One way to find potential revenue is to simply go bigger.

[Roger Simons]

Bigger houses often have rooms that may not be a bedroom now but could be repurposed for that.  The larger the house the more likely you'll have a large living or dining room, a library, a home office, a rec room, a media room.  Anything that is fire code safe can be rented as a room.  Here's an example of a description of a 2100 sq ft house renting for about 40% more than the median in this area.
Main floor: living room, dining room and family room.
Second floor: 4 bedrooms.
Basement: 2 bedrooms + large rec room.
So we see 4 rooms that can be repurposed and we haven't even seen the house yet.  Its advertised as a 6 bedroom but you know the living and dining rooms are going to be much larger and nicer and probably have nicer views than the bedrooms.  So in my middle class area where bedrooms are averaging about $550, we have (6 x $550 = $3300) + living room ($700) + dining room ($600) + family room ($600) + large rec room ($700).  $5900 total and its renting for about $2400.  You need one of the rooms so say $5900 - 550 = 5350.  

In a small house you almost never have "extra" rooms you can repurpose.  
In large apartments you sometimes see rooms that can be repurposed but usually houses are just far better for this.  Plus you generally have far more privacy in a house.  Neighbors can ruin things.