A router broadcasts roughly outward in all directions, so a router in the corner of a house wastes half its signal on the garden. Moving it toward the centre often doubles usable coverage without spending anything.
Height matters too. Signals travel better outward than through floors, so a router on a shelf reaches further than one on the floor behind a sofa.
Materials decide the rest. Plasterboard barely matters; brick, concrete, mirrors, metal cabinets and water tanks matter enormously. A single mirrored wardrobe can create a dead spot on the far side of it.