Income boost becomes second income (tax? etc?)

Skyhigh

I've already looked for similar threads but can't spot them, if there are any please point them out

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I can boost my income offering website development and design outside my work hours. The average site I build could be charged anywhere between £500 for a basic site of a few thousand for a more complex page (i.e. corporate sales site).

Getting work, doing the work, etc isn't a problem.

What I'm not sure about is exactly how to go about this in regards to tax and such.

[I worked with a friend who did this, he had his own company - so it all went through that I assume].

Do I need to set up a business (and therefore business bank account) for the money to go through?
Or can it just go into my account and then I declare it as 'additional income' on my personal tax return at the end of the year (and then pay all the tax charges, etc along with it?).

If I did either, how would my tax code be affected - if at all?
My basic rate is under the higher tax bracket, so I'm on a specific tax code (522L?) for it. But if I declare my additional earnings at the end of the tax year and it takes me over the high tax bracket, then....will my tax code be different for my 9 to 5 job the following year? I'm not sure if it would be, since the additional work is not guaranteed, only my basic salary at work is...?


Please ignore me if I seem ignorant, especially in regards to personal tax returns, I've never filled in a tax return form (since I've only been in employment for a few months after graduating from Uni in June).