I've wondered the same thing. They do look very similar.

There was 2 or 3 years between the end of Soap and the beginning of Who's the Boss. It's highly unlikely any studio would have left a set standing for that amount of time. They may have reused some of the elements from the Tate's set for WtB but that layout is pretty standard sit-com fare.
I agree with the other poster that the entrance and stairwell are very similar but the same could be said about Step by Step and Valerie/The Hogan Family.
Also, even if shows are on the same network they may not be shot on the same lot. If I remember right Roseanne shot at least a few of their seasons at the CBS Studio City lot, even though they were on ABC.

Studios tend to use the samew soundstages many times over,as well as the sets.
In this case,on Who's the Boss,the actors enter Angela's house through a front door to (our) left. Like SOAP,there's immediatley a saircase,with room to the side of it (our left again).
The living room is smaller of course.
Those are the only similarities I see though.
Since both shows were on ABC,there might be more sets re-used.
happipuppi13
arf,man!!