Accessory Dwelling Units (“ADUs”) in Philadelphia

Accessory Dwelling Units (“ADUs”) in Philadelphia

One topic I get at least one call about per week is the accessory dwelling unit. While it’s a cool idea and awesome when it works, it’s a rare circumstance when it does, and they are not the backdoor to getting that duplex that you’ve been desperately searching for.


Accessory Dwelling Units, or “ADU”s in zoning nerd speak, historically called granny flats or backyard cottages, are zoning approvals that allow a second dwelling within the same building or on the same property footprint as the main house. In theory, this would change the single family home to a duplex, but only sort of, because the code requires you to live in one unit in order to qualify for renting out the other! BUT! You can Airbnb, which is of course attractive (though see our article HERE for the Philly Airbnb rabbit hole).


ADUs have been included in the Philadelphia Zoning Code since 2012, when language was added HERE. However, at that time, the City failed to indicate that these units would or could be approved without obtaining a dreaded zoning variance, spending money and time and MAYBE getting the ADU approved. It basically just defined what an ADU is. Not helpful.


In 2019, updates were made to the zoning code which approved ADUs by-right, but only in certain circumstances/locations. That language:


(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.


So, for a while ADUs were limited to extremely small, historically-zoned areas, and required deference to the historical commission of Philadelphia, for their insight into whether the ADU was appropriate on any given block. Fast forward to 2021 and we got this language:


(.2) Large Lots in the RSA-5 and CMX-1 Districts.


In the RSA-5 and CMX-1 districts, an accessory dwelling unit is permitted, provided that:


(.a) The area of the lot is a minimum of 1,600 sq. ft.; and


(.b) No more than one dwelling unit is otherwise permitted.


This is where the code sits today. If you have a very large single family residential or CMX-1 lot, you can get one by right. Lots this size are fairly rare, so we still have a ways to go.


The whole idea of these flats is to prevent displacement of residents living in single family homes, who may see the value of their lot rise and may feel the pressure to sell and downsize, but with an ADU may find another source of revenue in their granny flat. I would expect that in practice, if the restrictions on location get a little looser, it would lead to builders and investors working towards figuring out how to capitalize on these, which I am not saying is a bad thing, but I could see it expanding beyond the intent of the bill.


There are a couple of other important restrictions to building an ADU, should you qualify. First, you’ve got to have an entrance either accessible through the primary home, or through an alley or through the backyard. It cannot be two doors on the front facing street. Finally, the max size of the ADU is 800 square feet, limiting the sexiness of the dwelling, but is the perfect size for a couple or an airbnb.


If you have a property you think might be a good candidate for an accessory dwelling unit approval, you can always call me at 267-603-2493, or email me at I’m happy to help!



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