
Short assured tenancy is normal practice when letting houses. It is really what is says A short assurance agreement. If a tenant is a nightmare you can generally give them notice etc easily with it. It also means for the person renting they have to get fair notice.
Generally assured tenancies can only be given by say councils etc.
Generally a first time let to a person is 6 months, if everything is satisfactory then the tenancy is generally then done yearly. Very few people will commit anything else. Most mortgage lenders of "buy to let homes" only allow 1 year assured tenancy agreements some are only 6 months. That doesn't mean you cant live in the house for 20 years if the owner is happy to let it for 20 years.
If the bank had to make a repossession of a house they have to give the tenant the same notice and can end the tenancy at the end of the tenancy agreement.