Let's say I expect to generate $500/month of steady income from my side company (single member LLC). I work FT during the day and taxes are paid accordingly. Do I need to worry about estimated taxes, assuming my withholdings are reasonable (S1)?
What happens if I don't file my estimated taxes?
EDIT: I live in the US and recently got married. My wife's W4 is S0 while I'm M1. I may change mine to M0 to offset her large refund.
No. If you are employed, and your employer is withholding enough taxes to account for all of your income, you do not have to file estimated taxes.
If your employer is currently not withholding enough, all you have to do is file IRS Form W-4 with your employer to increase the amount they withhold.
This is assuming you are in the US.
You haven't indicated if you live in the US, as Zuly noted, so I too am assuming this.
If you wind up owing more than $1,000 in taxes, you have not withheld enough and are subject to penalty: (Source: IRS Website ).
So the question is, how close is your withholding to your actual tax liability? If every year you get a refund of $1,000 and you're in the 28% tax bracket, you don't need to worry about prepayment on an extra $6,000/yr of income. But if you're in a 28% tax bracket, and you pay $1,000 every year when you mail your tax return, then you should be looking into making quarterly payments (of, say, $400).
You can read about how to make quarterly payments by looking into the 1040 ES. This will become more important as your side income grows, which hopefully it will!
You are employed and holding a side business. Have you looked into potential conflict of interests? Your employer can potentially claim IP developed at your side business without the legal aspects resolved.