Hi,

I work part-time for the EU company and use a payroll service to pay taxes and social deductions.
I was surprised that the tax in my latest salary statement was doubled. The payroll company explained to me that they have to tax me like I'm working for 100%. Since they do not know if I'm working anywhere else.

"If the employer does not know the employee’s total degree of employment, the wage for the rate-determining income is calculated on the basis of a degree of employment of 100%."

Does anyone have the same case? Is there any option how to pay withholding tax based on my real income?

See also this thread: New laws about withholding taxes effective 1/1/2021

Once it is 2022, but before March 31 2022 request a reassessment.
As explained in Section 6.5 of Kreisschreiben 45.

This means, that you will have to do a full tax return (An other change which comes with the new Kreisschreiben 45).
See 2021 Changes in tax at source reassessment [Kreisschreiben 45]

Their argument makes no sense - on that basis, if you worked 20% you'd have no income at all after source tax.

There must be some way you can make a statement that this is your only job.

No employer can know for certain that it is your only job, unless you literally work 100% meaning 24/7 !

That is not how math and tax works. The tax rate will never be 100%.
There is always the taxable income (steuerbares Einkomen) and a tax rate determining income (satzbestimmendes Einkommen).

Example: If some body makes 40 CHf /h and works 15 h per week they will get around 4 ⅓ weeks/month · 15 h/week · 40 CHF/h = 2580 CHF/month while working 65 h/month. This is the taxable income.

In Canton Zurich, on tariff A0 (single, no kids, no church affiliation) the tax rate would be, or was under the old regime where ax rate determining income was the same as the taxable income, 2.14%

Now, for the determing the aplicaple tax rate the employment gets extrapolated to a full time employment which is 180 h/month. The applicable taxe rate is thus the one for a income of 2580 · 180 / 64 = 7256.25 CHF. This is the tax rate determing income. This results in a applicable tax rate of 8.83 % and the tax at source deducted from 2580 CHF, the taxable income, is accordingly 227.80 CHF.

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"Okay, I just hope we don't wake up on Mars or something surrounded by millions of little squashy guys."

I was assuming they are taxing the full 100%, not just setting the rate based on that.

Seems surprising the tax rate would double, but as your calculation shows the rates go up pretty quickly.

My employer asked for self-declaration if you have additional employment in Switzerland or elsewhere, if yes then what is the workload/salary to determine the correct tax rate. I think this new rule of determining the tax rate is for everyone paying withholding tax irrespective part-time or full-time job.

E.g. In the past a person having 100% job with a monthly salary of CHF 6000.- had to pay more withholding tax than a person having two 50% job with a monthly salary of CHF 3000.- each. Now it will the same. My two cents!

Assuming that you have no other employers, declaring that they are the only employer should be sufficient, in my opinion.

The employer is now just covering its ass, as they would be liable missing tax at source payments under the new law.