Low-Rise or High-Rise Multifamily: Which Applies?
Count habitable stories. Three or fewer is low-rise multifamily; four or more is high-rise. Both are governed by Section 170, but the requirements diverge at that line — and a high-rise building is often treated much like a nonresidential one.
The count is habitable stories
Not floors, not height in feet — habitable stories. Three or fewer puts the building in the low-rise category; four or more makes it high-rise.
That wording matters on real buildings. A podium structure with parking below habitable levels is counted on its habitable stories, so the total number of floors can be misleading. It is worth establishing this before anything else, because it determines which requirements you are reading.
Both are Section 170 — this is the part that gets confused
A common misconception is that low-rise multifamily is residential and high-rise multifamily is nonresidential, sitting in different chapters. They are both in Section 170. The section splits internally by height rather than handing high-rise buildings to the nonresidential chapter.
Section 140 applies to nonresidential buildings. It does not apply to multifamily. Reaching for a Section 140 requirement on a multifamily project is a straightforward category error, and it happens because high-rise multifamily is treated like nonresidential in a number of respects without actually being it.
Where the split shows up
Solar is the clearest example: low-rise multifamily PV sits at 170.2(f) and high-rise at 170.2(g). Two subsections, two sets of rules, one section.
Water heating splits too, and in a way that surprises people: individual gas or propane instantaneous water heaters may no longer be used in low-rise buildings, and that restriction does not apply to midrise. Here the low-rise category is the more constrained one, which is the opposite of the usual assumption that taller means stricter.
Why the line sits at three stories
Below four habitable stories a multifamily building behaves thermally much like a house — each unit has significant exterior envelope, its own systems are small, and the physics are residential. Above it, buildings tend toward central systems, corridors, shared plant and a much lower ratio of envelope to floor area. The requirements follow the physics rather than an arbitrary threshold.
Establish it first, then read the code
Almost every other question on a multifamily project depends on this answer — which ventilation rules apply, how water heating may be configured, which PV subsection governs. Counting habitable stories takes a moment and prevents reading the wrong half of a section for weeks.