Tre's been here since October 2019. He's currently in his third job. In two or all three he's paid into the standard company pension but that's it. Jobs are all <£25k per annum so not high earning and I imagine within any limits set by the US government.
Company pension is all good, nothing to worry about here.
We took out a LISA in his name to benefit from the 1k per year bonus from the UK government for first time home buyers. This was after much research and also reading someone on here saying that their accountant just put the bonus down as additional income.
Cash or Stocks & Shares LISA? Cash is totally fine, you just report the interest as US-taxable, as well as the UK government bonus - no big deal. S&S you would want to avoid all funds, as they'll be PFICs and subject to punitive US tax plus onerous filing requirements. Individual stocks are fine, if you're comfortable managing them.
Further to that, we're about to start house-hunting for somewhere bigger. New place will probably be maximum 300k, so capital gains tax on that probably not going to be anywhere the $250k per person threshold as I guess we'd need a profit equivalent to $500k in total to trigger that? We'd like to buy together in both our names, but despite the above argument, is it best not to just to make sure US government can't take a slice? I presume if he renounces before we sold they couldn't take a slice then? If we remortgage at the end of say a fixed, 2-year term, is that classed as paying off a mortgage, and so the IRS would look to tax us at the point of remortgage?
Agree, no worries on capital gains tax, assuming you meet the requirements:
https://www.irs.gov/taxtopics/tc701 (no UK capital gains tax on a primary residence). If your husband renounces and exits the US tax system, then US taxes cease to be any concern, but there are checks on exit tax in that process. The IRS would only tax at the point of remortgage if there are phantom currency gains (
https://www.bogleheads.org/w/index.php?title=US_tax_pitfalls_for_a_US_person_living_abroad&mobileaction=toggle_view_desktop#Phantom_currency_gains), otherwise remortgaging isn't a taxable event - it's the FOREX transactions that might be taxable, from the IRSs point of view, not the remortgaging itself.
We're also hoping to get a small investment buy-to-let property. I guess same questions there in terms of capital gains tax. How does income from a buy-to-let mortgage get treated? If it's in both our names do we just class his half of the rental income as an addition to his salary? Again, is it viable to buy in both our names or is it just asking for trouble?
I'll skip this one, as I'm not versed enough in buy-to-let to comment. Do look into the recent/upcoming changes in UK taxes on buy-to-let and make sure it makes financial sense, I've heard anecdotes/noise that it's getting much harder to actually make money with buy-to-let, even for UK-only taxpayers.
Regarding investing in a small stocks and shares portfolio, again is it best just done in my name?
Does your husband file as Married Filing Separately or Head of Household? If so, yes, this would be ok, as long as he's ok with everything being in your name. But if you're in the US tax system as well, as with Married Filing Jointly, the same rules apply to you, with PFIC being the big scary one to avoid.
How difficult/expensive is renouncing once he has ILR/British citizenship?
The renunciation fee is $2,350. There may also be expatraition (aka exit) tax due:
https://www.irs.gov/individuals/international-taxpayers/expatriation-tax Many countries shut down renuciantions during COVID, not sure if that's gotten better in the UK yet. Note that you would NOT want to renounce until he has British citizenship - ILR doesn't count. He'd wind up stateless, no passport, very bad juju.
So far we've just returned a tax form to the US detailing his salary for the tax year and that's been it, not had any letter back so assuming everything's ok so far but if we're missing anything urgent please let us know so we can try to fix!
Did you also report any other income? Interest on a savings account, dividends or capital gains from any stocks, that kind of thing? And do you know if you used the Foreign Earned Income Exclusion or the Foreign Tax Credit?