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 Pennsylvania (PA), the lease agreement is indeed a crucial document outlining the rights of tenants and the responsibilities of landlords. A written lease provides clarity and legal protection for both parties more so than an oral agreement. Tenants are encouraged to read and understand the lease before signing. Regarding late fees, Pennsylvania law allows landlords to charge them as specified in the lease agreement. However, the fees must be reasonable and not punitive. The state's Landlord and Tenant Act of 1951 does not set a specific cap on late fees, but courts may interpret excessive fees as illegal penalties. Additionally, under general contract law principles, there is an implied duty of good faith and fair dealing in contractual relationships, which means that a landlord's actions in enforcing late fees should be fair and not overly harsh. For instance, charging a late fee for rent that is only an hour late without a clear stipulation in the lease could be seen as a breach of this duty. It is important for tenants to review their lease terms regarding rent due dates and late fees to understand their obligations and the potential consequences of late payments.