Where the water appears matters
An outside tap can leak from the spout after it is shut, around the spindle behind the handle, from the joint to the wall plate or from pipework inside the property. Dry the fitting first, then watch where fresh water forms. A drip from the spout usually means the internal valve is not sealing. Water around the handle points more towards the spindle or gland. Dampness at the wall can be more important because the leak may be behind the fitting or inside the wall rather than on the tap itself. In winter, any sign of splitting or distortion deserves particular attention because frost damage often shows itself after a cold spell.
A dripping spout is often a simple valve fault
Traditional outside taps commonly use a washer to close against a valve seat. As the washer hardens or wears, the tap may need increasing force to stop the water and eventually begins to drip. Scale or grit can also prevent the valve from sealing. If the tap has been overtightened for years, the seating surface can become damaged as well. A replacement washer may solve a sound fitting, but an old corroded tap is often cheap enough that complete replacement gives a neater result. The condition of the wall plate and supply pipe should be checked at the same time.
Frost damage can cause a very different leak
Water expands as it freezes, which can split an exposed tap body or the short pipe serving it. The split may not become obvious until the weather warms and the ice thaws. A frost-damaged pipe can leak inside the wall, garage or kitchen cupboard while only a little water is visible outside. If the tap suddenly started leaking after freezing conditions, shut off its local isolation valve if one is fitted and inspect the internal connection from a safe position. Do not assume a new tap alone will cure a split further back in the pipe.
Check whether there is an internal isolation valve
A properly arranged outside tap often has a valve inside the house so it can be isolated for repair or during severe winter weather. Make sure you know which valve it is before turning anything. Old screwdriver-slot isolation valves can seize and may start leaking if forced. If the outside tap is supplied by pipework under a sink, look for dampness around that connection too. A leak that appears only while the outside tap is being used may be on the supply pipe rather than the tap body.
When a new tap is better than repeated repairs
If the body is heavily corroded, the spindle is loose, the hose union is damaged or the wall connection moves when the handle is turned, replacing the tap can make more sense than servicing one small part. Modern outside taps can also include backflow protection arrangements that should not be defeated by improvised fittings. Ask the plumber whether the existing wall plate and pipe support are sound. A secure replacement should turn smoothly without twisting the supply pipe behind it.
Useful information for a quotation
Tell the plumber whether the leak is from the spout, handle, wall connection or hidden pipework. Mention any recent freezing weather and whether an internal isolation valve is available. A photograph from outside plus one showing the pipe connection indoors can reveal how accessible the job is. If the tap is on an extension, garage or outbuilding, say how the supply reaches it. That can affect whether the repair is a straightforward tap change or involves replacing a damaged section of pipe as well.
Think about winter protection at the same time
An outside tap repair is a good opportunity to look at how exposed the supply is. Pipework passing through an unheated garage, external wall or draughty cupboard can be vulnerable in severe cold. A local isolation valve inside the house is useful because the tap can be shut off and drained when necessary. Insulation may help exposed sections, but it should not hide joints that need to remain accessible. If the previous failure followed freezing weather, ask whether the replacement arrangement leaves less water trapped in the exposed section. A small leak is easy to fix; a split pipe inside a wall is far more disruptive. The aim is not to make the outside tap complicated, but to leave it secure, easy to isolate and less likely to fail for the same reason next winter.
This page is general guidance about why is my outside tap leaking; the cause in an individual property depends on the actual plumbing arrangement.