If the lease allows you to carry out alterations to the premises or improvements, you may find that the landlord has also included a clause in the lease saying that you must put the premises back into their original condition at the end of the lease. (Sometimes, it is qualified by saying “if the landlord requires it”.)
This may seem completely insane to you, if your improvements have substantially increased the letting potential of the premises, so that, as a result of your alterations, the landlord will be able to let them at an even higher rent to a new tenant.
One perfectly sensible reason for such a clause is that if you make alterations which suit you but wouldn’t suit a prospective tenant, then the landlord will have good reason to want you to have to remove the alterations and reinstate the premises as they were before you added the replica Sistine Chapel ceiling.
But there is also a legalistic reason for landlords to want a ‘reinstatement clause’. The purpose of the reinstatement clause is to get round paying tenants compensation for improvements they have made. The Landlord and Tenant Act 1927 provides that in some circumstances a tenant must be given compensation for improvements by the landlord at the end of the lease. It is believed that a reinstatement provision might get round the need to pay any such compensation. It is certainly a potential lever for the landlord in any negotiations.