Millcreek, PA Accessory Structures: ADU Rules (2026)

Section 145-58 of the Millcreek Township Code governs the In-Law Apartment. Under § 145-11 it is a separate living space within a Single-Family Dwelling, Detached with separate sleeping, cooking and bathroom facilities, occupied by an in-law or a member of the family unit living in the main portion of the dwelling. The house must be owner-occupied, and the dwelling unit occupied by the owner must be greater than 70% of the total living space, including the In-Law Apartment. The apartment's living space cannot exceed 720 square feet or 40% of the principal dwelling's total area, whichever is less.

For this purpose living space excludes any garage, while total area includes the garage and all other portions of the dwelling. Only one In-Law Apartment is allowed within any single-family detached dwelling, no more than two bedrooms are permitted, and the apartment can be located only within the primary dwelling. That last rule rules out a detached garage or backyard unit. Section 145-27B backs it up by barring use of an accessory structure for living space except as provided elsewhere in the chapter. Any addition or modification to the principal dwelling to create the apartment must be architecturally integrated with the principal dwelling's use as a single-family detached dwelling, and the work must meet Pennsylvania Construction Code requirements.

The section also requires the use to meet every element of the definition and any conditions imposed at all times. Occupancy or rental of the apartment by persons other than in-laws as defined in the chapter is expressly prohibited, so it cannot become a rental unit for unrelated tenants.