Tenant refuses to sign joint tenancy agreement?

You need to be careful not to queer the pitch. There is every chance that the girlfriend does not want to be named on your tenancy agreement. They may have their own property, rented or otherwise. They may be claiming benefits and being acknowledged as living at your property may well upset their own finances. As someone else has pointed out it may cause Council Tax difficulties.
Tenants have a right to freedom of choice, and no tenancy agreement can expect to control or monitor who your tenant sleeps with, lives with, entertains or has round for sleep-overs as friends, partners or family. You will never be able to evict your tenant for breaching their AST on the basis of who they are sharing a bed with, and unless their living arrangements cause a clear breach of other laws, such as overcrowding or antisocial behaviour, I have never understood a need for a landlord to want to formalise a change in circumstances, which may well be temporary anyway. Whilst I have heard of cases of partners ‘inheriting’ the rights of tenancy when an original tenant dies or moves on, I think this is rare.
I would be grateful if other contributors could identify at which point Courts treat a tenant’s partner as being a tenant in their own right, and whether this is an established point, or decided upon on a case by case basis.

Nobody has mention the Protection from Eviction Act 1977.
1 Unlawful eviction and harassment of occupier.
(1) In this section “residential occupier”, in relation to any premises, means a person occupying the premises as a residence, whether under a contract or by virtue of any enactment or rule of law giving him the right to remain in occupation or restricting the right of any other person to recover possession of the premises.
I think that a letter making the girlfriend a permitted occupier might constitute a contract within the meaning of S.1(1), the counter argument being that no consideration is being given.
My standard lease defines permitted occupiers closely including limiting how long they can stay in the property and excludes anyone who is using the property as their only or main residence. If a tenant was in breach of this provision I would end the tenancy.
Also be careful about permitted occupiers if there is any risk whatsoever of creating an HMO – a risk that doesn’t apply in the OP’s situation unless a second long-term guest arrives.

Read this thread with real interest.
AS for rights I think creating and having acknowledged the PO status actually gives LLs additional rights under the Prevention from Eviction (sic) legislation.

Reply to the comment left by Jim K at 10/01/2026 – 13:04
How does acknowledging the ‘PO’ (?) status give the landlord additional rights under the Protection from Eviction Act 1977?
https://www.legislation.gov.uk/ukpga/1977/43/contents