ADU rules in Allentown, PA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Allentown expressly allows accessory dwelling units (ADUs) on lots occupied by a single dwelling unit. An ADU may not exceed 800 square feet or 40% of the principal dwelling's floor area (whichever is less), only one is allowed per lot, at least one unit on the lot must be owner-occupied, and the combined floor area of the principal unit plus the ADU must total at least 1,750 square feet.
The City of Allentown Zoning Ordinance (Ord. No. 16166, adopted Oct. 15, 2025, effective Jan. 1, 2026) governs accessory dwelling units in Section 660-42. ADUs are allowed only on lots occupied by a single (one) dwelling unit; the combined floor area of the principal dwelling unit and the ADU must be at least 1,750 square feet; and any lot with a detached ADU (an ADU within an outbuilding) must have access to an alley or street abutting the lot's rear or street-side lot line (Sec. 660-42.C(1)). No more than one ADU is allowed per lot (Sec. 660-42.C(3)). An ADU may be created by converting interior floor area of the principal dwelling, adding floor area, constructing an outbuilding under Section 660-49, converting space within an existing outbuilding, or building a new principal dwelling that includes an internal or accessory ADU (Sec. 660-42.C(4)). ADU floor area may not exceed 800 square feet or 40% of the principal dwelling's floor area, whichever is less (Sec. 660-42.C(5)). At least one off-street parking space is required for the ADU in addition to parking for the principal dwelling (Sec. 660-42.C(7)). At least one unit on the lot must be occupied by a record owner as their permanent residence for a majority of days each year (Sec. 660-42.C(10)), neither unit may be used as a short-term rental (Sec. 660-42.C(11)), and the owner must record a deed restriction before a permit issues (Sec. 660-42.C(12)). Pennsylvania does not have a statewide ADU mandate, so these municipal rules control.
Establishing or occupying an ADU without complying with Section 660-42 (including without recording the required deed restriction, or in violation of the owner-occupancy or short-term-rental limits) is a zoning violation. The City may bring a civil enforcement proceeding seeking a judgment of up to $500.00 plus court costs and reasonable attorney fees, with each day of continued violation treated as a separate offense, and the Zoning Officer may order the unauthorized unit removed or vacated.
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