Bonus month will be a rude shock. The way your withholding tax is calculated each month is based on monthly earnings, then annualised.

So in most months, you'll be taxed on the basis of your income of ~9000CHF per month.

But supposing you got a 15K bonus in December (for a total pay of CHF24K). You'll be taxed at a rate that assumes you earn 288k per year. And the rate of withholding in December will be much higher than you're used to.

And watch your job contract as well. If they pay you a 13th month, you'll get screwed in that month, too. Even worse if your 13th month and bonus are both paid at the same time. Relocation allowances (above the CHF 5K which are tax free) also cause the same problem -- best to spread them across a number of months if your HR department will allow it.

If the bonus takes you over CHF120k per year, you'll be required to file a tax return (which may actually work in your favour, particularly if you've been hit hard by a bonus payment in a single month). As a B-permit holder, you will continue to have your tax withheld, with any balance owing adjusted at the end of the year. You may end up owing, but you can also claim back any tax you've overpaid.