The most basic difference between a section 8 and section 21 is that a section 8 notice is served when a tenant is in breach of contract (eg,, rent arrears), and a section 21 is served to end a tenancy agreement, simply so that the landlord can regain possession.
A “section 8 notice to quit”, also known as a “section 8 possession notice” is used to terminate an Assured Shorthold Tenancy Agreement (AST), and can be used at any time during a tenancy. Rent arrears are the most common cause of a section 8 notice.
A landlord cannot evict their tenant without first obtaining an order for possession from a court. Before you do this, you must serve a “Section 8 notice to quit” form on the tenant. This notice states that you intend to seek possession of the property and states the ground(s) on which possession is sought.
A “section 21 notice of possession” is served to end a tenancy agreement, and requires no other reason for serving the notice other than they want the tenant to vacate.