Real Estate /
Tax on UK rental income, while residency in Poland [11]
In general, tax is paid in the country in which you are resident. There are a few cases where it is paid in the country in which the money is earned, rental income being one of these. I am not an expert, but my reading of the UK-Poland double taxation treaty is that the other country does not have the right to tax that income
at all, meaning that if you have a tax allowance in the country where the property is, you get to keep it. I tested this by downloading some Polish tax-declaration software and trying some imaginary scenarios and it although I was required to declare the UK income, it did not affect the amount of tax payable. I wasn't asked how much tax I had paid in the UK so that the Urząd Skarbowy could use that as an allowance against tax payable in Poland for example. Since I am still officially UK resident, I have not tested this conclusion in real life.
However, it seems to me that your situation is different since it is not you, but your mother who is earning the income from the property. The fact that you are receiving the money means she is passing it on as a gift. I don't know if there is any special treatment for gifts, but I would guess that they count as normal income, which would be liable for taxation in Poland.