Something on the smaller side usually works best at first so as to not be overwhelming or discouraging. For people in wheelchairs or using walkers, provide smooth, wide pathways and beds built high enough to reach into easily. Design beds so that the center can be reached comfortably without stretching. Twelve inches is a good depth for planting boxes, but remember that wet soil is heavy. Construction must be sound.
Shallower boxes are sufficient for many plants such as annual flowers and herbs.
Provide a place for tool storage that is close to the garden, either a small shed, a cabinet, or even a large mailbox mounted on a fence or fencepost, or on the edge of an elevated garden bed. Adapt tool pouches into hanging pouches that can be hung from wheelchairs, walkers, and the edges of raised beds. Keep an eye out for tools that make tasks easier. Garden centers, gardening catalogs, and specialty catalogs are good places to look for specially adapted hand tools. Use drip irrigation or soaker hoses and mulches around plants to cut down the time required for general maintenance such as watering and weeding.