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Allentown, PA Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified June 2026

ADU rules in Allentown, PA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

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

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Pennsylvania under recent ADU streamlining laws. The Allentown, PA rules below cover local size, height, and setback requirements.

Summary

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.

660-42.C REGULATIONS All accessory dwelling units are subject to the general regulations of this subsection. (1) Where Allowed. The number and location of dwelling units tables for each building type in Article 4 identify the zones in which accessory dwelling units are allowed, provided further that: (a) ADUs are allowed only on lots occupied by a single (one) dwelling unit; (b) The total floor area of the principal dwelling unit and accessory dwelling unit, combined, must be at least 1,750 square feet; and (c) Any lot to be occupied by a detached ADU (i.e., ADU within an outbuilding) must have access to an alley or street that abuts the subject lot's rear or street side lot line. ... (3) Number. No more than one accessory dwelling unit is allowed per lot. ... (5) ADU Floor Area. The floor area of an accessory dwelling unit may not exceed 800 square feet or 40% of the floor area of the principal dwelling unit, whichever is less. ... (10) Owner Occupancy and Rental. At least one of the dwelling units on a lot occupied by an accessory dwelling unit must be occupied by a record owner of the subject property. The owner must occupy either the principal dwelling unit or the accessory dwelling unit as their permanent residence for a majority of the days each calendar year. (11) Short-Term Rental. On lots with an accessory dwelling unit, neither the principal unit or the accessory unit may be used as a short-term rental.

Source: City of Allentown Code of OrdinancesView official code

Full Breakdown

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.

C(1)). No more than one ADU is allowed per lot (Sec. 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. C(4)). ADU floor area may not exceed 800 square feet or 40% of the principal dwelling's floor area, whichever is less (Sec. C(5)). At least one off-street parking space is required for the ADU in addition to parking for the principal dwelling (Sec.

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. C(10)), neither unit may be used as a short-term rental (Sec. C(11)), and the owner must record a deed restriction before a permit issues (Sec. C(12)). Pennsylvania does not have a statewide ADU mandate, so these municipal rules control.

Violations & Fines

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.

Frequently Asked Questions

Are ADUs (in-law suites) allowed in Allentown?

Yes. Section 660-42 expressly allows accessory dwelling units on lots occupied by a single dwelling unit, subject to size, parking, owner-occupancy, and deed-restriction requirements.

How big can an ADU be?

An ADU may not exceed 800 square feet or 40% of the principal dwelling's floor area, whichever is less (Sec. 660-42.C(5)).

Do I have to live on the property if I build an ADU?

Yes. At least one of the two units must be occupied by a record owner as their permanent residence for a majority of the days each calendar year, and a deed restriction must be recorded before a permit is issued (Sec. 660-42.C(10), (12)).

Sources & Official References

[1]City of Allentown Zoning Ordinance (Ord. No. 16166, eff. Jan. 1, 2026) - Sec. 660-42 Accessory Dwelling Units (.gov PDF)

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All Allentown rules

How Allentown compares: Easiest Cities to Build an ADU·Compare Allentown to another location·View the Pennsylvania accessory structures overview

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