The most common question we get about additions is whether the existing air conditioner will handle the new space. The most common assumption behind it is that a system is either big enough or it is not. Both the sensible and the latent parts of the answer are more interesting than that.
Square feet per ton is a rule that should be retired
The old rule of thumb, 500 or 600 square feet per ton, came from a narrow band of houses in a narrow band of climates. It does not survive contact with the range of houses we work on. A tight new addition with low-E impact glazing and a sealed envelope can comfortably exceed 800 square feet per ton. A 1960s room with a west-facing glass wall and thin ceiling insulation can need fewer than 400.
The load depends on orientation, glass area and type, insulation, air leakage, ceiling height, occupancy and where the ducts run. That is what a room-by-room Manual J calculation accounts for, and it is why we run one on the finished plan rather than on a guess.
Latent load is the part people skip
In a hot-humid climate, a large share of the cooling job is removing water from the air rather than lowering its temperature. That is the latent load, and it behaves differently from the sensible load.
Oversized equipment is the classic Florida mistake. It satisfies the thermostat fast, shuts off, and never runs long enough for the coil to condense much moisture. The house hits its set point and still feels clammy. People respond by setting the thermostat lower, which makes it colder and clammier. The system is not broken, it is simply too big for the load.
If the house is cold and still feels damp in August, the equipment is almost certainly oversized.
The duct is a separate question from the box
Even where the equipment has capacity, extending the trunk into an addition only works if the duct system has static pressure headroom. Most residential air handlers are rated at about half an inch of water column of total external static pressure. We routinely measure 0.9 and higher on existing systems, which means the equipment is already fighting its own ductwork before anyone adds a branch.
A system running against high static moves less air than its nameplate suggests, dehumidifies worse, and wears out sooner. Adding load to it makes all three of those worse. Measuring static pressure takes fifteen minutes and tells you whether you are looking at an equipment job or a duct job.
Return air decides comfort
The single most common defect we are called to fix in converted garages and bonus rooms is a supply register with no return path. Air pushed into a closed room has nowhere to go, so it stops moving. The room runs hot in summer and stale all year, and the rest of the house gets pressure imbalances.
The fix is a transfer grille, a jumper duct or a dedicated return. It is not expensive and it is not optional. If a contractor proposes a conditioned room with supply only, ask where the air goes.
When a separate zone is the better answer
On the San Marco addition we finished in 2024, the calculated load for the new 620 square feet was about 1.6 tons of sensible plus a meaningful latent load. The existing three ton system was near its limit and its trunk had no headroom. Re-ducting the whole house would have cost noticeably more than installing a dedicated system for the addition, and it would have meant opening ceilings in rooms that were otherwise untouched.
We installed a separate system for the new space. The owners got independent control of a suite used on a different schedule from the rest of the house, and nobody had to demolish a finished ceiling.
What to ask your contractor
- Will you run a room-by-room Manual J on the finished plan, and can I see it?
- What is the measured total external static pressure on the existing system today?
- Where does the return air for the new space come from?
- If the calculated load lands between two nominal sizes, what do you propose and why?
Any contractor who answers the first question with a square footage rule is estimating your comfort rather than calculating it.
Written for this demo site. Code references, tolerances and figures are illustrative and vary by jurisdiction, by product and by house. Always confirm with the authority having jurisdiction and a licensed design professional.
