What decides whether a room addition is feasible
Setbacks, lot coverage, HOA architectural review, and where the services run, all confirmed with Orange County or your own city. Those four get checked before anything is drawn, because any one of them can make an addition impossible on a particular lot and none of them are obvious from looking at the house.
Foundation and tie-in
An addition needs its own footing and slab, and the join to the existing slab is a detail rather than an afterthought. Above ground, matching the roof pitch and blending the stucco are what decide whether the finished house reads as one building or as a house with something attached to it.
Services
New conditioned space needs to be on the HVAC, and the existing system may or may not have the capacity. It needs circuits, and the existing panel may or may not have room. If the home is on septic, adding a bedroom can trigger a drain field question. All of this is settled before the estimate, not during the build.