Even if you allowed skyscrapers, most existing houses and buildings would be illegal because of code. The map on this particular page mentions it only looks at zoning, so it's still very valid.
But my point is saying "X amount of homes would be illegal to build" is completely pointless and just meant to rile people up. OF COURSE most buildings are illegal to build today. We don't build like we use to for a variety of reasons. Even if you forget about building code, there's also rules around minimum amount of affordable units that older buildings didn't need to follow. Zoning is a specific one and yes, it's often problematic and regressive. Let's focus on that.
The building code and fire code thing is not true. Building codes are actually very flexible. There are plenty of ways to build a building to look exactly like almost any old building and fully comply with all codes.
Look like old buildings? Sure. But actually be the same? No. Hell, the building I live in is less than 12 years old and the electric panel is no longer to code, the HVAC condensate line is no longer to code, the damn stairs are no longer to code, the freagin dryer vent is no longer to code. They were all to code when they were built.
But my point is saying "X amount of homes would be illegal to build" is completely pointless and just meant to rile people up. OF COURSE most buildings are illegal to build today. We don't build like we use to for a variety of reasons. Even if you forget about building code, there's also rules around minimum amount of affordable units that older buildings didn't need to follow. Zoning is a specific one and yes, it's often problematic and regressive. Let's focus on that.