Sub-letting without permission is normally considered being a matter of civil, rather than criminal law.
However, if you are a tenant that is living in social housing and you decide to sub-let your property or assist someone else to sub-let such a property, you may have committed a criminal offence.
It is the responsibility of Local Authorities, rather than the police, to prosecute any instances of unlawful sub-letting.
If you sub-let all or part of your rented accommodation without obtaining permission from the landlord, then you would risk breaking the terms of your tenancy agreement, and you could be evicted. The landlord can serve you with a ‘Section 8 – notice’, seeking possession and the matter could go directly to court.
The Citizens Advice provide comprehensive information relating to both the civil and criminal liabilities which can arise when a property is sub-let without the relevant permissions.
Please see the link in Related Information.
