if someone lives abroad and rents out their property in the UK how do they declare any tax that might be due? Assuming they were not registered for self assessment here?
The property is physically located in the UK, their money is being earned here, they must pay tax on it.
The way that seems preferable to me is for the non resident to be registered for self assessment, although there is also the option for income tax to be deducted at source by the letting agent.
The issue there (besides trusting an agent) is that tax will be deducted at basic rate by the agent but the taxpayer who is not resident will be throwing away their personal allowance.
If the personal allowance was meant that they would be paying no tax the likelihood is that they would not be sent a self assessment anyway but they would still receive more of their money than if the agent deducted UK income tax at basic.
Responses are not meant as a substitute for professional advice. Answers are intended as outline only the advice of a qualified professional with access to all relevant information should be sought before acting on any response given.