Mega Backdoor Roth

After-tax 401(k) contributions can either be converted in-plan to Roth 401(k) or rolled out-of-plan directly to a Roth IRA. The contributions themselves have already been taxed, so neither conversion nor rollover triggers additional tax except on any pre-conversion earnings. If you roll after-tax dollars straight to a Roth IRA, they enter as Roth IRA contribution basis, which you can withdraw anytime since the tax was already paid. If you instead do an in-plan Roth 401(k) conversion, then roll it to a Roth IRA later, the dollars become Roth conversion basis—already taxed, always income-tax-free to withdraw, but subject to the 5-year penalty window if you take them out before age 59½.

The strategic tradeoff is: direct Roth IRA rollover = maximum immediate access to principal, while in-plan conversion → later rollover = maximum long-term Roth basis, because all the growth that happens inside the Roth 401(k) becomes conversion basis in the Roth IRA (fully withdrawal-eligible after 5 years). A Roth 401(k) has structural drawbacks while employed: no contribution-basis access before 59½ or separation, typically limited investment menus, and sometimes higher fees. All of these disappear once you leave your employer and roll the Roth 401(k) to a Roth IRA.

Inside Roth accounts (Roth IRA / Roth 401k), the tax has already been paid on contributions, so all future growth and withdrawals are tax-free — meaning you want your highest-expected-return, highest-tax-inefficiency assets there. In Traditional accounts (Traditional IRA / Traditional 401k) where withdrawals are taxed later as ordinary income, you concentrate your lower-return, ordinary-income-heavy assets to minimize portion of total growth the government takes. In taxable accounts, hold long term assets with low ongoing tax drag.

UK Residency Investing

A U.S. citizen living in the UK is taxed by both countries on worldwide income. The U.S. taxes based on citizenship, and the UK taxes based on residence. The treaty and foreign tax credits prevent double taxation of the same income, but the two systems classify pensions, taxable accounts, and investment funds differently.

Retirement accounts are the most straightforward because each country generally treats the other’s pensions the way they are treated in their home system. While money remains inside these plans, neither system treats the individual as directly owning the underlying investments. As a result, U.S. retirement accounts such as a 401(k), Traditional IRA, or Roth IRA operate normally while you live in the UK: the UK does not tax their internal growth or require reporting. Similarly, the U.S. treats UK pension plans as pension arrangements rather than taxable accounts, so U.S. foreign-fund rules do not apply to the investments held within them, and no annual U.S. tax arises while the assets remain inside the pension.

You generally cannot contribute to U.S. pensions while working solely in the UK because a 401(k) requires a U.S. employer, and IRA contributions require U.S.-taxable earned income, which UK salary does not create once foreign exclusions or credits are applied. However, working in the UK does allow full participation in UK pensions, since UK pension eligibility is based on employment in the UK rather than citizenship. As a result, new retirement saving usually occurs through UK pension structures, while existing U.S. pensions remain invested but inactive for contribution purposes.

UK workplace pensions and Self-Invested Personal Pensions (SIPPs) operate within the same tax framework. Both accept pre-tax contributions, both are locked until the UK’s pension access age—which is scheduled to reach 57 in 2028—and both count toward the same total annual allowance, which is the lower of £60,000 or 100% of UK earnings, with significant tapering for very high incomes (reduced by £1 for every £2 of income above £260,000, down to a minimum of £10,000). Workplace pensions receive employer contributions that vest immediately. Employee contributions enter the pension as pre-tax income, either by deducting the contribution before payroll tax is calculated or by contributing from take-home pay and having HMRC add back basic-rate tax relief. SIPPs follow the same contribution and tax-relief rules but do not include employer contributions and allow a wider choice of investments and greater flexibility in how much is contributed. In both cases, the pension trust—not the individual—is treated as the investor for U.S. tax purposes, so the underlying holdings do not trigger PFIC rules. U.S. foreign-account reporting obligations such as FBAR and Form 8938 apply to UK financial accounts but affect only reporting, not tax.

Taxable investing is where the two systems diverge most sharply. Almost all UK-domiciled pooled investment vehicles—OEICs, UCITS ETFs, accumulation share classes, and nearly all funds held inside ISAs or ordinary UK brokerage accounts—are classified by the U.S. as Passive Foreign Investment Companies. PFIC status does not always create annual tax, but under the default regime the U.S. taxes the investment’s accumulated gains, including unrealized gains, at the highest marginal rate when the asset is sold and adds interest charges intended to approximate tax that would have been owed in each prior year. PFICs also require detailed annual reporting. Because these consequences are economically significant and structurally burdensome, UK-domiciled pooled funds are generally unusable for a U.S. citizen outside of a UK pension, where PFIC rules do not apply.

