Basically the question, nothing more to add
the whole amount (text added to reach 20 chars min)
Social insurance and tax law are generally not concerned about what percentage you’re working, but instead how much you earn (an exception would be partial unemployment when registering as a job seeker and/or applying for unemployment benefits).
If you’re not covered by a pension fund, you can only deduct 20% (subject to a maximum of CHF 34’416) of your employment income.
I guess, in most cases, that’s true.
But: It’s not a question regarding “deducting in your taxes”, it’s a question regarding “paying into a 3a pillar at all”. If you are eligible to pay a certain amount into a 3a pillar, you can deduct this amount at 100% in your taxes…
What can you pay into a 3a pillar (and therefore deduct 100% in your taxes)? The whole amount ONLY if you are paying into a 2nd pillar. Otherwise, it’s 20% of your net income (or max. 34’416 - not relevant in this case). A 2nd pillar is mandatory if your gross income is > 21’330 p.a (as of 2020).
So, given a 50% job paying < 21’330 p.a. you don’t contribute to a 2nd pillar and therefore you can only pay 20% of your net income towards a 3a pillar (and only deduct this amount in your taxes)…