Most venues believe they're at capacity. The seating chart says so, the posted capacity says so, and nobody has looked at the building differently in years. But a seating chart only shows where seats are. It doesn't show where they could be.
In most stadiums there's room hiding in plain sight. It sits in the corners, behind the end zones, along concourse edges, and in the dead space everyone walks past on game day. The question isn't whether it exists. It's whether anyone has looked for it before they needed it.
Where the room usually is
What makes a space usable
Not every empty square foot can carry seats. A few questions decide it:
Why it's worth knowing now
Added capacity only matters when demand shows up, and demand rarely gives notice. A home playoff game, a sellout, an opponent whose fans travel. When that happens, the venues that respond fastest aren't the ones that build fastest. They're the ones that already know where the room is and have already answered the questions above.
A site walk early in the season turns a guess into a plan. It maps what the venue could hold, what each area would take, and what the building department will want to see. None of that commits anyone to building anything. It just means that when the moment comes, the decision is whether to add capacity, not whether you can.
The room is usually already there. The work is finding it before you need it.