Subletting and assignment are different acts with different consequences

Three things people call the same thing

A tenant who wants somebody else living in the property is usually contemplating one of three quite different arrangements, and the words used rarely distinguish them. Taking in a lodger or a flatmate who shares with you is one thing. Letting a whole property or a self-contained part of it to someone who occupies it in their own right is another. Handing the tenancy itself to a replacement is a third.

The legal consequences of these are not similar, and the agreement almost certainly addresses them in separate clauses. Getting the category wrong is how a tenant ends up personally liable for rent on a property they left months ago, or in breach of a term they did not realise they were touching.

What a sublet actually creates

A sublet builds a second tenancy underneath the first. The original tenant remains a tenant of the landlord, owing everything they always owed, and simultaneously becomes a landlord to the subtenant, owing that person the obligations a landlord owes. Two relationships now exist where one did, and the person in the middle is exposed on both sides.

That middle position is uncomfortable in ways people do not anticipate. If the subtenant stops paying, the rent to the head landlord still falls due. If the property needs repair, the subtenant may look to the person who let to them. And in many systems the subtenant has occupation rights of their own, so the middle tenant cannot simply ask them to leave when convenient.

Assignment transfers the position itself

An assignment is an attempt to step out entirely, transferring the tenancy to somebody who takes over the rights and obligations. Where it is done properly and with consent, the incoming tenant deals directly with the landlord and the outgoing one is generally released, though several systems allow a landlord to require continuing guarantees in some circumstances.

Because assignment changes who the landlord has to deal with for the remainder of the term, it is almost always made conditional on consent, and the formalities can be strict. An assignment attempted informally may fail entirely, leaving the original tenant liable and the incoming occupier in an uncertain position that suits nobody.

Consent clauses, and the limits on refusing

Most agreements prohibit subletting and assignment without the landlord’s written consent, and that is not unreasonable in itself. A landlord chose a tenant on the basis of who they were and has a legitimate interest in who occupies and who is liable. Insurance, mortgage conditions and licensing requirements may also depend on how the property is used.

Many jurisdictions temper this by providing that consent must not be unreasonably withheld, at least in defined kinds of letting, and sometimes that a decision must be given within a reasonable time with reasons. What counts as reasonable is judged against the landlord’s genuine concerns rather than their preferences, and a refusal based on an irrelevant consideration may not stand. Whether that protection exists where you are, and which lettings it covers, differs widely.

Doing it quietly is the expensive option

Unauthorised subletting is a breach of the agreement in most cases, and the consequences can be serious: grounds for possession, liability for any loss the landlord suffers, and in some places specific penalties where a tenancy is sublet for profit. Insurance cover may be invalidated, which turns a small saving into a large exposure if anything goes wrong.

The subtenant is exposed as well, and often more so, because they may have paid a deposit and moved in on the strength of an arrangement that the head landlord can attack. Where a middle tenancy ends, a subtenancy created under it frequently ends with it. Anyone taking a property from someone who is not the owner has a strong reason to ask what right that person has to let it.

Ask before doing anything, and ask locally

The rules on subletting, assignment, consent, and what happens to an occupier when the tenancy above them ends vary considerably between jurisdictions and between kinds of tenancy. Social housing, commercial leases and short residential lettings are frequently governed by quite separate regimes, and some places regulate short-term holiday letting on top of everything else.

This is general description rather than advice about your agreement. Read the clauses, ask the landlord in writing before making any arrangement, and if the answer is unclear or the sums involved are significant, take it to a qualified lawyer or a housing advice service in your own jurisdiction. Move promptly if a dispute has already started, because time limits apply to claims on both sides and they run whether or not anyone is corresponding.