Can a landlord increase a tenant's rent anytime? Or does it have to be with the signing of a new lease?
Normally a landlord cannot increase a rental amount during a fixed term rental lease agreement unless the increase is specifically stated and agreed upon in rental lease agreement itself.
Beyond this, depending on your specific location, city, etc., there may be some additional local laws, rules and regulations, stating when and by how much you can increase the rental amount for your tenants. For example in an area with heavy rent controls, it's possible that you can only increase the rental amounts in very specific increments at specific intervals, for example by a maximum of 3% per year for existing tenants.
Therefore after a rental lease agreement has been completed and a tenant is renting month to month (or even if the tenant starts with a month to month rental agreement), it's always best to consult legal advice from your local legal representative to determine what's possible and allowed in your local area in terms of how much and when you can increase your tenant's rents.