Its legal (i.e. allowed) but it might be a breach of contract. So, the legal question is what does the writing say and is it something you can rely upon to for the basis of a contract for instance does it give a term that the rent will not increase? I would mail a copy of the writing you mention to the landlord with a friendly reminder/cover letter that he/she/it covenanted not to increase the rent for the renewal term at least and see what happens. You can always move.
Answered on May 06th, 2014 at 12:47 PM