Two adults and a child is a different brief from two adults. Storage, sound and a second door carry most of the difference.
A second closable space is not optional
A child needs somewhere to sleep that is not the room the adults are sitting in at nine in the evening. That is a door, not a curtain, and it means MD 3 or larger. Everything else in the brief follows from that one requirement.
The room does not need to be large. Two by two and a half metres with a window and storage is a genuine bedroom for a child; the constraint is that it closes.
Storage roughly doubles
Two adults need six to eight cubic metres. Add a child and the figure climbs by half again: clothes that change size annually, a pram or a bike, toys, school things and the seasonal equipment a family accumulates.
Plan an external store from the start. A weatherproof box on the terrace takes bikes and garden equipment out of a house that cannot spare the volume.
Sound between the two rooms
The partition between the child's room and the living space is the wall that decides whether the evening works. A filled cavity, a solid door and a threshold seal make it a room; a hollow stud with a hollow door makes it a screen.
The same applies to the bathroom, which in a compact plan is usually next to the bedroom.
Safety details worth specifying
Stair going and rise, a handrail a child can reach, restrictors on the upper windows and a guard at the loft edge. All are cheaper as a specification than as a retrofit, and the loft guard in particular cannot be added convincingly afterwards.
Thermostatic mixing on the hot water is a five-minute item at production and prevents the one scald that everybody remembers.
Outdoor space does the heavy lifting
A family in a compact house lives outdoors more than a couple does, so the terrace, the shelter over it and the route from the door matter as much as any internal dimension. A covered deck extends the usable season by months.
Plan the outdoor space at the same time as the plan, not as landscaping afterwards. Where the door faces decides how the house is used.