Most compact plans assume a ladder and a bathroom you turn sideways in. Both are fine at forty and neither is at seventy, and both are cheap to change on the drawing.
Sleeping on one level is the whole decision
Everything else about ageing in a compact house is manageable; a loft reached by a steep stair is not. A ground-floor sleeping space — MD 3 or larger — is what makes the house a twenty-year proposition rather than a ten-year one.
If a loft is the only option, specify a proper stair with a handrail rather than a ladder, and a going deep enough to descend facing forward. That distinction matters long before seventy.
Doors and clear widths
An 800mm door leaf and a 900mm clear route through the plan cost almost nothing at design stage and are the difference between a house that accommodates a walking frame and one that does not.
Avoid thresholds. A flush transition to the terrace helps everyone with a wheeled anything — a suitcase, a pushchair, a wheelchair — and it is the same detail that makes the house feel bigger.
The bathroom is where retrofitting is hardest
A level-access shower with no upstand, blocking in the walls for future grab rails, and a door that opens outward are three specification lines. Adding any of them later means stripping the room back to the frame.
The blocking in particular costs nothing now: a plywood pad behind the board where a rail might one day go. Nobody regrets it.
Reach, light and switches
Sockets at 450mm rather than skirting height, switches at 1000mm, and pull cords rather than rotary controls where possible. All of these are standard accessible practice and none of them look institutional when done at design stage.
Light levels matter more with age than most people expect. Layered lighting — ambient, task and a night route to the bathroom — is worth planning as carefully as the daylight.
The approach from the car
The path, the step at the door and the gradient of the plot are what actually decide whether the house is reachable. A perfect interior at the top of three steps and a gravel slope is not accessible.
Deal with it when the terrace is designed. A ramp added afterwards is a ramp that looks added afterwards.