Apartment buildings across American cities share an unusual family resemblance, typically four or five wood-framed floors sitting on a concrete podium. The shape is drawn by the building code rather than by any architect's preference.
Codes sort buildings by construction type
Model building codes classify structures according to the fire resistance of their materials, and each type carries limits on height, number of storeys and floor area.
Light wood framing falls into the least fire-resistant category, and its permitted height is correspondingly restricted.
Concrete and steel construction is allowed to go much higher, and it costs substantially more per square foot.
Wood is the cheapest way to build up to the limit
Wood framing uses widely available material, a large trained labour force and standard components, which makes it far cheaper than concrete for the same floor area.
The rational move for a developer is therefore to build in wood right up to the maximum the code allows, and no further.
Going one storey higher forces a different construction type for the entire building, which raises the cost of every floor rather than just the added one.
The podium buys extra height
Codes permit a fire-resistant lower structure to be treated separately from the wood-framed floors above it.
Developers use this by placing a concrete base containing parking or retail beneath the residential levels, which adds usable floors without pushing the wood portion past its limit.
The visible result is the recognisable form of a solid ground floor with lighter construction stacked on top.
Sprinklers and access rules shape the floor plan
Permitted heights and areas are larger when a building is fully sprinklered, so sprinkler systems are effectively standard in this class of building.
Requirements for two means of egress and maximum travel distance to a stair then drive the double-loaded corridor plan, with apartments on both sides of a central hallway.
Fire access for apparatus and the position of the stairs also fix where the building can sit on its lot.
Uniformity follows from shared constraints
When many developers optimise against the same code, the same construction costs and the same financing expectations, they converge on the same building.
Changes to the underlying rules, such as provisions for taller mass timber structures or revisions to single-stair requirements, are what actually change the shape of new housing.
The look is a byproduct of regulation and cost meeting at a particular point, and it moves when either one moves.