The three main permissions to seek are:-

1. If lease property, ensure the free-holder is made aware. Check lease, make any amendment, it might not be possible to let the property.
2. Inform any joint owners – be careful where the property has been inherited by more than one person, and only one decides to let without informing the others;
3. Inform the mortgage lender. Might be a criminal offence not to tell them

It is critical to ensure all the permissions are obtained if applying for a license (Licensing) and at time of purchase – use a specialist conveyancing clerk/company to ensure you don’t end up with a property un-rentable.