How should I budget a curbless accessible route during new construction?

I’m trying to price accessibility as part of the design for a new custom home rather than treat it as a series of future modifications. The proposed route would run about 45 feet from the driveway to the main living area, with an entry height change of roughly 18 inches from the exterior approach to the finished interior floor. The plan may need a zero-threshold exterior door, wider clear openings, and flush transitions throughout the route.

For budgeting purposes, would you price this as one coordinated accessibility package, or separate the entry, doors, flooring transitions, and interior circulation so the owners can phase some work later? I’m especially interested in where the cost difference becomes significant:

- How much do site slope and foundation type affect the entry solution?
- Which dimensions or structural details need to be fixed before framing and concrete work?
- Is it worthwhile to add blocking for future grab bars, lifts, or door operators even if those features aren’t installed initially?
- What electrical rough-ins are sensible for future automatic door operators or lift equipment?
- Which items are relatively inexpensive allowances now but disruptive to add after completion?

I’m not looking for a generic accessory checklist. The main question is how to compare the upfront construction premium with the cost and disruption of opening walls, rebuilding thresholds, or changing the approach later. A rough way to organize a builder’s quote by major cost drivers would also be helpful.