Generally, if you are under occupying your home, your benefits and entitlement may be affected as per Housing Benefit Rules and you may be already be liable for any additional rooms that is beyond your housing needs requirement.
Although the Key Guarantees would allow a tenant, if they wish, to have one extra room in additional to their housing needs. However, it does not have any power to change the rules. Therefore your entitlement would be as per prevailing Housing Benefit Rules.
Does this answer your questions? If not get in touch