A lodger is a person who rents a furnished room in a home; shares all the facilities of the home but do not have sole use of any part of the home and may have services such as cleaning or meals.  

A sub-tenant is a person who has been given sole use of a room or part of the home, and who normally lives separately from the household under a more formal arrangement.

A flatsitter is someone who the tenant asks to stay in their home as their agent if they leave on a temporary basis.  The home remains the tenants main and principal home.

Lodgers

Secure tenants have an absolute right to take in lodgers but introductory and demoted tenants do not (however, current tenancy conditions say that we may consider allowing a disabled introductory tenant to have a carer living with them).

Sub-tenants

Secure tenants have qualified right to take in sub-tenants but introductory and demoted tenants do not.

Legislation relating to lodgers and sub-tenants is covered by S.93.94 of the Housing Act 1985.   It is a term of every secure tenancy that the tenant:

• may allow any persons to reside as lodgers in the dwelling house, but;
• will not, without the written consent of the landlord, sublet or part with possession of  part of the dwelling-house.
• If the tenant under a secure tenancy parts with the possession of the dwelling  house or sublets the whole of it (or sublets first part of it and then the  remainder), the tenancy ceases to be a secure tenancy and cannot  subsequently become a secure tenancy.

It is unlikely that consent to sub-let will be withheld unless there is a good reason to do so.  If a secure tenant requests permission to sublet and this request is not replied to within a reasonable time, the consent is deemed to have been withheld.

The full procedure and FAQs are here.