How necessary is it to book hotels in Sep

Take a look at Booking.com and see what is available. Also check Hotels.com. It should give you an idea of what is left on those sites and prices. I’d get booking rooms ASAP.

It usually starts to get quieter in Switzerland the end of September but it depends on where you are. I personally would not take the chance as I am a bit fussy about my lodgings. I don’t need fancy but I need a good location and decent facilities among other personal criteria.

Do you have any idea what you want to go or where you want to go? Can you at least take a stab at an itinerary and maybe book the first week after Chamonix?

It seems likely that it will be weather dependent as to where we go so it would be hard to book something given that unknown.

You know that you need to end in Zurich. I would suggest that you make a reservation somewhere near Zurich for your last night(s).

I'm not picky about lodgings, but as a budget traveler I'd be very worried about being forced into a higher category of hotel than I wanted to pay for. However, I haven't been to Switzerland in ages, so this is just a theoretical possibility.

At the very least, I agree that keeping a sharp eye on booking.com would be prudent. It can give you an early warning signal that you may need to alter your itinerary. I failed to follow that advice on my current trip and ended up paying way too much money in one town due to an automobile rally and in another due to a music festival--neither of which interested me at all.

It seems likely that it will be weather dependent as to where we go

It seems likely that it well be weather dependent and availability dependent as to where you will go.

I think that this September it’s wise to book ahead because Europe is having a record-breaking season this year welcoming international travelers and September in Switzerland is still high season.