Section 21 notices and assured shorthold tenancies Assured shorthold tenancies Assured shorthold tenancies (AST) have been created since 15 January 1989. ASTs can be either a fixed-term contract, for example six, 12, 18 months etc, or periodic from the start, that is, monthly, weekly or three-monthly etc, depending on what the agreement says and how often the rent payment is due. When the fixed term ends, if the tenant wishes to stay on, the tenancy can either lapse into a statutory periodic AST, which will be automatic if the parties do nothing, or be renewed for a new term if the parties sign a new agreement on the