Doesn’t seem so for Basel (Stadt), as far as I read.
They stated clearly that only what can be proven incurred will be deducted. 
Source: https://www.steuerverwaltung.bs.ch/dam/jcr:89a3202f-8461-4642-bfc7-b0eada1e79bf/18009_mb_np_vermoegensverwaltungskosten_v2020_cd_bf.pdf
Last year I listed 120 CHF for IB, and 120 CHF for UBS (which was not really justified as no investments there) - but got accepted.
Perhaps they don’t care for my puny amounts.
You did this in BS and it went through?
According to the guidelines, the ZH Tax office accepts donations up to CHF 300.00 without evidence.
Seems right, my tax attorney added that without any evidence.
Yes I have the antimustachian tax attorney 
web translation of the BS document:
Deduction measurement
6 In principle, only the administrative costs actually incurred and proven are deductible. Not only does the taxable person have the existence and amount of of the capital costs incurred, but also their property as deductible administrative costs (extraction costs).
7 In the case of capital costs relating to investments or securities deposits, which are fixed in their amount but in which a separation into deductible administrative costs (extraction costs) and in non-deductible investment costs is not possible or only at disproportionate expense, the assessment practice leaves a flat-rate deduction of a maximum of 3% of the market value of the investments in question as administrative costs, without requiring proof of their mining cost status. Thus, up to the limit of 3’ only the the existence of effectively incurred capital costs, but not the their deductibility in the sense described below Rz 3 above.
8 If the capital costs claimed for deduction are higher than 3% of the market value of the investments, then the taxable person must also prove the cost of obtaining those costs. If the proof does not succeed, then only costs up to 3" can be deducted.
The evidence submitted shows that the deductible administrative costs are less than 3% of the market value of the investment, only the effective administrative costs are deductible; in this case, it is necessary to take into account the actual circumstances, not the flat-rate limit.
Proof of the cost of extraction is required on the basis of appropriate documents
(contracts, regulations, settlements) from which, in addition to the scope,
also shows the nature, purpose and composition of the costs related to securities and capital assets.
looks like under 3%o we should not prove anything? (the first sentence in german is a little bit ambiguous…I think I will leave it without any deduction)
That’s true. One friend is always including those 300 CHF as a cash donation on the street lol.
Also read the whole thing @valarmorghulis It seems that you can deduct up to 0.3% of your assets without proof.
I think the 0.3% is like the donation, it’s not technically a pauschal (unlike other deductions), just the amount where no documentation is required.
It’s morally dubious, I guess everyone decides for themself.
Is that so? So if they ask for justification, or simply ask me (I will not lie) I am not entitled to this 0.3%? That is news to me 
I am pretty sure my actual costs are a fraction of that…
They won’t ask tho, it’s documented as such.
(Also asking for 0.3% but I’m doubting myself every year wrt morality, at least in a few years I’ll hit the 6k limit)
I read the document provided by baltax and understood that no proof needed, then was told the same here so it quite confirmed my understanding. This is the first time I do this so can’t give feedback if it’s accepted (but I think it will)
the question is, what if they will ask you proof of these costs? what are you going to answer / provide?
Either you have transaction advices or ask the company/organization you donated to for a tax certification.
If you hand out 1000s of Franks to people on the street, you might simply not be able to deduct it due to no evidence presented.
sorry I meant for the management cost of investment account (so the 3%o argument)
from the (annual) investment account statement where they list the charged fees?
I’m taxed at source. However I’m interested in filing a tax declaration mostly to get 3a money back. Now, I have lived in BS until January but currently living in ZH. I assume the tax declaration has to be filed with the ZH canton, even if refers to 2020 during which I lived in BS, is that correct?
Tax declaration should be submitted to the tax office where you have been registered as of 31 December of the year the return refers to.
Here is my contribution for people who want to filed their Tax declaration in the Canton of Vaud with VaudTax20 
(I used a fake investment, though I will love to have that yet).
If you have VWRL:
If you have VT:
Hope it could help, and please correct me if I missed something as I didn’t need to filed this for 2020, but will do it for 2021
PS: You can have more detail on the blog of @_MP 
I believe it is very similar in Geneva with GeTax.
I’ve bookmarked your thread for my own use next year as I will need to fill it as well.
In Geneva Getax you can attach a summary from your broker and enter it as one line
Any summary? Or does it have to be in the official “relevé fiscal” form?
