaho offers a valuable retirement benefits deduction, capped for 2025 at $48,216 for single filers and $72,324 for joint filers. However, it applies only from age 65, or 62 if disabled, and only to a closed list of pensions. The list covers US military retirement, the federal Civil Service Retirement System, the Foreign Service system, and certain Idaho firefighter and police schemes. A UK workplace pension, a SIPP or a final salary scheme appears nowhere on it. Therefore UK pension income is fully taxable at 5.3% in Idaho, unlike Georgia, where a UK pension can reach the retirement exclusion.
The UK State Pension Is Not Social Security for Idaho
Idaho exempts US Social Security benefits. However, the IRS treats the UK State Pension as an annuity rather than as Social Security, as IRS Publication 915 explains, so it is fully includible federally rather than up to 85%. Consequently, the UK State Pension never reaches Idaho's Social Security subtraction either. Under Article 17(3) of the US-UK income tax treaty, only the United States may tax it while you live in Boise, so Idaho collects 5.3% on every pound.
The 25% Tax-Free Lump Sum Becomes Taxable
Britain lets you draw 25% of a registered pension free of UK tax. Article 17(2) of the treaty leaves lump sums taxable in the country where the scheme sits, yet the savings clause lets the United States tax its own residents and citizens anyway, and paragraph 2 is not among the carve-outs. In our view, the lump sum is therefore fully taxable federally, and Idaho follows federal AGI. Consequently, a £200,000 tax-free lump sum in Surrey can cost roughly $14,000 of Idaho tax in Boise. Any contrary treaty position needs disclosure on Form 8833.
Periodic Drawdown and HMRC Withholding
Article 17(1) gives the United States the exclusive right to tax most periodic UK pension payments to a US resident. Nevertheless, UK providers deduct PAYE until HMRC issues a no-tax code, which requires a treaty claim on Form US-Individual 2002 certified by the IRS. In the meantime, UK tax deducted is a non-compulsory payment that earns no federal credit and, of course, no Idaho credit either. Therefore filing the treaty claim before your first drawdown is an essential expat tax Idaho task.
UK Investments, Capital Gains and Expat Tax Idaho Rules
Idaho's capital gains relief is generous, but it is built for Idaho farmers and landowners, not for London investors.
The 60% Capital Gains Deduction Stops at the State Line
Idaho allows a deduction of 60% of capital gain net income from qualifying Idaho property, claimed on Form CG. Real property must sit in Idaho and be held for at least 12 months, and certain business property qualifies too. However, gains on stocks and other intangibles never qualify, wherever they arise. Consequently, a sale of a Boise ranch might be taxed at an effective 2.12% in Idaho, while a sale of a Fulham flat or a FTSE portfolio pays the full 5.3%.
Selling UK Property While Living in Boise
A UK property sale triggers three separate obligations. First, HMRC requires a non-resident capital gains tax return within 60 days of completion, even where no tax is due. Second, the federal return taxes the gain in dollars, using the dollar cost at purchase, so currency movements can create a US gain where none exists in sterling. Third, Idaho charges 5.3% on that dollar gain with no credit for UK tax. Therefore expat tax Idaho planning on timing and ownership should happen before you list the property.
ISAs, UK Funds and the PFIC Problem
The United States ignores the ISA wrapper entirely. Interest, dividends and gains inside an ISA are fully taxable federally and therefore fully taxable in Idaho, with no UK tax to credit. Moreover, most UK unit trusts and OEICs are passive foreign investment companies, reportable annually on Form 8621 and taxed punitively on disposal. Consequently, missed reporting on ISAs and investment accounts is among the commonest problems we find in expat tax Idaho reviews.
UK Rental Income While You Live in Boise
Most British families keep at least one UK property, and the rules change considerably in 2027, which reshapes expat tax Idaho cash flow.
The Non-Resident Landlord Scheme and the 2027 Rate Rise
Once you leave Britain, your letting agent must withhold basic rate tax from rent under the Non-Resident Landlord Scheme, unless HMRC approves you to receive rent gross. From 6 April 2027, separate property income rates of 22%, 42% and 47% apply, and HMRC's technical note confirms the scheme will withhold at the new 22% property basic rate. Therefore an NRL1 application for gross payment is more valuable now than ever.
