Over the past few weeks, the Center for Offsite Construction published two new versions of the Configure-to-Order file type specification — the open standard for describing buildings composed of pre-engineered, pre-certified offsite products. 

The .cto file type exists to advance Configure-to-Order construction — giving designers, developers, and manufacturers a shared data structure for working with configurators, referencing standard interfaces, generating immediate MSRPs, and deriving realistic project schedules from the moment a design takes shape. By encoding a building as a machine-readable composition of known products rather than a bespoke drawing, the file type also creates the structured foundation that AI agents and blockchain-based provenance tools will need to meaningfully support 21st-century construction.

Both releases move the spec closer to matching how buildings are actually designed, built, and delivered. Three additions stand out:

Levels are now first-class.

A building used to be described as a stack of floor plates. Now, it's organized around level datums — abstract horizontal planes that host cartridges, walls, and pods. The ground floor sits on a datum. The second floor derives its elevation automatically from the ceiling height below plus the cartridge thickness. A roof datum hosts the roof. Levels exist whether or not a floor has been placed on them, which mirrors how architects actually think — you lay out a second floor before you know exactly what's in it.

Stairwells became a real thing.

Previously, a stairwell was "the absence of a floor cartridge." Now it's a bounding volume — a void that spans levels, hosts a prefab staircase product, and lets adjacent cartridges dead-end cleanly against its edge. Furring walls attach to its perimeter. Shortened cartridges snap to its face. It behaves the way stairwells behave on drawings.

The roof is a prism.

A sloped roof is no longer a collection of angled panels floating in space. It's a prism — a geometric solid with a ridge, two sloped faces, and defined overhangs. Roof panels host to the sloped faces the way wall panels host to wall positions. Gable ends are handled by wall panels below, not by the roof system. Clean separation of concerns.

Smaller additions — entourage furniture, chain-of-custody fields, interface standards that distinguish between the dwelling-unit side and building-services side of a coupling — are now live in the spec as well.

The open schema is on GitHub. More to come.