Employed by a US company - Living in the UK

In short, no substantial changes compared to your previous PAYE job.

We can ignore state tax issues because Washington state levies no income tax, but even if it did you would qualify as non-resident provided you don't live or spend much time there. That leaves you US federal tax and HMRC to deal with.

Under Article 14 of the US/UK treaty, primary taxing rights fall to the country of residence, so (presumably) the UK. Because of the US's nasty "saving clause" in Article 1 Para 4, as a US citizen you then have the pleasure of filling out a US tax return, claiming either a Foreign Tax Credit for UK tax paid or the Foreign Earned Income Exclusion (FEIE) for UK salary, with the likely outcome that this wipes out any actual US tax liability. Same as your old job there, then.

Now, the tricky part is to get this to happen without being in the PAYE system.

If your company has a UK presence, becoming an employee of that is by far the simplest way forwards; you're done. If it really has no UK presence then it won't be able to handle UK payroll, UK tax PAYE, UK NI, and so on, and this could produce some difficulties.

One way around these difficulties would be for you to form a one-person UK corporation and have your US employer pay that as if a contractor, then you can handle UK PAYE and NI from your corporation. The wrinkle with that is US tax law, which pushes a lot of expensive tax reporting onto 'Controlled Foreign Corporations', which yours would be (Google "IRS CFC paperwork" and weep!). A one-person US corporation might be easier from the US tax perspective, but probably hard to open as a non-US resident. Or you can probably handle this through normal HMRC Self Assessment and perhaps some direct Class 1 NIC payments to HMRC. Armed with this set of options, HMRC really should be able to advise on suitable routes here.

Finally, you will want to pay close attention to how much time you spend in the US. Anything more than around 30 days in the US and you lose the ability to claim the FEIE (it requires at least 330 days of non-US residence in the year). That 36th day in the US could prove very expensive, then. Again, this was potentially already the case with your previous PAYE job, if you claimed the FEIE on that salary.

After five years in the UK, I assume that you are well aware of the US tax pitfalls of using perfectly vanilla UK accounts and investment vehicles such as ISAs, pensions, non-US domiciled unit trusts or ETFs, OEICs, and so on.

js90 wrote: »

... This job has nothing to do with the UK, besides the fact I will be resident here....

It's your residence that is the issue.

My advice would be, get an accountant.

As far as I can see, based on the following discussion;
https://www.taxation.co.uk/Articles/2013/07/17/310691/personal-paye

if the employer has no tax presence in the UK it is unable to operate PAYE. It is therefore your responsibility to register to pay tax through self-assessment. You may well be entitled to double tax relief if your employer is deducting US taxes.

The employer would have to comply with local English or Sottish employment law. The employer may have a corporate tax liability. Your position would be largely unchanged unless you have US workdays, because income attributable to US workdays would be US source; but the employer would doubtless withhold Federal & State tax on US source income.