1) No idea, did the same when we bought to give the seller a bit of time.
2) Now and then you own. If you do that add a rent to the contract that the seller needs to pay you rent and add who is responsible for Nebenkosten etc.
1) It’s up to buyer and seller to negotiate the date. Theoretically the customer is king, but this is Switzerland and there is no monarchy. So you should feel privileged to be able to find a property and buy it.
2) Depending on your canton, notary sets up payment time and possession time so it would normally be quite early on in the process.
3) I would say that logically that you should only have Eigenmietwert calculated on the time you live there, otherwise your local council would be paid twice for the year...
You get everything sorted out with the notary and finance and purchase contract where you agree the date of handover. That's the date you own the house. I.e. sometime in July.
1) As others said, it's by negotiation, but it's actually one of the things I like about the Swiss system (versus e.g the UK). It gives everybody certainty of timings.
2) Either or both. There are two steps: the purchase contract and the transfer of ownership. They can be done together on one day or separately. The transfer of ownership is when you actually start to own the place and would normally, in your case, be July. The contract is "merely" the irrevocable commitment for you to buy and the vendors to sell the apartment to you.
3) It's Switzerland so it might vary depending where you are, but I seem to recall that - unlike other taxes where your residence on 31 Dec is the reference date - for property it's 1 Jan. I sold an apartment some years ago and we couldn't get a notary date in December, so had to settle for 9 Jan. IIRC had to pay tax on the property for the whole of that year. If that applies to your case, then the vendor would pay for the whole of 2024 and you would start paying from 2025 onwards. But it also depends which taxes you're talking about (wealth tax? notional rental value? property tax?) and I'm sure someone more knowledgeable will come along. In any case, you could always check with the local tax office or a Treuhänder/accountant.