The core is why a tower costs more per square metre

What follows is an argument about the building core, and about where the received version of it stops being true.

Net to gross is the number that matters

The proportion of a floor that can be sold or let, after cores, corridors, risers and walls, is the efficiency ratio developers watch most closely. On a low building it can be very high; on a tall one it falls as more of each floor is consumed by vertical services.

Every lift shaft, stair and riser runs the full height regardless of how many floors it serves at that level. That is why the same architect produces a more generous plan at six storeys than at thirty.

Lift design is driven by the morning peak

Lift systems are sized to handle the busiest five-minute arrival period, not the average day. Meeting a target waiting time in a tall building requires more cars, faster cars or sky lobbies where passengers transfer between systems. Each option adds shaft area, machinery space and cost, and shaft area is subtracted from lettable floor on every level.

Destination control systems, which group passengers by floor before boarding, can raise throughput without adding shafts and are now common in tall buildings.

Wind load grows faster than height

A tall building must resist lateral wind load, and both the force and the leverage increase with height. Resisting it requires a stiffer structure, which usually means a larger core, outriggers or a perimeter frame designed for the purpose. Occupant comfort under sway is often the governing criterion rather than structural safety, since people notice movement long before anything is at risk.

Damping systems exist to address exactly this and represent pure cost with no floor area in return.

Foundations respond to concentrated load

Concentrating a great deal of floor area on a small footprint concentrates load into the ground beneath it. Deep piling, raft foundations and ground improvement follow, and the cost depends heavily on local geology. This is one reason certain cities build tall readily and others do not, independent of planning policy.

Site investigation results can change the viability of a height before any design is drawn.

Fire strategy sets non-negotiable minimums

Escape stair provision, firefighting shafts, refuge areas, compartmentation and sprinkler requirements all scale with height and occupancy. National codes differ substantially in how many stairs a residential building of a given height requires, and this has been actively debated in several countries.

Whatever the rule, it consumes core area and it is the least negotiable part of the plan. A change in this area of regulation can reshape residential floorplates across an entire market.

Local statute governs most of this, and it varies street by street.

The consequence for housing costs

Because cost per square metre rises with height while sale values rise more slowly on most sites, the tallest viable building is rarely the most profitable one. Developers build tall when land is scarce enough that the extra floor area is worth the extra cost per unit of it. The idea that building higher automatically makes housing cheaper does not follow from the cost curve.

At street level, cheaper homes come from cheaper land, simpler construction and lower finance costs, which point toward mid-rise more often than not.

The takeaway

The interesting part of a tower is the bit with no windows in it.

The design decision is visible long after the people who made it have gone.