About Optimise Accountants
About Simon Misiewicz: US & UK Expat Tax Specialist
Living between two tax systems should not mean paying tax twice. Yet for thousands of US and UK expats, investors and cross-border professionals, that is exactly what happens, usually because their accountant understands one country but not the other. I built Optimise Accountants to solve that.
I am Simon Misiewicz, FCCA, ATT, EA, CAA, MBA, and I specialise exclusively in US-UK expat tax advisory. I help clients avoid residency traps, double taxation, PFIC charges, GILTI exposure and costly property mistakes.
In short, I fix the structure that decides how much tax you pay and where you pay it.
For UK accountants, US EAs and CPAs, wealth managers and tax advisers, I also act as a specialist US-UK partner, so you keep the client and the relationship while the cross-border work is handled correctly. Most advisers lack the expertise to handle these cases confidently, and risk losing valuable clients or fees as a result.
The biggest risks come from structure, not income
After more than 20 years with international clients, the same structural problems come up again and again:
- ✓ US citizens in the UK accidentally triggering PFIC charges of 40%+.
- ✓ UK directors setting up US LLCs and facing double tax and GILTI.
- ✓ Clients becoming dual tax-resident without realising.
- ✓ Cross-border property investors paying unnecessary tax on sales.
- ✓ Expats receiving HMRC or IRS letters after years of filing incorrectly.
My story: why I created the Expat Tax Blueprint
After working with international clients for more than 20 years, one thing became obvious. The biggest tax risks were not caused by income. They were caused by structure. So I created the US-UK Expat Tax Blueprint, a clear, step-by-step method to prevent these issues before they happen, rather than fixing them after a letter arrives.
What I do
| Focus area | What I do | Your outcome |
|---|---|---|
| Residency & domicile positioning | Clarify whether you are tax-resident in the UK, the US or both, using the SRT, SPT, treaty tie-breakers and income-sourcing rules. | Correct residency status, double taxation avoided, and the right filings in each country. |
| Entity classification & business structuring | Assess whether your LLC, Ltd, partnership or hybrid is tax-efficient and compliant in both jurisdictions. | PFIC and GILTI traps avoided, double tax on profits prevented, and the right flow-through or corporate treatment. |
| Asset & investment positioning | Map your worldwide income, investments and capital gains so UK/US timing mismatches and PFIC issues are avoided. | Correct treatment of global assets, better-timed disposals and a tax-efficient investment structure. |
| Property & real estate structuring | Advise on cross-border US and UK property ownership, rental income rules, FIRPTA, UK Non-Resident CGT and ownership vehicles. | A clear view of the US/UK treatment of income and gains, correct CGT treatment, an optimal long-term structure and reduced FIRPTA and withholding. |
Who I work with
I specialise in helping:
- ✓ US citizens living in the UK struggling with PFIC and dual-residency issues.
- ✓ UK residents with US companies or investments.
- ✓ Expats with RSUs, stock options and global income streams.
- ✓ Cross-border landlords and property investors.
- ✓ High-net-worth individuals with multi-jurisdiction assets.
- ✓ Professionals relocating between the US and UK who need pre-arrival planning.
- ✓ Accountants, EAs, CPAs and wealth managers who want a specialist US-UK partner.
If you have income, assets or property connected to both countries, you will benefit from the structured clarity this blueprint provides.
My qualifications
| Qualification | What it means |
|---|---|
| FCCA | Fellow of the Association of Chartered Certified Accountants (UK). |
| ATT | Member of the Association of Taxation Technicians (UK tax). |
| EA | IRS Enrolled Agent, federally authorised to represent taxpayers before the IRS. |
| CAA | IRS Certifying Acceptance Agent for ITIN applications, able to certify passports directly. |
| MBA | Master of Business Administration, strategic finance and international business. |
This dual-qualification pathway is rare. Fewer than 1% of accountants hold expertise across both US and UK cross-border tax strategy.
Frequently asked questions
Is Simon Misiewicz qualified in both US and UK tax?
Yes. Simon holds the UK FCCA and ATT qualifications, the US Enrolled Agent and Certifying Acceptance Agent credentials, and an MBA. This lets him advise on both tax systems together rather than in isolation.
Who does Simon Misiewicz work with?
US citizens in the UK, UK residents with US companies or investments, expats with stock compensation, cross-border landlords, high-net-worth individuals with assets in both countries, and professionals relocating between the US and UK.
Can Simon partner with my accountancy or advisory firm?
Yes. Simon works alongside UK accountants, US EAs and CPAs, and wealth managers as a specialist US-UK partner, so your firm keeps the client relationship while the cross-border work is handled correctly.
What is the US-UK Expat Tax Blueprint?
A structured, step-by-step method covering residency and domicile, entity classification, asset and investment positioning, and property structuring. It is designed to prevent double taxation and cross-border errors before they happen.
What are the most common cross-border tax mistakes?
PFIC charges on UK funds for US persons, US LLCs causing double tax and GILTI for UK directors, accidental dual tax residency, unnecessary tax on property sales, and discovering errors only after an HMRC or IRS letter.
Let’s fix the structure before it costs you
Whether you are an expat, an investor, or an adviser looking for a US-UK partner, a short call is the quickest way to see where you stand.
This page is general information and not advice. Tax rules, rates and thresholds change. Take specialist advice before acting.
