Most people build a detached dwelling unit for one reason and end up using it for another. The office becomes a place for a parent, then a rental, then a guest suite. Designing for that sequence from the start costs very little. Retrofitting it later costs a great deal.
Put in the full kitchen rough-in
Even if you only want a coffee station today, run the plumbing, drainage and electrical for a full kitchen while the walls are open. Adding it afterwards means opening finished walls and often the slab.
Plan the bathroom for accessibility
A level-entry shower and a door wide enough to matter cost almost nothing extra at the framing stage. They are what make the building usable for an ageing parent later, and they are close to impossible to add cheaply afterwards.
Separate the utilities
If there is any chance of renting, consider separate metering or at least sub-metering. Splitting a shared bill with a tenant is a recurring annoyance you can design out once.
Think about privacy and approach
An office only needs a path. A rental needs an entrance that does not run past your kitchen window. Landscaping, fencing and where the door faces are what make the two uses compatible.
Storage is the thing people regret
Compact buildings live or die on built-in storage. It is the cheapest thing to include during the build and the most disruptive to add once someone has moved in.
