I have a medium sized web project that's growing. Here's what my ITCSS architecture looks like:
- Settings ...... global vars, config switches, brand colors.
- Tools ......... default mixins and functions, 3rd party tools.
- Generic ....... ground-zero styles (Normalize, resets, box-sizing).
- Elements ...... unclassed HTML elements (type selectors).
- Objects ....... cosmetic-free design patterns, agnostically named (wrappers, layouts, media).
- Components .... designed components, chunks of UI, group of objects.
- Themes ........ thematic styles, like seasonal or admin section.
- Force ......... Overrides, helpers, !important styles.
Where would you create the Layout related files?
- Objects/o-3-column-layout.scss
- Components/c-3-column-layout.scss
The ITCSS (and even your own description of the ITCSS levels above) recommends putting layout-related files under Objects.
Layout-related CSS are 'class-based selectors which define undecorated design patterns' - rows, columns, spacers, pushes/pullers, etc.