How Allentown Handles Accessory Structures: A Practical Guide

Allentown maintains 134 local ordinances across all categories, and 4 of those deal specifically with accessory structures. Here is a breakdown of what the city actually requires, what is prohibited, and where Allentown falls on the strict-to-permissive spectrum compared to other cities.

ADU Rules

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.

Key details: Max ADU floor area: 800 sq ft or 40% of principal, whichever is less. Number allowed: 1 per lot. Owner occupancy: Required (one unit, majority of year). Min combined floor area: 1,750 sq ft (principal + ADU). Code Section: Sec. 660-42.

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.

Garage Conversions

Allentown allows a detached garage or other outbuilding to be converted to living space as an accessory dwelling unit: an ADU may be created by converting space within an existing outbuilding on a lot occupied by a single principal dwelling, subject to the Section 660-42 ADU rules. Finishing a garage and any change of use require building permits, an outbuilding occupied by an ADU must sit on a permanent foundation, and the converted ADU may not exceed 800 sq ft or 40% of the principal dwelling, whichever is less.

Key details: Garage-to-ADU: Allowed via Sec. 660-42.C(4)(d). Foundation: Permanent foundation required for ADU outbuilding. Max converted ADU size: 800 sq ft or 40% of principal, whichever is less. Permits: Building permit + change-of-use required. Code Section: Sec. 660-42.C(4) and 660-49.

Converting a garage to living space without permits, or creating an ADU that violates Section 660-42, is a violation. The permit guidance states a $100.00 fine may be imposed for each type of work performed without a permit; zoning violations are also enforceable in a civil proceeding for up to $500.00 plus costs and attorney fees per day. The building official may require an unpermitted conversion to be corrected, and an ADU created without the required deed restriction may be ordered vacated.

Shed & Carport Rules

In Allentown a storage shed is an accessory structure that generally may not exceed 15 feet in height, may not stand in any front yard, and must be set back at least 3 feet from side and rear lot lines and 10 feet from any street lot line (unless the Article 4 outbuilding tables set a specific figure). A shed of 150 square feet or less may sit on an abutting commonly owned lot. A detached shed not attached to a one- or two-family dwelling or townhouse, or any shed 1,000 sq ft or larger, requires a building permit.

Key details: Max height (accessory): 15 ft (unless otherwise specified). Side/rear setback: 3 ft min (or per Article 4 table). Street lot line setback: 10 ft min. Front yard: Not allowed. Permit threshold (detached): 1,000 sq ft, or if attached to a house.

Building a shed without a required permit, in a front yard, over the height limit, or too close to a lot line is a code violation. The City's residential permit guidance states a fine of $100.00 may be imposed for each type of work done without a permit, and zoning violations are separately enforceable in a civil proceeding for up to $500.00 plus court costs and attorney fees per day. The Zoning Officer may order an offending shed relocated or removed.

ADU Rental Restrictions

Under Allentown's zoning code, any lot with an accessory dwelling unit loses the right to rent either unit short-term. Section 660-42.C(11) blocks Airbnb-style stays on ADU lots outright, and the owner must sign a recorded deed restriction promising compliance before the ADU permit issues.

Key details: Governing section: § 660-42.C(11), Zoning Ordinance. Scope: Bans short-term rental of both units on ADU lots. Enforcement: Zoning Officer; permit revocation possible. Civil penalty: Up to $500 plus costs, per day. Deed restriction: Recorded, runs with land, binds future owners.

Running a short-term rental on an ADU lot violates § 660-42.C(11) and can trigger revocation of the ADU permit under § 660-123.C for misrepresentation or breach of a permit condition. Separately, the Zoning Officer can pursue civil enforcement under § 660-123.D: a court judgment of up to $500 plus costs and the City's attorney fees, with each day the listing stays active counted as its own violation.

This is one of the stricter rules in Allentown's municipal code. If you are unsure whether your situation complies, it is worth checking with the city before proceeding.

The Bottom Line

Allentown's accessory structures rules are a mixed bag. Some areas are strict, others are relaxed, and the details matter. The best approach is to check the specific rule that applies to your situation rather than assuming Allentown is broadly strict or permissive.

All of the above reflects Allentown's municipal code as of our last review. If you need specifics on fines, exemptions, or filing requirements, the detailed ordinance pages linked above have the full breakdown.