ADU rules in Allentown

A detached ADU (an ADU inside an accessory outbuilding) is allowed only where the house is one of the four house building types, and only in that type's zones: Twin House in N3, N4, GX-C and GX-N; House A in N3 and N4 only; House B in N3, N4 and N5; House C in N5. The three rowhouse types that permit an ADU allow it only inside the principal building, and Lane Rowhouse (N1, N2), Small General Building and every commercial, mixed-use and industrial building type allow no ADU at all. Three gates then apply citywide: the lot may hold only one dwelling unit, the principal dwelling and ADU combined must total at least 1,750 square feet, and a lot with a detached ADU must have access to an alley or street abutting its rear or street-side lot line. Once those are satisfied the approval is administrative, not a hearing.

An ADU inside the principal building, created either by converting existing interior floor area or by adding floor area, is allowed in more building types than a detached one: Town Rowhouse (GX-C, and GX-N/N2), Urban Rowhouse (GX-N, N1) and Suburban Rowhouse (GX-C, NX, N3) each allow '1 in principal building', and all four house types allow it in the zones listed for detached ADUs. It is still not universal. Lane Rowhouse (N1, N2) and Small General Building allow none, and House A allows none in GX-C, GX-N, NX or N2, because those columns already permit two to six principal units and § 660-42.C(1)(a) restricts ADUs to lots occupied by a single dwelling unit. The same 1,750 square foot combined-floor-area gate applies, and only one entrance to the principal building may face a street unless a second street-facing entrance existed before the ADU was created.

§ 660-42.C(5) caps an ADU at 800 square feet or 40% of the principal dwelling unit's floor area, whichever is less, so the 800 square foot headline is only reachable when the house itself has at least 2,000 square feet of floor area. At the bottom end the ADU is exempt from the general minimum unit sizes of Table 660-5 but must still meet the alternative minimums of Table 660-6: 400 square feet for an efficiency or one-bedroom, 550 for a two-bedroom, 750 for a three-bedroom. Separately, § 660-42.C(1)(b) requires the principal dwelling and the ADU combined to total at least 1,750 square feet. A detached ADU occupies an outbuilding, which may not exceed two stories or the height of the principal building, whichever is less.

At least one of the two dwelling units on the lot must be occupied by a record owner of the property, who may live in either the principal dwelling or the ADU but must occupy it as a permanent residence for a majority of the days in each calendar year. This is an ongoing condition, not just a test at application: before any permit to establish an ADU issues, the owner must file an affidavit with the Zoning Officer and record a deed restriction promising to comply with the owner-occupancy and rental restrictions and to notify prospective purchasers, and that restriction runs with the land and binds heirs, assigns and later purchasers. Pennsylvania has no statewide ADU statute preempting this, so the local requirement stands unqualified.

§ 660-42.C(7) requires at least one off-street parking space for the ADU on top of whatever the principal dwelling already needs. Article 9's parking exemptions, reductions and credits are expressly unavailable to ADUs under § 660-86.A, which matters in the older rowhouse neighborhoods where principal units in Lane, Urban and Town Rowhouse buildings are themselves exempt from off-street parking; the ADU space is still owed. Note a drafting slip: § 660-86.A cites the ADU parking rule as '§ 660-44.B', but § 660-44 is Electric Vehicle Charging and the operative ADU parking requirement is § 660-42.C(7).

Where the building-type table allows an ADU, it is allowed by right with administrative review: a zoning permit from the Zoning Officer under § 660-114.A (required for a new use, and for interior alterations creating an additional dwelling unit), plus a building permit whose plans the Building Official reviews for building and life-safety compliance under § 660-42.C(9). No special exception or conditional use hearing is involved; the building-type tables list ADUs in the 'Number and Location of Dwelling Units' table with a plain unit count, not under the 'Requires Special Exception' key used for accessory structures. A certified site plan is not automatically triggered, since § 660-114.B(1)(f) reaches new residential buildings of three or more units, though the Zoning Officer may require one. The one extra step is administrative rather than discretionary: the owner must record the § 660-42.C(12) deed restriction before the permit issues. Applications are filed through the city's EnerGov self-service portal.

Renting the ADU long term is contemplated by the ordinance (one of its stated purposes is letting aging residents 'obtain extra income'), but § 660-42.C(10) requires a record owner to occupy one of the two units as a permanent residence for most of the year, so only one of the pair can ever be a rental. § 660-42.C(11) goes further than most cities: on any lot with an ADU, neither the principal unit nor the accessory unit may be used as a short-term rental. § 660-42.C(6) also caps combined occupancy at the number allowed for a single household plus two additional adults, and at two students total inside the Student Residence Overlay. The ADU is accessory to the principal dwelling on a single lot and the code makes no provision for conveying it separately; the recorded deed restriction binds successors in title. A tenanted unit also falls under the city's residential rental unit licensing program in Chapter 455, Article II.