There's no reason why they can't as we've said. But most landlords are likely to be looking for long term rentals of more than 3 months which is the tourist limit. As the OP already has an apartment so long as the landlord is happy for him to stay as a tourist he should be fine.

However orangeblue, remember that after the L permit runs out you will come under the non-EU hiring rules which make it much more difficult for employers to be able to hire you. And you may be required to return to your home country to obtain a Type D visa to enter Switzerland long term legally again.

A tourist can rent an apartment. Nobody is saying that they can't, and if they are saying that, they are completely wrong!
'You can rent an apartment in Switzerland without needing to produce any residence permit at all', ... means essentially the same as ... 'a tourist can rent an apartment'.
Illegal immigrants and/or non-residents can and do rent apartments!

One more thing I am wondering.. is about deposit.
I have 3month rental fee as deposit in UBS.
Even though I de-register my self from Switzerland, does the bank account with deposit still safe?
Some of my friends said that it's not allow to have an account at UBS without permit..
(Nobody is sure about it though)
Do you know about this?

Best,

A special locked rental deposit account should fine. It'll take a little while for the landlord/agency to release the money back to you, depending on whether there's any work to be done on the apartment or not.

UBS would never know you don't have a permit unless you tell them.
Plenty of foreigners not living in CH have accounts at UBS, they have for years....

Our UBS manager is very friendly and approachable, very helpful- and he knows all his clients personally. He ceertainly would know if we went to live abroad - we pop in for a coffee and natter often.

Not everybody wishes to fraternise with their servants.

If it's not written in the contract that the tenant should have a permit, then I don't see why not

I don't know in what part of Switzerland you live, but here in Zurich i was asked for a copy of my permit every time i applied. And what's more, they would not rent me a flat on my L permit. It had to be at least a B permit. Maybe the law does not say that you must have a permit, but until now, i have not met a landlord that has not asked to see my permit. But in this case the OP will probably get away with it because the contract is already signed, and they do not have to show the permit again. So the landlord will not know about the expired visa. Now if that is legal or not, i couldn't say.

It's not a legal requirement to have a permit before you can rent somewhere. But what each landlord/agency decides to do regarding permits is entirely up to them.

It can also depend on other factors like whether you already have an employment contract, your employer has actually arranged temp accommodation on your behalf, etc.