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How can I work out how much a side-job contracting will be taxed for?
I would say you can file your taxes on your own, but you will probably want the advice of an accountant if you need any supplies or tools for the side business that might be tax deductible. IIRC you don't have to tell your current employer for tax reasons (just check that your contract doesn't state you can't have a side job or business), but I believe you'll have to tell HMRC. At the end of the year you'll have to file a tax return and at that point in time you'll have to pay the tax on the additional earnings. These will be taxed at your highest tax rate and you might end up in a higher tax bracket, too. I'd put about 40% away for tax, that will put you on the safe side in case you end up in the high tax bracket; if not, you'll have a bit of money going spare after paying your taxes.