Thanks for your enquiry.
For private rentals, the most common type of tenancy is an Assured Shorthold Tenancy (AST).These tenancies usually begin as fixed term tenancies where the duration is defined from the outset, typically anywhere between 6 months to three years and mutually agreed between the landlord and tenant.
However, there is not a maximum length to an AST, so the only thing I can think of is that IF the Landlord has a Mortgage over the property,Banks often have different stipulations if a Tenancy is over 3 years in duration, and this is why he has asked for such a Tenancy.
At the end of the day, it is up to the parties to formally agree the term of the AST, and even if he may have previously agreed to a 5 year term, until the AST is signed, either party is free to withdraw.
Please let me know if you have any further query.
Can I help you any further? Kind Regards Al