The most important source of information about a residential tenant’s rights and a landlord’s responsibilities is the written lease agreement. Some landlords prefer oral agreements, but it is more common for them to require the tenant's signature on a written lease—and a written lease protects the tenant more than an oral agreement. Be sure to read the lease carefully before you sign it.

A residential landlord’s ability to charge late fees generally depends on the specific language in the written lease agreement; the state’s landlord-tenant laws that may limit the amount of late fees the landlord can charge and the circumstances in which they can be charged; and the state's contract law.

Contract law in most states recognizes an implied duty of good faith and fair dealing between parties to a contract, and a residential landlord who charges late fees because the tenant is an hour late paying the rent may be in breach of the landlord’s implied duty of good faith and fair dealing—unless, for example, the lease agreement specifies an hour (12:00 p.m.) of the day by which rent is due and states that time is of the essence.

In Arizona, the relationship between residential tenants and landlords is primarily governed by the Arizona Residential Landlord and Tenant Act. The written lease agreement is indeed a crucial document as it outlines the rights and responsibilities of both parties. While oral agreements are legal, a written lease provides clearer protection and is advisable. Tenants should read and understand the lease before signing. Regarding late fees, Arizona law allows landlords to charge them as specified in the lease agreement. However, these fees must be reasonable and are subject to limitations under state law to prevent them from being punitive. The concept of 'good faith and fair dealing' is also recognized in Arizona contract law, which means that landlords should exercise fairness in enforcing lease terms, including the imposition of late fees. If a lease specifies a particular time by which rent is due and emphasizes that timing is critical ('time is of the essence'), then the landlord may be justified in charging late fees for rent paid past that time. However, charging late fees for minor delays without such a clause could potentially breach the duty of good faith and fair dealing.