A countertop is one of the few components in a bathroom that is fabricated off-site to a fixed dimension and then has to fit exactly. There is no scribing a stone slab once it arrives. Everything therefore depends on the measurement it was cut from, and the only measurement that can be trusted is one taken from the cabinetry after it has been installed and levelled.
Why templating after installation matters. Cabinets get scribed to walls that are out of plumb, toe kicks get packed on floors that are out of level, and a run drawn as 1,820mm ends up 1,814mm because the return wall bows. A top cut from the drawing is then either too long — and re-cutting quartz or stone on site leaves an edge that needs polishing in place — or short, leaving a gap filled with silicone. Templating on installed cabinets picks up the real wall profile, the real length, the faucet hole positions relative to the basin, and the actual overhang at each end. It costs a return visit and a few days in the schedule.
Choosing between the materials. Engineered quartz is the default for bathrooms for good reasons: it is non-porous, needs no sealing, resists the cosmetics, hair products and cleaners that actually damage bathroom tops, and comes in consistent slabs so a second vanity matches the first. Its weakness is heat and prolonged UV, neither of which is usually a bathroom problem. Natural granite is harder, has genuine depth of pattern, and needs periodic sealing. Marble is the most beautiful and the least forgiving: it is calcareous, so anything acidic — a splash of toilet cleaner, some skincare products, lemon oil — etches the polish, and it will develop a patina rather than staying pristine. In a guest powder room that is often a fair trade; in a family bathroom it usually is not. Solid surface material is seamless, repairable by sanding, and allows an integrated basin moulded into the top with no joint at all, which is the easiest surface in the room to keep clean and a genuinely useful choice in an accessible bathroom.
Edges, thickness and overhang. A 20mm slab with a mitred edge reads as 40mm or more without the weight of a full-thickness top, and that mitre is where fabrication quality shows — the joint should be invisible and the pattern should run continuously around the corner. A pencil round or a small eased edge suits most bathrooms; heavy profiles collect product and date quickly. Overhang past the cabinet face is usually 10–20mm, enough to shed water clear of the drawer fronts without becoming something to catch a hip on in a narrow room.
Sink type changes everything upstream. An undermount basin gives an uninterrupted surface that can be wiped straight into the bowl, and it needs the cutout polished and the reveal decided — positive, flush or negative — because that decision affects how easy the rim is to clean. A vessel basin sits on top, raises the effective working height by 100mm or more, and therefore requires the cabinet to be built lower and the tap to be tall enough to clear the rim. A drop-in basin is the most forgiving of an imperfect cutout and the hardest to keep clean at the rim. An integrated basin has no joint to fail at all. Each of these also determines the faucet: deck-mounted holes have to be drilled in the correct number and spread, and a wall-mounted faucet means no holes at all but a valve body that had to be set in the wall at rough-in, at a height and projection that suits the basin chosen months earlier.
Installation details that decide longevity. The cabinet has to be dead level before the top goes on, because a stone top will telegraph a twist and can crack over an unsupported span. Support is added under any unsupported run and around the sink cutout, which is the weakest part of the slab. Seams, where a run is too long for a single piece, are placed at a sink cutout or a corner where they are least visible and the pattern is matched across them. The joint at the wall gets a flexible sealant rather than grout, because the cabinet and the wall move independently. Undermount basins are mechanically supported, not held up by adhesive alone.
Replacing a top on its own. Swapping a countertop and basin without touching the rest of the bathroom is often the whole scope, and a sensible one. The constraint is the existing cabinet: it has to be sound, level and able to carry the weight, and the faucet hole spread has to suit the new basin.