Conn. Gen. Stat. § 47a-15a: Nonpayment of rent by tenant: Landlord's remedy. Charges for late rent. Extension of grace period.

(a) If rent is unpaid when due and the tenant fails to pay rent within nine days thereafter or, in the case of a one-week tenancy, within four days thereafter, the landlord may terminate the rental agreement in accordance with the provisions of sections 47a-23 to 47a-23b, inclusive, except that such nine-day or four-day time period shall be extended an additional five days if a landlord's online rental payment system prevents the payment of rent when due. Any extension of such time periods shall apply only for the week or month, as applicable, when such rental payment system prevents the payment of rent when due. For purposes of this section, “grace period” means the nine-day or four-day time periods or the extension of such time periods identified in this subsection, as applicable.

(b) If a rental agreement contains a valid written agreement to pay a late charge in accordance with subsection (a) of section 47a-4 a landlord may assess a tenant such a late charge on a rent payment made subsequent to the grace period in accordance with this section. Such late charge may not exceed the lesser of (1) five dollars per day, up to a maximum of fifty dollars, or (2) five per cent of the delinquent rent payment or, in the case of a rental agreement paid in whole or in part by a governmental or charitable entity, five per cent of the tenant's share of the delinquent rent payment. The landlord may not assess more than one late charge upon a delinquent rent payment, regardless of how long the rent remains unpaid.