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Please specify your status in the US - are you US citizen or permanent resident (green card)?
Are you a resident or non-resident in the US?
As a resident of the US - you pay income taxes on all income worldwide.
An endowment contract is a policy under which you are paid a specified amount of money on a certain date unless you die before that date, in which case, the money is paid to your designated beneficiary. Endowment proceeds paid in a lump sum to you at maturity are taxable only if the proceeds are more than the cost of the policy.
In general, your cost (or investment in the contract) is the total of premiums that you paid for the life insurance policy, less any refunded premiums, rebates, dividends, or unrepaid loans that were not included in your income.
You should report the total proceeds on the line 16a and the taxable part on the line 16b of Form 1040.
Endowment proceeds that you choose to receive in installments instead of a lump-sum payment at the maturity of the policy are taxed as an annuity. For this treatment to apply, you must choose to receive the proceeds in installments before receiving any part of the lump sum. This election must be made within 60 days after the lump-sum payment first becomes payable to you.
Does this take into account the tax harmonisation between the UK and US?
In the UK this would be tax free.
The US-UK tax treaty apply for UK residents.
US residents generally are not subject of the tax treaty (there are some exemptions for UK government pensions paid to green card holders)
Tax harmonization refers to the process of making taxes identical - as the best of my knowledge - there is no such activities between US and UK.
There is an agreement to avoid double taxation - when the same income is taxable in both in UK and in US. In this case as a resident of the US you would claim a credit for taxes paid in UK, however as you stated - that income is not taxable in UK - there is nothing to claim.
In the US - you need to determine the basis - the total of premiums that you paid for the endowment contract. The difference between the total proceeds and the basis - if positive - is your taxable income.