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 New Hampshire, the lease agreement is indeed a crucial document for outlining the rights of residential tenants and the responsibilities of landlords. Tenants should thoroughly review and understand the lease before signing, as it provides more security and clarity than an oral agreement. Regarding late fees, New Hampshire's landlord-tenant laws allow landlords to charge late fees as specified in the lease agreement. However, the fees must be reasonable and not punitive. The state does not set a specific cap on late fees, but they should be outlined in the lease and agreed upon by both parties. Additionally, New Hampshire recognizes the implied duty of good faith and fair dealing in contracts, which means that landlords are expected to exercise fairness in enforcing lease terms, including the imposition of late fees. A landlord charging a late fee for rent that is only an hour late could potentially be seen as acting in bad faith, especially if the lease does not explicitly state that rent is due by a specific hour and that time is of the essence.