A landmark lease renewal case under the Landlord and Tenant Act 1954 - O'May v City of London Real Property Co Ltd (1982). Frequently referred to in lease renewal negotiations where one party, usually the landlord, seeks to vary the terms of the renewal lease to reflect 'modernisation' of the lease covenants, as the result of changed market practice since the grant of the original lease. In summary, the case held that the tenant's statutory protection under the 1954 Act means that the starting point in any renewal is a lease on substantially the same terms, and in effect imposes a 'presumption against' change. The party proposing the change must prove that it is 'fair and reasonable in all circumstances' (including the existing covenants, and not merely current open market practice), and that the tenant can be compensated for the proposed change via an adjustment in the rent.

Some of the tenant's covenants in a lease may impose restrictions on its occupation of the premises which, when assessing the rental value of the property at rent review, warrant an adjustment in the rent. If the premises were offered in the open market on the same tenancy terms (and a discount could be expected in any open market bids), to reflect the onerous lease provisions in comparison to the 'market norm' for the class of property concerned, a similar discount may be applied to the rent review.

Open market rent is the most common basis of valuation at rent review (also known as open market rental value - OMRV). It is commonly defined as the rent at which the premises might reasonably be expected to let, in the open market, at the review date, on the terms of the hypothetical lease. Typically, this is framed with primary reference to the terms and covenants of the actual lease of the premises, although this is not always the case (for example, in relation to the assumed length of term) and due diligence is essential in understanding the particular definition of open market rent in any review clause.

A rent review to open market rent.

Overage is also known as a clawback. It is a right to receive future payments which are triggered by future events – for example, achieving planning permission for change of use or development, practical completion of a development, or the sale or lease of the completed development.