Check the rent review notice carefully, including the notice period, the calculation used and any comparable rents provided, as the applicable rules can depend on when the increase was proposed. Write to your landlord promptly, explain why you believe the increase is unlawful and keep copies of the notice, tenancy agreement and correspondence. If it is not resolved, you can refer the dispute to the Residential Tenancies Board (RTB), paying close attention to its time limits and evidence requirements; do not simply withhold rent. The RTB, or a solicitor based on your individual circumstances, will determine the outcome.