ADU rules in Harrisburg

A detached backyard ADU is permitted by right in the RL (Residential Low Density), RM (Residential Medium Density) and RF (Riverfront) districts and by special exception in the CN (Commercial Neighborhood) district. Section 7-309.2.A(1) states that the unit 'may be created within the principal residential unit or may be located as a detached unit on the same lot,' so the detached form needs no extra approval beyond what an internal unit needs. The Use Table at 7-305 Attachment 1 marks Accessory dwelling unit (7-309) as N P P SE N N P N N across OSR, RL, RM, CN, CG, DC, RF, INS and IND, so ADUs are simply not allowed in the OSR, CG, DC, INS and IND districts, including the Downtown Center. Ordinance No. 6-2021, adopted 4-13-2021, is what made the use by right: before it, an ADU was a special exception in RL, RM, CN and RF. Two conditions ride along with by-right approval, a 33% size cap and owner occupancy of the principal structure.

The same section covers the attached and internal forms: an ADU 'may be created within the principal residential unit,' which is how a basement, attic or wing conversion qualifies, again by right in RL, RM and RF and by special exception in CN. The unit has to be a genuine second dwelling unit, defined in Chapter 7-333 as 'a single habitable living unit occupied by only one family' with 'its own toilet, bath or shower, bathroom sink, sleeping facilities, refrigerator, kitchen sink, and only one stove and/or oven.' Watch the code's structural vocabulary: an 'accessory structure' by definition 'shall not be attached to the principal building,' and § 7-307.6.C provides that any portion of a building structurally attached to the principal building is treated as part of that principal building, so an addition built to hold an ADU is measured against the district's principal-building setbacks and height rather than any accessory-structure standard.

Section 7-309.2.A(2) is the only ADU-specific size rule: the unit 'may not be larger than 33% of the square footage of the principal residential unit.' Ordinance No. 6-2021 raised that ceiling from 25%. There is no absolute square-foot cap and no ADU-specific height limit, so on a 1,800 sq ft house the ceiling is roughly 594 sq ft, while a large house yields a proportionally larger unit. A detached unit is otherwise shaped by the district development standards in 7-307 Attachment 1 (RL: 5,000 sq ft minimum lot, 20 ft front, 15 ft rear and 5 ft side yards, 50% maximum impervious coverage; RM: 10 ft rear, 4 ft side yards, 70%; RF: 15 ft rear along Front Street or 5 ft along State Street, 10 ft side yards, 75%), because § 7-307.6.A applies the district setbacks to every accessory building and § 7-307.6.B bars accessory structures from a required front yard. The published height maxima (36 ft in RL, 45 ft in RM, 36 ft minimum and 45 ft maximum in RF) are written for the principal building.

Section 7-309.2.A(3) requires flatly that 'the principal residential structure shall be owner-occupied.' Ordinance No. 6-2021 added this clause in the same amendment that converted ADUs to a by-right use, replacing the old subsection (3) that had required one off-street parking space. The Code writes it as a criterion of the use rather than as a one-time application test, and § 7-309.1.A puts the burden on the applicant to 'demonstrate compliance with these criteria,' so it operates as a continuing condition: an ADU on a property whose principal house stops being owner-occupied is outside the terms of its zoning permit and enforceable under § 7-303.12. The practical consequence is that the ADU, not the main house, is the unit a Harrisburg owner can rent. Pennsylvania has no statewide ADU statute overriding local owner-occupancy rules; HB 2186 of the 2025-2026 session would create one, but as of 2026-08-04 it had only passed the House.

The off-street parking table at § 7-327.6A lists, under 'C. Accessory residential and nonresidential uses,' 'Accessory dwelling unit (7-309): 1 space per unit.' Ordinance No. 6-2021 deleted the parking sentence from § 7-309.2.A(3) but left this table entry in force, so the one space still applies in RL, RM and RF. Section 7-327.2 provides that 'the off-street parking requirements in this chapter do not apply to uses within the Downtown Center or Commercial Neighborhood Zoning Districts,' so a CN ADU (the special exception case) needs no off-street space at all; the Downtown Center exemption is academic because ADUs are not permitted there. In RL, RM and RF the space may not sit in the required minimum front yard, and any reduction or waiver requires a special exception from the Zoning Hearing Board, which weighs nearby public parking, transit adequacy, car-share provision, a transit subsidy agreement, or shared parking within 400 feet.

In RL, RM and RF the ADU is a by-right use decided administratively: § 7-305.7.C defines 'P' as 'Permitted by right (zoning decision by Zoning Officer),' and § 7-303.6.A(1) directs that the Zoning Officer 'shall issue a zoning permit for a use that is permitted by right if it meets all of the requirements of the Zoning Code.' A zoning permit is needed before construction reasonably valued at $1,000 or more and before any change of use, and § 7-303.7.E(1) sets a seven-day target for deciding a complete by-right application. The exception is CN, where an ADU is a special exception: the Zoning Hearing Board decides after a hearing (with an opportunity for Planning Commission review) and the Zoning Officer issues the permit only on the Board's written approval. A Uniform Construction Code building permit from the Bureau of Codes Administration is required in addition to the zoning permit, and work in a municipal historic district also needs review under Chapter 7-317.

Because § 7-309.2.A(3) fixes owner occupancy on the principal structure, the ADU is the unit that may be leased, and the Zoning Code sets no minimum or maximum lease term. A rented ADU is a 'residential rental unit' under § 8-511.3 ('any dwelling unit or room that is not occupied by the owner'), which pulls it into the Residential Rental Unit Registration Program: an annual registration application filed with the Codes Bureau between October 31 and December 31 for the coming calendar year, a business privilege license, a designated responsible agent living or working within 50 miles, and inspection at least once every five years leading to a certificate of compliance. The owner-occupied exemption in § 8-511.7.D(1) covers renting rooms inside the owner's own dwelling unit, not a separate ADU. Harrisburg has adopted no short-term-rental ordinance (a full-code search returns no STR provisions, and the only Council activity on record is a March 29, 2023 policy work session), so the constraint is definitional: a 'dwelling' is 'a building used as nontransient living quarters,' and the one nightly-rental use available in RL and RM is a bed-and-breakfast, permitted there only by special exception as an accessory use and forbidden to have 'separate cooking facilities in any guest room,' which every ADU has by definition.