If you are buying the property under a Right to Buy Mortgage or you intend to sub-let the property then permission from the freeholder / landlord will be required, at additional cost. If there is a clause against sub-letting, or permission is needed and not applied for, then you will be in breach of at least one of the terms of your lease and forfeiture proceedings can be issued against you.
This is a list of covenants that the Landlord makes with the leaseholder and can include the following:-
Obligation to impose similar regulations and covenants on any other person granted a lease of part of the building. If required, and if its costs are paid, the landlord will enforce such regulations and covenants against any person in breach of the terms of their lease. Obligation to insure the whole building and common parts. Obligation to maintain and decorate the structure and exterior of the building.
You might like to read another great article: Lease Ganger