How Long a Modular Cabin Lasts, and What Fails First

Ask how many years a cabin will last and the answer is usually a round twenty. That figure refers to the steel structure, while the service life the occupant actually experiences is the shortest of three: envelope, storage and generator.
Steel is rated by corrosivity category. Inland C3 with galvanising and a coating gives 20–25 years; coastal or industrial C4 drops to about 15; salt spray and permanent damp in C5 — an island or a rocky headland — gives 10–12. The difference lives entirely in detail: stainless fasteners, zinc-rich repair on cut edges, isolating washers at dissimilar metal contacts, and a ventilated void under the floor. Storage life is set by calendar ageing rather than cycles, and a standby application cycling 20–50 times a year should plan a replacement at 8–10 years. A diesel genset is rated 15 years or 20,000 hours, and hours usually arrive first. On top of that sits a five-year consumables list: gaskets, door hardware, air filters and refrigerant couplings, coolant, UPS batteries.
Laid out like this the conclusion is clear: cabin life is not extended by building it heavier, but by making it replaceable. Three things belong in the design — a 600 mm service aisle on the equipment side so someone can stand while changing a rack rather than crawl; battery and power modules sized to slide out as a unit, not welded to structure; and a consumables list with part numbers and intervals handed over with the building. Those three decide the twenty-year cost of ownership far more than the sentence in the warranty.