Drainage and septic tanks
Where a report pairs an anti-siphon trap with the basin, what stays open about gurgling the surveyor heard once?
What stays open is what one gurgle means. An anti-siphon trap under the basin is a trap meant to keep its water seal if pressure in the waste pipe fluctuates. The survey paired that trap with the basin and recorded a gurgle heard once. One noise does not show that the seal was lost, that it will happen every time, or that the stack ventilation is at fault. The survey has not repeated the test and has not inspected the stack. The gurgle stays a single observation.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
Gurgling at a basin usually means air is being pulled through the trap, or is bubbling back, as water runs away. An anti-siphon, or resealing, trap is fitted to resist that. Hearing it once tells you air moved on that occasion. It does not tell you the trap failed to reseal, and it does not tell you a vent further up the stack is missing. The surveyor heard the noise at the basin. They did not take the trap apart and did not open a stack vent.
Tell the solicitor the gurgle was heard once and was not investigated. If you need to know whether it repeats, a plumber can run the basin several times and look at the trap and the vent. The survey has not done that and has not specified a new trap. Do not replace the anti-siphon trap on a single noise. The seal may be fine, or the stack may be the real limit, and this visit did not separate those causes. Keep the trap type and the one gurgle apart.
What should you ask your surveyor?
- What was running when you heard the gurgle: this basin, or another fitting?
- Did the trap lose its seal, or did you only hear the noise?
- Was any vent or air-admittance valve seen on the same waste?
What are the typical next steps?
- Treat the gurgle as one noise, not as a failed trap.
- Ask a plumber to repeat the discharge before anyone changes the trap.
- Tell the solicitor the stack was not investigated.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Is an anti-siphon trap a sign the basin was already troublesome?
Not by itself. The trap is a type chosen to resist seal loss. Fitting one does not prove the basin has a history of trouble, and this survey only heard a single gurgle. It did not take the trap apart. The reason it was fitted is unknown. Do not read the hardware as a record of past complaints.
Will the gurgle happen every time the basin is used?
The survey cannot say. It was heard once and the discharge was not repeated. An anti-siphon trap can reseal after a noisy emptying, or the noise can return if the waste is pulling air. Neither outcome was tested. A plumber can run the basin again. Until then one gurgle stays one gurgle.