The Den Residential is a cut location in Fallout 2, but it is still possible to actually enter this part.
Reaching the area
First, you must enter the map screen (with green triangles). After this, you must just press the number 3 (the Den Residential is the 3rd area in The Den, so you have entered the 3rd area). After entering this part of the map, you have entered the cut version of The Den. This means you can get what you want, and get away with it. Meaning you can kill Rebecca and get away with it, because you are in the different version of The Den, not the original one.
This map contains an early version of Smitty's junkyard (without the car) and a few rather boring houses with people with no dialogue. The only dialogue that is implemented on this map is that of the ghost girl, Anna Winslow. She appears at night in the easternmost house and behaves much like she does on the Den East Side. There are some collectibles in Smitty's locker.
There is a quest that will require you to get the orphans from the streets. A kid is looking for a place where they can live without stealing from other people. It requires Fallout 2 Restoration Pack.
Den Residential was to appear in Fallout 2, but was cut.