When you look depends on the kind of accommodation that you are looking for.
YSA Houses
London is a very transient place, especially for students and young professionals. YSAs are constantly moving in and out of London. As such, it's a good idea to keep an out for housing on the Facebook pages.
This is a link to the OFFICIAL London YSA facebook page. Here you will find postings for flatmates wanted and people looking for housing. Post your details here so that people can contact you if they have a room available or if they are looking for a group of people to start a new flat.
http://www.facebook.com/#!/groups/hydeparkysa/
Here is a link to a historic London YSA facebook page that is still in use. Most people post to both pages, so there will be overlap, but it's a good idea to check both pages in the event that one page has a listing that the other does not!
http://www.facebook.com/#!/groups/londoncfya/
If you are looking for a flat/house or a room with a resident landlord, you will not normally be able to arrange this more than 6 weeks in advance. The same is true for YSA housing. Most people who are not students will not list their housing availability sooner than a few weeks out.
Students
If you are looking for a place in an independent student hostel or private hall of residence, you may need to apply months in advance. Contact your university to find out more about these options.
Some university accommodation offices will encourage their landlords to advertise early for September. This means that some lucky students are able to arrange their housing as early as May. However, it is a small minority of students that are able to do this and there is no real evidence to show that housing found in May is any better than that found later on.
The majority of students do not arrange their housing until August/September. They will usually arrange temporary housing in London either in a hotel, college hall of residence (usually open to guests both within and outside the University over the summer) or hostel whilst they are looking. Provided you are well organised, it shouldn’t take more than two weeks to find your housing.
It's easiest to look for housing while in London, but some people are able to arrange for housing prior to arriving. Just know that you may end up with something that isn't your ideal situation. Do you research so that you are not surprised by the location, commute to your university or workplace, and the cost of living.
