I'm in the UK and planning to start a side business. The income from my current job doesn't exceed my tax-free Personal Allowance of £12,500 a year, nor do I currently expect the income from this business to do so.
I believe this means that I won't have to pay taxes on any income I make from the business. Therefore, since this income isn't taxable, do I still need to declare all of it as income?
Once you've got your business up and running (and exceeded the £1k allowance mentioned in richardb’s answer), you will need to do a self-assessment (SA) tax return for the tax year in question.
As part of the tax return, yes, you declare your PAYE income as well.
HMRC in all likelihood will already have this data and, if you are completing the SA return online, you will go through the steps of simply acknowledging / confirming it.
You still benefit from the Personal Allowance, and the calculation is provided for transparency at the end.
You have a tax free allowance of £1000 to work on side projects to encourage (and often legalise) them. However, if it's more than that you need to declare income under self assessment.
