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....

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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.