To plan a bathroom layout, put the fixtures on one wall where you can, order them along that wall from the one that needs the most room to the one that needs the least, then check the clearance in front of and beside each one. The door swing is the next check, storage...
To plan a kitchen layout, fix the positions of the sink, the cooktop and the refrigerator first, set the aisle widths that let someone work and someone else pass at the same time, then build prep and cleanup zones around those three points. Storage goes in afterward,...