I want to start another business as a sideline to my day job. At some stage I know I’ll have to declare earnings from this, but will this then affect my Tax Code for my regular job?

Ideally I don’t want to alert my employer to my out of work business.

They don’t see how your tax code is make up, just the code itself, so they can’t tell what you’re up to.

They don’t see how your tax code is make up, just the code itself, so they can’t tell what you’re up to.

This, and I doubt payroll will be able to say anything.

When my wife does supply for other LEAs, she just gets taxed by them at the most punitive rate and has to claim it back (although I think they have actually just refunded her without any effort on her part). If it’s ongoing though they may adjust your primary tax code to balance out the income from your other employment.
I think it might be down to what somebody somewhere feels like doing on that day… it seems too random to be an algorithim.

I want to start another business as a sideline to my day job. At some stage I know I’ll have to declare earnings from this, but will this then affect my Tax Code for my regular job?

It shouldn’t, unless you start earning a total of over £100k and your tax code needs to be adjusted accordingly. You will however not get any tax free allowance on your “additional” income so your tax code for that job might be a bit odd.

Ideally I don’t want to alert my employer to my out of work business.

Whether you’d be in breach of contract with your current employer is another matter

Whether you’d be in breach of contract with your current employer is another matter

Fortunately I have no contract of employment.

Employer need not know at all.

Your new venture will presumably be self employed, so you will do a tax return each year for that venture, also fill in total tax return including earnings from PAYE. Revenue then calculates how much is owing and you pay direct.

Done it myself and tax code in PAYE stays same.

Also if you start new venture now, your first tax return for 2015/16 not due until end of 2016, so there’s no way they would know about it

Tax code is current year telling your employer how much tax allowance to give you against your earning there.
Self Assessment is retrospective for the tax yearend taking into account all your earnings. You maybe able to reclaim some of the taxes you have paid through your employment in certain circumstances.