The lease for your business premises should state when the rent will be reviewed and on what basis the new rent will be established. Typically rent reviews are based on ‘open market rents’ — the rent that the landlord could expect if the lease was offered on the open market — though alternatives include indexing the rent to inflation.
A rent review based on open market rents may justify a sizeable increase in the rent, particularly if the general level of rents in an improving neighbourhood rises significantly. Improvements to the premises can also lead to a higher rental value — in some cases, even if you have paid for the improvements yourself.
If you think the proposed rent is too high, you should appoint a local surveyor to advise you and negotiate on your behalf.