In my case, the seller had the license of 1st occupation when she moved in - she was the 2nd owner, and has lived there for 10 years. Before selling to me, she should have asked for the license of 2nd occupation, but didn't. Her agent didn't advise her to do this.
My first solicitor was a clown, who didn't even ask for either 1st or 2nd occupation licenses, and told me everything was legal and ok, so I signed the contract, and paid the deposit (yeah, I know.. slaps forehead).
Seller then agreed (thank god) to get the license of 2nd occupation before notary date, even though it wasn't listed as a necessary document in the contract we signed.. it should have been. Again.. solicitors fault. It isn't a legal requirement to have them, as others have already said, but it really is best that you get one, and make it a condition in the contract you sign with the seller.
For me, the complication was then that the first owner had done an extension, so my seller then had to get an architects report to prove that the work has been done to all norms, it wasn't a new 'level' etc, then submitted that report to the council with the request for the 2nd occupation license.
The council ok'd it - the extension had to be signed off by the municipal architect at the council who reviewed the seller's architect's report. Then finally the 2nd occupation license was issued (phew).
It took exactly 3 weeks from the submission of the application to me receiving it. So my guess is that you won't get it any faster than that / cannot rely on it since yours has an extension too.
The big question is whether you have signed a contract or not ? Is your completion date just a verbal thing, or have you actually signed the contract. And is the requirement for the 2nd occupation license in that contract ?
If you have signed a contract, and it doesn't stipulate the 2nd occupation license, then I suspect you're on a sticky wicket, since you have (by signing) agreed to pay the deposit. And if you are now trying to withhold the deposit, and basically break the contract, then expect legal complications and expect them to come after you for the deposit. I was told the deposit must be paid within 48 hours of signing the contract.
If you haven't signed anything, then you're (unlike I was) in a good position.. just say that nothing will be signed and deposit won't be paid until you get the 2nd occupation license.
Or to be more flexible, you don't have to insist on the 2nd occupation license up front, but make it a condition of the contract you sign, saying it has to be there before notary. And set latest completion date to be many weeks into the future to give the seller time to get the 2nd occupation license.
Disclosure: I'm not a legal eagle, this is just based on my recent experience / hope it's useful. Your solicitor really should be advising you on this stuff.. what you can and can't do, based on what you have or haven't signed.
I got very lucky in spite of me being an idiot in signing and handing over the deposit without checking the lawyers work. I now have all the docs.. hope it works out for you.