The Personal Allowance Depends on Your Passport
British nationals keep the £12,570 UK personal allowance as non-residents. By contrast, an American who holds no British or EEA passport loses it entirely. Consequently, in a mixed-nationality marriage, holding UK property in the British spouse's name can save over £5,000 a year in UK tax. For a dual national US UK couple, both spouses usually retain the allowance, which changes the ownership calculation.
The Disregarded Income Cap
Section 811 of the Income Tax Act 2007 caps a non-resident's UK tax on disregarded income such as UK dividends and bank interest. Since April 2026, the notional dividend credit for non-residents has gone, which makes the ordinary computation dearer. As a result, more Boise households should elect the disregarded basis, accepting the loss of the allowance in exchange for removing UK tax that would only generate unusable US credits. In our experience, this election is among the most valuable expat tax Idaho decisions for portfolio investors.
Federal Reporting After the Move to Boise
For expat tax Idaho clients, moving to Idaho changes federal reporting thresholds immediately, and that catches many families out.
Form 8938 at the Lower Resident Thresholds
Because you now live in the United States, the higher thresholds for taxpayers abroad no longer apply. Instead, single filers report specified foreign financial assets above $50,000 on the last day of the year or $75,000 at any time, while joint filers report above $100,000 and $150,000. The IRS comparison of Form 8938 and FBAR sets out which assets each form covers. Notably, a UK pension, an ISA and a UK brokerage account all count.
Missed FBAR Filings After Arrival
A green card holder becomes a US person for FBAR purposes from the start of residence. Consequently, a British arrival whose UK accounts together exceed $10,000, roughly £7,600, at any point in the year will need to file through the FinCEN BSA E-Filing System by 15 April, with an automatic extension to 15 October. The IRS withdrew its Delinquent FBAR Submission Procedures in July 2026, so a missed FBAR now needs a reasoned approach rather than a box-ticking late filing. Non-wilful penalties can reach $16,536 per report.
Accidental Americans and Dual Nationals Moving to Idaho
Some of our Boise clients discover only on arrival that they were US citizens all along, often through a US-born parent. For an accidental American, years of missed US tax returns and missed FBAR reports may already exist before any Idaho issue arises. Fortunately, the IRS Streamlined Filing route can still apply in the right facts, but the domestic version carries a 5% penalty. Therefore resolving prior years while you still qualify as resident abroad is usually far cheaper.
Boise's Local Taxes for High Earners
Idaho keeps its local tax picture simple, which is part of the expat tax Idaho appeal.
No Local Income Tax Anywhere in Idaho
Boise, Eagle, Meridian and Ketchum levy no city or county income tax. Consequently, unlike Portland or Philadelphia, your only income tax charges are federal and the 5.3% state rate. Idaho's state sales tax is 6%, with a few resort cities such as Ketchum and Sun Valley adding local option taxes.
Property Tax and the Homeowner's Exemption
Idaho's homeowner's exemption removes 50% of the value of an owner-occupied primary residence, plus up to one acre, capped at $125,000. Ada County's effective property tax rates are moderate by national standards, typically well below 1% of market value after the exemption. However, the exemption requires an application and does not apply to a second home or a rental. Notably, property tax is deductible federally only within the SALT cap, so it offers limited expat tax Idaho relief for high earners.
Visa Status and Idaho Residency
Most British arrivals in Idaho hold an E-2 treaty investor visa, an L-1 transfer or a green card. Idaho does not care which, because its tests turn on domicile and days. Federally, however, a green card makes you a resident from day one, while a visa holder becomes resident under the substantial presence test. As a result, the federal residence start date and the Idaho domicile date can differ, and both must be documented in your expat tax Idaho file.
A Worked Case Study in Expat Tax Idaho: The Harris Ranch Household
Consider Richard and Claire, who moved from Surrey to Harris Ranch in Boise in January 2025. Richard holds British citizenship and a green card. Claire, aged 57, holds both British and US passports, so she had filed US returns throughout her years in London. She earns $380,000 from a Boise technology employer. They kept a Fulham flat producing £48,000 of net rental profit, a UK dividend portfolio yielding £35,000, ISAs producing £9,000 a year, and Claire's SIPP, from which she drew a £100,000 tax-free lump sum in February 2025.
