Section 8 and a Section 21 notices are both used to serve notice on a tenant, however they are very different, and it is vitally important that the correct notice is served to avoid delay and expense.
A Section 8 notice (also known as a Notice to Quit) is served when a tenant has breached their contract and therefore the landlord has grounds for possession. The most common ground for eviction is rent arrears, although there are of course many other ways in which a contract can be breached. If you are serving a Section 8 on the grounds of unpaid rent, you should note that the tenant must be at least 2 months in arrears. If the tenant reduces their arrears to less than 2 months at any time from the date that the notice is served to the date of the court hearing, your Section 8 notice will no longer be valid.
If the landlord has a legitimate ground for regaining possession via a Section 8 notice, the notice can be served at any time during the tenancy, including the fixed term. The notice can be immediate, 2 weeks, or 2 months, depending on the grounds for the notice.
It is often easier to serve a Section 21, even if the tenant is in arrears, as the process for a Section 8 can be costly and any grounds you give can be contested by the tenant in court. It is possible to serve both notices at the same time.
A Section 21 notice (also known as a Notice of Possession) can not be served in the first 4 months of a tenancy (or any subsequent renewal). The notice cannot end before the fixed term, and is valid for up to 6 months. A landlord does not have to give any reason for serving the notice in order to regain possession.