The UK system is simpler but not necessarily more favorable from a tax-rate perspective. The UK divides pooled funds into “reporting funds” and “non-reporting funds” based on whether the fund supplies HMRC with annual income data. Reporting-fund gains are taxed as capital gains. Most U.S.-domiciled ETFs and mutual funds are non-reporting funds. The UK applies no annual tax to unrealized appreciation in non-reporting funds and requires no special reporting, but dividends and interest are taxed annually, even if reinvested, and all gains are taxed at ordinary-income rates upon disposal rather than at the lower capital-gains rate available for reporting funds. Reinvested dividends increase the UK cost basis and thereby reduce the taxable gain at sale.

Gains can be realized without UK tax during two periods. The first is while eligible for the UK’s Foreign Income and Gains (FIG) regime, which generally applies during the first four UK tax years if you were not UK-resident in the previous ten. During this period, foreign investment gains are not taxed by the UK if the proceeds are not remitted to the UK. The second period applies after leaving the UK. If you remain non-resident for five complete UK tax years, the UK does not tax gains on investments sold thereafter. Outside these windows, selling U.S.-domiciled funds while UK-resident results in UK ordinary-income taxation of the gain, with the U.S. liability typically eliminated through foreign tax credits.

Taxable investing is therefore best handled through a U.S. brokerage using U.S.-domiciled funds, since UK-domiciled funds trigger PFIC rules and U.S. funds receive predictable UK treatment. Many U.S. citizens abroad maintain existing U.S. brokerage accounts by keeping a U.S. mailing address on file, a widely used grey-area practice. Interactive Brokers provides a fully compliant alternative, accepting UK residents and offering low-cost GBP-to-USD conversion. A UK bank account is required for salary and local expenses, and a U.S. bank account is usually needed to fund the U.S. brokerage account. The practical sequence is receiving salary in the UK, converting GBP to USD, transferring funds to the U.S., and investing through the U.S. account.

US Taxes Abroad

A U.S. citizen is taxed on worldwide income regardless of residence, and tax treaties generally prevent double taxation rather than eliminate U.S. tax. In high-tax treaty countries such as the UK, France, Germany, Canada, and Japan, local income-tax rates exceed U.S. rates, so the U.S. foreign tax credit eliminates the U.S. liability and you effectively just pay the foreign rate. In lower-tax countries such as Singapore or Hong Kong, you pay the local rate and then U.S. federal tax on the amount by which your total foreign-source income exceeds the U.S. rate. The only major domestic relief is the Foreign Earned Income Exclusion (FEIE), which allows roughly $126,500 of earned income to be excluded from U.S. federal tax, and the Foreign Housing Exclusion, which allows additional exclusion of housing costs above a base amount, raising the total exclusion to roughly $150k–$160k of salary in high-housing-cost locations such as Dubai, Doha, and Bahrain. In zero-tax jurisdictions, this means salary up to the FEIE + housing limit avoids U.S. tax entirely, while all salary above that threshold is fully subject to U.S. federal income tax. In low-tax countries, you pay the foreign rate and then U.S. federal tax on income above the FEIE + housing exclusion; in high-tax countries, the foreign tax credit eliminates U.S. tax and you simply pay the foreign rate. FEIE does not apply to dividends, interest, capital gains, rental income, or business profits that are not taken as wages, and self-employment income remains subject to U.S. self-employment tax unless the individual is a true employee.

Puerto Rico has its own tax system because Section 933 excludes Puerto Rico-sourced income of bona fide residents from U.S. federal tax. Under ordinary Puerto Rico law, wages are taxed at progressive rates up to about 33 percent, long-term capital gains at 10 percent, short-term gains at ordinary rates, and corporate income at effective rates of roughly 18–37 percent.

Act 60 adds two separate incentives meant to attract new residents and investment, but only for individuals who qualify as bona fide Puerto Rico residents—meaning they spend roughly 183 days per year on the island, maintain a Puerto Rico tax home, and have a closer personal and economic connection to Puerto Rico than to any U.S. state.

The Export Services incentive applies to operating businesses: a Puerto Rico corporation performing services in Puerto Rico for clients outside Puerto Rico pays 4 percent corporate tax and its dividends are taxed at 0 percent, with the income entirely excluded from U.S. tax because it is Puerto Rico-sourced.

The Individual Investor incentive applies only to capital gains: post-move, Puerto Rico-sourced gains are taxed at 0 percent, while pre-move appreciation on assets owned before residency is taxed at 10 percent. Most U.S. equities, ETFs, and crypto purchased after moving qualify for the 0 percent rate; U.S. real estate and other statutorily U.S.-sourced assets never qualify. The Individual Investor decree is available only to people who have not been Puerto Rico residents in the prior 10 years, which makes the 0 percent rate accessible to newcomers and returning Puerto Ricans who lived abroad long enough, but not to long-time locals.