At the 2025 IRS average rate, their UK income came to roughly $253,000. Idaho charged 5.3% on all of it, about $13,400, with no credit for a penny of HMRC tax. The lump sum alone cost $6,983 in Idaho tax. Federally, the same lump sum faced about $46,100 of tax at Claire's 35% marginal rate, with no UK tax to credit. Furthermore, Richard had filed no FBAR for 2025, neither had filed Form 8938 despite crossing the $150,000 joint threshold, and their ISAs held four UK funds that were PFICs.
We worked the case on four tracks. First, Claire's general-category foreign tax credit carryovers from her UK salary years, around $70,000, sheltered almost all the federal tax on the lump sum, saving roughly $44,000. Second, we filed Richard's 2025 FBAR and the joint Form 8938 well before the October deadline, closing a non-wilful penalty exposure of up to $16,536 per report. Third, we made Form 8621 elections, sold the PFIC funds and replaced them with individual shares. Fourth, we secured NRL1 gross payment approval on the Fulham flat ahead of the 2027 rise to 22% withholding.
The federal saving in year one came to about $44,000, and the compliance exposure closed entirely. However, the $13,400 Idaho charge on UK income could not be reduced, because no Idaho mechanism recognises foreign tax. Notably, we also advised Claire to defer any further UK pension drawdown until her credit carryovers were fully mapped, since those carryovers expire after ten years. That is the joined-up expat tax Idaho calculation that state-only guides never attempt.
How TaxYork Can Help With Expat Tax Idaho Compliance
We prepare US and UK returns together, in one process, for British households throughout Idaho. Our expat tax Idaho work covers federal returns, Idaho Form 40 and Form 43 filings, FBAR and Form 8938 reporting, PFIC elections, UK Self Assessment, non-resident landlord filings, 60-day property disposal returns and treaty claims. Furthermore, we deal with catch-up filings where returns or reports have been missed, including missed US tax returns, missed UK tax returns and missed reporting on pensions, ISAs and investment accounts.
Clients come to us because one team runs both sides of the Atlantic. Consequently, nothing falls between two firms, and no position taken in London contradicts a position taken in Boise. Our FBAR and FATCA reporting service handles the federal disclosure side, while our tax treaty optimisation service addresses pension, residence and credit questions. For wider relocation structuring, see our cross-border planning service. Our standards follow guidance from the Chartered Institute of Taxation and the ICAEW tax faculty.
Conclusion
Idaho looks like a low-tax state, and for income earned in Idaho it largely is. A flat 5.3% with no local income tax compares well with California and Oregon. However, Idaho gives no credit for UK tax, keeps its retirement deduction for US pensions, keeps its capital gains deduction for Idaho property, and taxes the UK lump sum that Britain leaves tax-free. Therefore the expat tax Idaho saving is real only for households that plan the interaction rather than assume it.
Compare Idaho with Montana's graduated system or with Oregon, where local taxes add another layer, and Boise sits comfortably in the middle. Ultimately, the quality of your expat tax Idaho preparation, not the postcode, decides the final bill.
Contact Us
Speak to us before your arrival year closes, before you draw a UK pension lump sum and before you sell UK property. Early expat tax Idaho advice costs a fraction of late remediation. To begin, book a consultation with our cross-border team.
Email hello@taxyork.com or telephone 020 3488 8606. We act for British households throughout Boise, Eagle, Meridian, Ketchum and Coeur d'Alene.
Disclaimer
This article provides general information on expat tax Idaho matters and does not constitute tax advice. Tax legislation changes frequently, and the rates, thresholds and figures cited reflect our understanding at the date of publication. The treaty treatment of UK pension lump sums is a technical position on which views differ, and individual circumstances vary considerably. You should obtain professional advice tailored to your position before acting, and TaxYork accepts no liability for action taken on the basis of this article alone. Further guidance is available from IRS Publication 519 for US tax on aliens, from HMRC on tax on UK income while living abroad, and from the IRS yearly average exchange rates.