What is joint and several liability in a tenancy?
Where tenants sign one tenancy agreement together, each is liable for the whole of the obligations — not for their share — which means one person's default becomes everyone's problem.
What it means in practice. If four people share a house on a joint tenancy at £2,000 a month, each is liable for the full £2,000, not for £500. If one stops paying, the landlord can pursue any of the others for the whole amount, and will pursue whoever is most likely to pay.
The same applies to damage. Damage caused by one tenant can be deducted from the whole deposit, and the others' recourse is against that person rather than against the landlord.
Why landlords prefer it. It removes the need to establish who owes what, and it provides several people to pursue rather than one.
The specific traps:
One tenant leaving does not end their liability. They remain a party to the agreement until it ends or they are formally released, so a departing housemate can still be liable — and, conversely, the remaining tenants cannot assume the landlord will simply substitute someone.
Replacing a housemate requires the landlord's agreement, usually by a deed of assignment or by surrendering and granting a new tenancy. Agents frequently charge for this, subject to the limits on permitted fees.
Notice by one can end it for all, on a periodic tenancy — a genuinely serious consequence.
The deposit is returned as a whole, to whoever is nominated, and disputes between tenants about its division are not the landlord's concern.
One tenant's breach can be grounds against everyone, including for possession.
The alternative: individual tenancies. Each tenant signs a separate agreement for their own room, with shared use of common parts. Each is liable only for their own rent, and one leaving does not affect the others. Common in purpose-built student accommodation and some professional house shares, and generally far better for tenants — though landlords take on more risk and administration.
What sharers should do: understand which arrangement they have; agree in writing between themselves how rent, bills and replacement will work, since the tenancy will not address it; and be cautious about signing for someone whose circumstances you do not know.
General information, not legal advice.