If the tenancy is a fixed term or contractual periodic tenancy, the landlord can only change the terms of the tenancy if the tenant agrees. It is best to agree any changes in writing.
Normally any changes are made by getting the tenant to sign a new tenancy agreement, incorporating the new terms and conditions. If the tenancy is an assured shorthold tenancy, and the tenant refuses to co-operate you will have the option of serving a section 21 notice [see section 5.2.6] and ending the tenancy.
After the fixed term of a tenancy has ended, assured and assured shorthold tenancies will automatically run on as a statutory periodic tenancy, on the same terms and conditions as the preceding fixed term tenancy. The ‘period’ will normally be either weekly or monthly depending on how rent is paid.
There is also a procedure whereby the landlord or the tenant can propose new terms, including a new rent. This can be done, within a year of the statutory periodic tenancy starting, and annually thereafter using a special procedure under the Housing Act 1988. There is a special form which needs to be used, and this needs to be served on the tenant. This procedure is often used for rent increases, particularly for assured tenancies (see 1.7 below), but rarely for amending the terms of the tenancy agreement. You can obtain the forms from law stationers and from some of the online services for landlords.
Although rarely exercised, the landlord and the tenant both have the right to apply for an independent decision by a Rent Assessment Committee if new terms cannot be agreed.