Plumbing and maintenance
Water dripping from an outside overflow pipe
An overflow discharging outside means a fill valve somewhere is not shutting off. It is a warning sign rather than the fault itself.
How the system behaves
Point 1
Trace which appliance the pipe serves, WC cistern, storage tank or another fill point, before assuming which valve has failed.
Point 2
A steady drip usually means a worn valve seal; a heavy flow usually means a float or valve has failed outright.
Common questions
Is it urgent?
It should be dealt with promptly, both to stop waste and because a failed valve can worsen quickly.
Can I stop it temporarily?
Isolating the supply to that fill point stops the discharge until it is repaired.
Not sure which option suits your property?
Most of these decisions come down to what is behind the wall and under the floor. A site visit settles it quickly.