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understand taxes when geting money from a project built long time ago plus my full time job
You need to register as self-employed with HMRC (it is perfectly fine to be self-employed and employed by an employer at the same time, in exactly your kind of situation). Then, when the income arrives you will need to declare it on your yearly tax return.  HMRC information about registering for self-employment and declaring the income is here: https://www.gov.uk/working-for-yourself/overview There's a few extra hoops if your clients are outside the UK; the detail depends on whether they are in the EU or not. More details about this are here: https://www.gov.uk/online-and-distance-selling-for-businesses/selling-overseas .