Accessory Dwelling Unit Laws in Pennsylvania

Individual cities throughout the Keystone State have differing laws regarding accessory dwelling units (ADUs). Below is list of some of the most populous areas in Pennsylvania and the ADU rules that apply to them. This list includes:

  • Philadelphia
  • Bethlehem
  • Pittsburgh


In cities throughout Pennsylvania dwellings have to abide by general building codes. However, there is one city that has recently become more accommodating to Accessory Dwelling Units, ‘The City of Brotherly Love’ Philadelphia.

According to the National Trust for Historic Preservation the recent bill that makes specific provisions for ADUs in Philadelphia is geared towards having them constructed at historically designed structures. The trust states that only ADUs can be built at properties listed on the Philadelphia Register, which includes mainly housing in the historic districts. The allowance of ADUs in these districts will allow for an increase of housing supply in the area, while maintaining the character of the areas. It will also allow people access to potentially an extra source of income by renting out the ADUs. This money could then be used in the maintenance and upkeep of the main resident’s buildings. 

Under Title 14 of the Philadelphia code there are details as to why ADUs are now being promoted in the city and the general requirements you must be mindful of if wishing to construct them. Below is an extract from the code that highlights them:

(c)   Where Allowed. 

Accessory dwelling units are allowed only on lots occupied by a single-family use contained in a detached or semi-detached building in the permitted areas described in § 14-604(11)(d) (Permitted Areas), except within historic structures (as described in subsection (d)(.1) below), where accessory dwelling units are also permitted on lots occupied by a single-family use contained in an attached building. Accessory dwelling units must be located within the interior of the principal building or within the interior of a detached accessory building, such as detached garages, that are in existence as of the effective date of this Zoning Code.

(d)   Permitted Areas. 

  • (.1)   Historic Structures. Accessory Dwelling Units are permitted within any building or structure that, pursuant to Chapter 14-1000, has been designated as historic; or that is located in a district that has been designated as historic and that contributes, in the Historical Commission’s opinion, to the character of such district.
  • (.2)   Reserved.

(e)   Owner Occupancy.

The principal dwelling unit or the accessory dwelling unit must be occupied by the owner of the subject lot. Before final occupancy of the accessory dwelling unit, the property owner shall record an affidavit and deed restriction, stating that the property owner will reside on the property, in either the principal or accessory dwelling unit. Once recorded, the deed restriction (requiring owner occupancy) may not be removed or modified without Zoning Board approval.

(f)   Number.

No more than one accessory dwelling unit is allowed per lot.

(g)   Density (Minimum Lot Area per Unit).

No additional land area is required for the accessory dwelling unit beyond the minimum lot size required in the subject zoning district.

(h)   Location of Entrances.

Only one entrance to a detached or semi-detached house containing an accessory dwelling unit may be located on the front facade that faces a street, unless the house contained an additional street-facing entrance before the accessory dwelling unit was created.

(i)   Size.

The floor area of an accessory dwelling unit may not exceed 800 sq. ft., except in historic structures (as described in subsection (d)(.1) above).


There is another city in Pennsylvania that seems to allow certain ADUs within it’s limits however the regulations regarding there construction is even more limited. Bethlehem allows individuals to construct ADUs in certain circumstances for relatives you wish to care for. 

Below is an extract from the City of Bethlehem Zoning Ordinance that explains the situation and requirements that allow for the construction of an accessory dwelling unit:

‘1302.140 Unit for Care of Relative. A dwelling unit that: a) is especially created for and limited to occupancy by a close “relative” of the permanent residents of the principal dwelling unit, b) is necessary to provide needed care and supervision to such relative, and c) meets the requirements for such use in Section 1322.03.

City of Bethlehem Zoning Ordinance Article 1322

(bbb) Unit for Care of Relative.

  • (1) See the definition.
  • (2) The accessory unit shall be occupied by a maximum of two persons, who shall be “relatives” of the permanent residents of the principal dwelling unit. At least one resident of the accessory unit shall need such accommodations because of an illness, old age or disability.
  • (3) The applicant shall prove to the Zoning Officer that the accessory unit has been designed and constructed so that it can be easily reconverted into part of the principal dwelling unit or is a modular cottage that will be completely removed from the lot after the relative no longer resides within the unit. Such accessory unit may be converted into an additional bedroom(s), permitted home occupation area or similar use. A lawful detached garage may be converted into a Unit for Care of Relative, and then be reconverted to a garage or permitted home occupation area or otherwise function as part of the principal dwelling unit.
  • (4) The applicant shall establish a legally binding mechanism in a form acceptable to the City that will prohibit the use of the accessory unit as a separate dwelling unit after the relative no longer resides within the unit. Such mechanism shall also be binding upon future owners.
  • (5) The owner of the property shall be required to annually renew the permit for the use. Such renewal shall be conditioned upon the owner providing a written and statement that such relative of the occupants of the principal dwelling unit continues to reside within the accessory unit.
  • (6) Such accessory unit shall not decrease the residential appearance of a one family dwelling, as viewed from exterior property lines.
  • (7) Additional parking for the accessory unit is not required if the applicant proves that the resident(s) of the accessory unit will not routinely operate a vehicle.
  • (8) If the unit is attached to the principal dwelling, there must be an internal doorway that connects the main dwelling unit to the unit for the relative.
  • (9) There shall only be one entry door facing onto a public street.’


As of this moment there are no specific laws in place on a city level in Pittsburgh that allow for the construction of Accessory Dwelling Units on residential plots in the city. However, that is not to say there are not considerations for a change in this reality. 

The city is planning for a 24 month trial period that will allow residents of single family residents in the neighborhood to construct ADUs. 

Some keys points from the ADU Overlay District Planning Commission Presentation that took place in April 2018 include: 

  • The City proposes a 24 month pilot ADU overlay district for the single-family zoned portions of the Garfield neighborhood.
  • Unless the overlay is made permanent through a separate legislative process, the overlay would expire after 24 months.
  • During this time period, the City will study the impacts of ADU’s on the neighborhood, and will assess whether the ADU concept should be considered in other neighborhoods.
  • Ordinance sunsets in 24 months from effective date.
  • ADU’s are limited to 800 sq. ft.
  • ADU’s are limited to one per lot.
  • ADU’s may not be leased for less than 30 days.
  • Owner of the property must reside on-site.
  • ADU’s are limited to two-stories, or 30 feet in height.
  • ADU’s are not subject to lot size and parking requirements

To find out more about the ADU overlay and when the pilot begins it’s probably best to contact the city directly through the website or by phone.

Conclusion on Pennsylvania ADU laws

Localities throughout Pennsylvania have different laws revolving around accessory dwelling units. In Philadelphia, accessory dwelling units are allowed only on lots occupied by a single-family use contained in a detached or semi-detached building in specific permitted areas.

To find out more about ADU laws in areas that weren’t mentioned above, make sure to contact the relevant city and county in question directly.

This article is written for informational purposes only and is not to be construed as legal advice.


  • City of Philadelphia, The Code of Philadelphia § 14-604.  (2020)
  • City of Bethlehem Zoning Ordinance § 1302.140 (2018)
  •, ADU Overlay District, Planning Commission Presentation (2018)

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