[contingencies]

Re. Ceiling height. Lumber, insulation and boards are supplied with 3m max height typically. (Some lumber, metal structural beams and sheet metal are readily available at 6m max length above this). Therefore, the generally expressed internal heights for cost efficient construction are 3m less inter-floor allowances for ceiling/utilities/floor/inter-floor insulation. You can of course do anything you like, but to your builder it means more work and materials.

In apartment buildings, it is common to squash non-ground floors vertically to obtain more houses within the available building height envelope in order to maximize returns. Local regulations in decent jurisdictions typically specify a limit to this type of viciously commercial construction outlook.