> It’s not clear that condo units have higher long-term capex costs than single-family homes; arguably economies of scale make their capex lower
Economies of scale have to do with manufacturing identical units. Making 1000 is cheaper than making 10. But stacking units atop each other is not making identical units, so it is always cheaper to build 10 SFH than to build a single building with the same size 10 units stacked atop each other -- ignoring the extra price of land. When you go vertical, you need to set aside more space for services such as hvac and elevators. You need a stronger foundation. You need to deal with wind and earthquakes. You need to add emergency stairs in case the elvators go down. The walls have to be stronger, noise abatement needs to be added. You have systems in place to deal with vibrations. There is no attic in which to run HVAC. Air recirculation is a big concern in condos -- many don't even have windows that open, but they have enormous air filtration and pumping systems to get fresh air to all the units and maintain an air pressure difference. There are entire systems condos have to have that aren't even an issue in SFHs. And those systems require their own control systems, controlled by complex software and networks of sensors, with dedicated building staff to monitor and operate them. This is if nothing goes wrong. Thus the costs per livable square foot go up rapidly as you stack units atop each other, which is why you see condos only in places where land is very expensive. Everywhere else, it is cheaper to build a SFH. It is also why condos tend to be much smaller than SFH in terms of livable area per unit.
In terms of maintenance, all those extra systems condos need are more failure points. Structurally, you have much higher stresses, which requires additional supports and more complexity. All of that increases the damage when there is a failure and increases maintenance costs as there are more systems to maintain. Even something as simple as washing windows becomes really expensive in a condo. Electrical and water repairs that are simple and fast in a SFH can become major engineering projects in a condo. More complex systems that are harder to service and more dependencies add up to higher maintenance costs to match the higher construction costs.