With all Assured Shorthold Tenancies landlords can increase the rent after the initial fixed period if it is stated in the tenancy agreement or if the tenant agrees to the increase. There may be a rent increase clause in the tenancy agreement which would have been agreed and signed at the start of the tenancy.
However if the tenancy agreement makes no provisions for the landlord to make rent increases, then you have two options to increase the rent:
Please note - the amount of increase should be considered in relation to the rental charges for other similar properties in the same area. If increases put the rent well above market rents, tenants can apply for the rent to be assessed by a Rent Assessment Committee. If they then reduce the rent, you are bound to this figure for 12 months.
IMPORTANT : If the property in question is located in England, Section 13 (2) - Form 4B must be used.