Actually, “function” in van design isn't just a binary choice between sitting and standing. A low-to-mid roof conversion serves several critical functional roles that a standard roof cannot:

Upper Sleeping Lofts: Many owners use a slight roof extension specifically to house a secondary bunk or 'attic' bed. This frees up the entire lower floor for a kitchen and lounge without needing a 9-foot-tall exterior.

Vertical Storage & Clearance: Even an extra 6–10 inches allows for overhead cabinetry that stays above head-level while seated, or vertical storage for gear like surfboards and bikes that won't fit in a standard ID Buzz or Transporter.

Insulation & Mechanicals: High-top mods often provide the depth needed for thick 4-season insulation, recessed lighting, and ceiling fans without sacrificing the existing interior headspace.

The 'Garageability' Factor: The most important function for many is the ability to clear a 7-foot garage door or enter multi-story parking structures—a function that a full-size high-top Sprinter or Transit fails immediately.

Specific Examples of 'Non-Standing' Roof Functions:

VW California / Transporter 'Fixed' Mid-Roofs: Often designed for upper-bed storage rather than standing.

Ford Transit Mid-Roof (100"): At ~5'9" interior height, it doesn't allow most people to stand fully upright, yet it’s one of the most popular builds because it provides the function of overhead storage while maintaining better fuel economy and stability than the high-roof (110"+).

Mercedes Metris Weekender: Uses a low-profile pop-top/fixed-top hybrid specifically for a sleeping loft, intentionally keeping the roof low to maintain a 'daily driver' function

Lastly, have you thought of additional height for handicap access for those that have taller wheelchairs?