I'm moving to Zurich in September and by pure luck I've found an amazing apartment to sublet, fully furnished, between Sep and April.

It's quite big and I'll have 2 spare rooms so I'm considering letting one of them out to people who need temporary accomadation (the odd week here and there) at an affordable price.

Do I need to inform the tax man about this and declare it as income?

If it should be declared, surely nobody would find out as long as it was cash in hand?

Does anyone have any experiences of this?

Thanks im Voraus

It's income. All income is to be declared.
Whether you pay tax on it or not is another matter.
Tax evasion, is an all together different matter.

Extra cash is added to your tax declaration in the appropriate line.

If you have a hobby or another source of extra money there is a limit to how much you can make before you need to list it on your tax declaration. I'm guessing this extra rent would be in the same category.

Someone else on the forum had more or less the same question but was about part time work over the internet and he was paying his taxes at source. He went and spoke to the authorities who told him that he only needed to declare if he went over a certain amount. You could do a search to find that thread. And also you should go talk to your commune as well to get confirmation. (not getting into the cash in hand, how will they find out thing.... )

Now, for me, your problem is not with the tax man but with the regie and/or the person you are renting from. You'll have to ask them if you can sublet a sublet. If it were my place & my furniture I wouldn't be none too pleased. But again, you need to work that out with the legal tenant/landlord/regie etc.

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I would think that it depends on how this is done. For example, (assuming your rental contract allows for sublet) if you sublet a room to someone for a month or even a few months, that surely would not be considered as income and would therefore not need to be declared with the tax authority, no? Wouldn't this just be considered a "flat sharing"?

Precisely! if you have a roommate, and pay the rent together (ie. i pay into my partner's account, and he pays the landlord), this, presumably, isn't considered income.

It might also depend a bit as to whether this is an arrangement between friends, where, as you say, one gives the other the money... or whether there is a specific contract between the two parties.

Sometimes it turns out it would have been better to make a written agreement, signed by both, with the ruling made clear.

...and here I didn't mean just about paying the rent. There are plenty of other things which can cause friction; expensive friction. 'But you said...' is very difficult to prove.