When the fixed term (minimum of 6 months) period of an assured shorthold tenancy ends there are several options for both landlord and tenant: 

(1)  If your landlord has served a valid Section 21(1)b (notice requiring possession) giving at lease 2 months' notice before the last day of the fixed term, then you will have to leave immeditely after your 2 months' notice ends, if the landlord still requires it.  (Note: landlords sometimes serve notice as a matter of course early in the tenancy and will not require you to leave at the end) If the notice was served less than 2 months to the end of the fixed term you will not be required to leave until after the 2 months is up.

(2)  If your landlord offers you a new tenancy term by asking you to sign another agreement, then obviously you can stay until the end of that new tenancy term.

(3)  If the landlord (whether he served a Section 21 notice or not) does nothing to indicate that he wants you out, then your tenancy will automatically (without the need for either party to do anything) become a Periodic Tenancy.

A periodic tenancy is for a period equal to your rent payment periods, e.g, if you pay rent monthly then the tenancy becomes a monthly periodic tenancy. In other words you enter into a series of one-month tenancies which will continue on indefinitely until one or other party decides to terminate.

If the landlord wants a periodic tenant to leave he must now serve a different Section 21 Notice - a Section 21(4)a. Here the notice must be for a minimum of 2 months ending on the last day of a rent period, which in practice is usually more than 2 months. Note the notice should identify a specific date (the last day of a period) AFTER which the tenant must leave.

If the tenant decides to leave during a periodic tenancy (the tenant cannot leave during the fixed term) she must give the landlord one month's notice in writing (where rent is paid monthly) or 4 weeks' notice in writing (where rent is paid weekly).

Note: never rely totally on these standard answers. Before taking action or not, always seek professional advice with the full facts of the case and all documents to hand. LandlordZONE.co.uk