Accommodation Process

Is the accommodation first come first serve? How are you supposed to apply? And when can you apply? I have an offer from Sussex but not from my other choices yet, and it doesn't say where/how I can apply for their accommodation. Do I have to wait until I get all of my offers and I can choose my firm and insurance? I'm just worried that by time I receive all of my other offers, that my top accommodation choices will fill up if they go by a first come first serve basis...

Also, sorry if this has been already asked or mentioned elsewhere. I couldn't find it on the Sussex thread.