Check whether the notice is in writing, gives the required notice period and effective date, and includes the information and calculations required for your tenancy; rules can also differ depending on when the tenancy began. Tell the landlord promptly, in writing, why you believe it is defective and keep the notice, rent records and all correspondence. Do not simply stop paying your existing rent, but you can ask the RTB about disputing the proposed increase or any arrears claim; the outcome depends on the tenancy’s facts and the RTB’s process. For complex cases, consider getting advice from a housing adviser or solicitor.