Under the RPZ rules, a landlord generally cannot increase the rent more than once in any 12-month period. The 12 months usually runs from when the tenancy began or when the rent was last set, rather than from the date the landlord sends the notice. The landlord must also give the required notice and follow the applicable RPZ rent-setting rules, including using the RTB’s calculator where relevant. If the increase is disputed, the RTB or a solicitor would assess whether it was valid based on the tenancy’s facts and the law in force.