The visit starts with a walkthrough of the whole house, not just the mechanical room. We look at window area and orientation, insulation, attic conditions, how the finished basement was framed around the returns, and where the rooms that never cool properly actually are. Those measurements feed a Manual J load calculation, which is what determines the correct capacity for your home rather than the nameplate on the failing unit.
Ductwork gets its own inspection. Many homes here still run their original trunk and branch systems, and undersized returns are the norm in split-levels and older colonials. If the duct system cannot move the air a new high efficiency system needs, we tell you before you buy the equipment, because a well matched system on bad ductwork will underperform and no warranty covers that.
From there you get options rather than one price: a straightforward single stage system, a two stage or variable speed system that runs longer at lower capacity and controls humidity far better, or a ductless solution for additions and converted spaces that the main system never reached.