Have a notebook. Watch yourself living over the next few weeks. Make dozens of notes.
Where do you spend time? Round the kicthen table? On a squishy sofa in front of TV? At a computer desk?
Do you have kids/plan on having kids, and how old, watch how you use space with them. Young kids play downstairs near you, as they get older things like playroom come into their owna nd then teens, a hang out space that isn't their rooms is amazing.
Things like - are you a cook and how do you use kitchen, where would you put prep area, sink, fridge etc.
How do you do washing? All tumble dried or do you like to hang it, where will you hang it, where do you need space?
I am a gardener, so I always want outside taps in right places, and this links to indoor taps.
I always like a proper sink in utility, for dirty stuff (pet bowls, paint brushes, muddy trainers)
Do you do craft? Do you need a storage/work area?
Do you iron? My Mum has spce for iron permamently up in utilty room, she irons things as she needs them.
Do you wfh? Where will you do that?
Chargers? phones? Computers? Where will they go?
Do you buy in bulk? Large larder/freezer? or eat all ready meals?
Do you socialise? Dinner round table? Party and dance space? Hot tub? or sofas and wine glasses on coffee table?
etc etc. Keep making notes, and watching yourself, to see how YOU use a house and what YOU need.