What causes low water pressure at one tap?

Pressure low at a single tap, while others are normal, means the problem is local to that tap or its supply rather than the incoming main. That distinction saves a great deal of unnecessary investigation.

The aerator is the first thing to check and the most common cause. It is the small screen in the tap outlet, and it collects grit, limescale and debris. Unscrewing it, usually by hand or with cloth-protected pliers, and rinsing or soaking it in vinegar restores flow immediately in most cases. In hard water areas this is routine maintenance rather than a fault.

The isolation valve under the sink is the next candidate. These small in-line valves are often partly closed, either from installation or from being knocked. A screwdriver slot turned so the slot aligns with the pipe is fully open.

A flexible tap connector hose can collapse internally, particularly cheaper ones after several years, restricting flow while looking perfectly normal from outside. It is an inexpensive part and worth replacing if the first two checks find nothing.

Cartridge or valve debris inside the tap itself is common after any work on the supply, because disturbing pipes dislodges scale and sediment that then lodges in the tap.

If only the hot side is low, the problem is likely on the hot supply rather than the tap: a partly closed valve at the cylinder, an airlock, or scale in the hot pipework, which scales faster than cold.

If pressure is low at every outlet, that is a different problem, pointing at the incoming supply, a stopcock not fully open, or a pressure-reducing valve.