We have a larger property and are considering adding a detached accessory dwelling unit to the back of the property for rental income.

Philadelphia also passed some special rules on Accessory Dwelling Units ("ADU").

It could be a good accessory apartment if there was access from the back

Most zoning laws in most cities do not allow accessory dwelling units, (ADUs) in neighborhoods with single family zoning.  

(iv) The junior accessory dwelling unit complies with the requirements of Section 65852.22.

Exemptions include "Housing accommodations in which the tenant shares bathroom or kitchen facilities with the owner who maintains their principal residence at the residential real property.(5) Single-family owner-occupied residences, including a residence in which the owner-occupant rents or leases no more than two units or bedrooms, including, but not limited to, an accessory dwelling unit or a junior accessory dwelling unit."

If the lot is big enough with the rental you already own, you can add an "accessory dwelling unit" and a "junior accessory dwelling unit" on the property you already own, you can convert a garage, etc. 

I made sure the buyer knew about this.In the town I live in they allow accessory units restricted to family member use in some zones. 

If you had some type of accessory building, then sure, but not in the house.

But, I need to find complementary accessories that wont make the green look tacky.