I have a job offer from a Swiss company to start in 4 months. I am aware that on arrival I have to register for a B permit for EU nationals. I'll be living in temporary accommodation for a few weeks when I first arrive (B&B) until I sort out a 1/2 year rental agreement. I've been told by HR to make sure that I have proof of accommodation for a 'long period of time' for the B permit application. I am a bit confused about this requirement.

If someone has recently filled in this application, can you please help? What kind of proof of accommodation do you need and how long should it be for?

At least a month and somewhere you can receive post at since you need to be able to put your name on the postbox as your permit will be posted to you when its ready. Or alternatively your post is addressed to you c/o whoever at the address. Hotel isn't a problem since people can receive post in those, but you'd need to check with the B&B owner if it's possible. As for proof, a booking form showing dates for your period of stay or an e-mail with same possibly. And if you're only going to be here for half a year you won't get a B permit, those are only issued for employment contracts of over a year. If it's less it'll be an L permit.

Thanks for the information.
The work contract does not have a time period in it. I'm looking at AirBnB and also bnb.ch/en for accommodation. Receiving post at these places shouldn't be a problem. I guess I'll just have to make sure I have a booking receipt and maybe also a letter from the hosting family.

I would also check with the cantonal authorities that the accommodation would be accepted before you book it. Some cantons accept AirBnB, others don't.