In order to rent out a tenancy, approval from the original landlord or the Rent Tribunal regarding subleasing is required. The primarily tenant must have noteworthy reasons for the rental in order to get approval and the original landlord should not possess any justified reason to refuse the rental. The lenght of the rental period must also be determined. 

Exampels of such noteworthy reasons may be:

• Work, studies or care for familymembers elsewhere
• Disease
• Rent to one's own children whose intentions for example are to study
• If you intend to settle there for retirement
• Moving in "on try" with a partner
• Long stay abroad

Above listed information is only valid in cases where not only a room in the tenancy is rented out. However, the same rules of good character applies to lodgers.