Allow or Refuse Untervermietung? Landlord Guide (2026)

Untervermietung: what it means for landlords

Untervermietung (subletting) means your tenant – the Hauptmieter (primary tenant) – lets the flat, or part of it, to a third party, the Untermieter (subtenant). Any such transfer of use to a third party needs your Erlaubnis (permission) under § 540 BGB. The decisive question for you is not whether the tenant must ask – they always must – but whether you are obliged to grant the Erlaubnis.

The answer depends on whether it concerns a part of the flat or the whole flat.

The claim to Erlaubnis for part-subletting (§ 553 BGB)

For a part of the flat your tenant has an enforceable claim to the Erlaubnis. § 553 Abs. 1 BGB states: if, after the rental contract was concluded, the tenant develops a berechtigtes Interesse (legitimate interest) in letting part of the dwelling to a third party, they may demand the Erlaubnis from you.

Three conditions must come together:

• It concerns only a part of the flat – the tenant keeps possession themselves, for example by subletting one room and continuing to live in the flat.
• The berechtigtes Interesse arose after the contract was concluded.
• There is no wichtiger Grund (good cause) to refuse (see below).

Where these conditions are met, your discretion is bound: you must grant the Erlaubnis. A berechtigtes Interesse means economic or personal reasons of some weight – for example the tenant's wish to reduce rent costs through a Untermieter, a partner or flatmate moving in, or an extended stay elsewhere for work or study.

When may you refuse the Untervermietung?

You may refuse the Erlaubnis only where a wichtiger Grund (good cause) exists (§ 553 Abs. 1 Satz 2 BGB). The statute names three cases:

• A wichtiger Grund lies in the person of the Untermieter – for instance serious earlier breaches of duty or concrete indications that this person will cause disturbance.
• The dwelling would become overcrowded (Überbelegung).
• The arrangement is otherwise unreasonable for you (sonstiger wichtiger Grund).

Note: a bare "I don't want a stranger in the building" is not enough. There must be a concrete reason in the person or the situation. For letting the whole flat, § 553 BGB does not apply – there you consent, or not, under § 540 BGB without being bound by the claim.

Achtung: If you refuse the Erlaubnis without justification, the tenant may terminate the tenancy extraordinarily with the statutory notice period (§ 540 Abs. 1 Satz 2 BGB) and may, in some cases, be entitled to damages from you (for instance lost sublet rent). Conversely, letting the whole flat without your Erlaubnis is grounds for Kündigung: you can issue an Abmahnung and, on continued unauthorised transfer of use, terminate without notice (§ 543 Abs. 2 Satz 1 Nr. 2 BGB).

Untermietzuschlag: when may you charge more?

If the arrangement is reasonable for you only against a reasonable rent increase, you may make the Erlaubnis conditional on the tenant agreeing to an Untermietzuschlag (sublet surcharge) under § 553 Abs. 2 BGB. The surcharge is meant to offset the additional wear and the more intensive use of the flat.

In practice a surcharge of roughly 10 to 20 percent of the rent attributable to the Untermieter is considered reasonable. A flat surcharge without a factual basis is not permitted – it must reflect the concrete additional burden.

Part vs. whole flat: what you must allow and what you may refuse

Situation	Legal basis	Must you allow it?	When may you refuse?
Part of the flat, berechtigtes Interesse arising after the contract	§ 553 Abs. 1 BGB	Yes – the tenant has a claim	Only for wichtiger Grund: person of the Untermieter, Überbelegung, unreasonableness (§ 553 Abs. 1 S. 2)
Part of the flat, reasonable only against a rent increase	§ 553 Abs. 2 BGB	Yes, against a reasonable Untermietzuschlag	If the tenant refuses the reasonable surcharge
Whole flat	§ 540 BGB	No – no statutory claim	At your discretion; transfer without Erlaubnis = grounds for Kündigung
Commercial short-term letting (e.g. Airbnb)	§ 553 BGB (BGH)	No – regularly no berechtigtes Interesse	Always; without consent = unauthorised transfer of use

Step by step: handling a request

1. Review the request – Who is to move in (name), for what period, and is it part of the flat or the whole flat?
2. Assess the berechtigtes Interesse – Did it arise after the contract was concluded, and is it of some weight? For part-subletting with a berechtigtes Interesse there is a claim.
3. Check for a wichtiger Grund – Is there a reason in the person of the Untermieter, a risk of Überbelegung, or unreasonableness for you?
4. Decide on an Untermietzuschlag – If the arrangement is reasonable for you only with an added burden, make the Erlaubnis conditional on a reasonable surcharge (§ 553 Abs. 2 BGB).
5. Decide in writing – Grant or refuse the Erlaubnis verifiably, with reasons where you refuse, naming the person and the period.

Common mistakes

• Refusing without cause. For part-subletting with a berechtigtes Interesse there is a claim – a refusal without wichtiger Grund can entitle the tenant to a Sonderkündigung and to damages.
• Demanding a flat surcharge. The Untermietzuschlag must reflect the actual additional burden (§ 553 Abs. 2 BGB), not a desired figure.
• Confusing part and whole. The claim applies only to a part. You never have to allow letting of the whole flat.
• Terminating without notice and without an Abmahnung. For unauthorised Untervermietung, a fristlose Kündigung normally requires a prior Abmahnung (§ 543 Abs. 3 BGB).
• Not documenting the decision. Keep the request, your reasons, the Erlaubnis and any surcharge in writing – in a dispute this is your evidence.

Mit HausMaus geht das einfacher

In HausMaus your tenant's Untermiete request reaches you, with its stated reason, directly within the tenancy; you decide in one step whether to grant or refuse it and can record your wichtiger Grund when you refuse. The Erlaubnis, the named person and any agreed Untermietzuschlag stay attached to the tenancy in the document vault. Through messaging you settle follow-up questions with the tenant without anything getting lost.

See pricing – 1 % of collected rent, free for tenants →