Any gas appliance that the landlord owns and provides for the tenant’s use is included in the landlord’s legal duties.
If a tenant has their own gas appliance that the landlord has not provided, then the landlord has responsibilities for parts of the associated installation and pipework but not for the actual appliance.
There are some good practice measures that a landlord could adopt with appliances that tenants own:
1. At the start of the tenancy, advise the tenant of any flues or chimneys that are unsuitable for the installation of a gas appliance. You may also wish to consider regulating the installation of any appliance by a tenant through the conditions of the tenancy agreement.
2. If a tenant installs a heat producing appliance (e.g. fire) in the property ensure you are provided with a Building Regulations Compliance Certificate.
3. If a tenant installs a flueless gas cooking appliance (such as a cooker or hob) in the property ensure you are provided with a Declaration of Safety Certificate.
4. Free-standing cookers connected by a flexible connector (bayonet fitting), are not considered to be ‘readily movable’, but can be moved, temporarily, eg to clean the space they normally occupy; this type of activity is not regarded as ‘work’ within the meaning of these Regulations. Any other type of installation/reinstallation is regarded as gas work and must be carried out by a Gas Safe registered engineer.
5. Send a reminder to the tenant that their appliances should be serviced and checked for safety each year by a Gas Safe registered engineer, and where possible, offer to include these (at reasonable cost) within gas safety maintenance undertaken on your behalf.
6. It is also recommended to include all flues (eg chimneys) connected to gas appliances within your landlord’s gas safety check, even where they do not serve appliances provided by the landlord. This may also help to fulfil other legal duties under the Health and Safety at Work etc. Act 1974.
More information here