Drainage and septic tanks

What is a drainage field, and can a home survey see whether it works?

In the English general binding rules, a discharge to ground is through an infiltration system, often called a drainage field, after a septic tank or a treatment plant. The pipes are buried. A surveyor walking the garden cannot see whether they are complete, waterlogged or missing. A soggy lawn is a clue, not a test.

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?

The rules require that infiltration system for a discharge to ground. They do not require the surveyor to excavate it. Smells, reeds, or a wet patch in dry weather are reasons to ask questions. They are not a percolation test. Approved Document H is about how drainage should be constructed for building work. It is not a retrospective certificate for a field laid in an unknown year.

Replacing a failed field is groundworks, sometimes with building control and with space the garden may not have. Ask a drainage contractor what they would need to investigate, and ask the seller for any plan. Costs vary with access, soil and length. Do not let a quote for a new tank ignore a field that may be the part that has failed.

What should you ask your surveyor?

Did you see any sign of a drainage field, or was the outlet unknown?
Was the ground wet, and had it been raining?
What investigation would show whether an infiltration system is there and working?

What are the typical next steps?

Ask the seller for a plan or an invoice that names a drainage field.
Do not dig a trial hole on a house you do not own.
If the purchase depends on the outlet, include the field in any drainage scope, not only the tank cover.

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 a soakaway crate the same as a drainage field?

Not by name. The rules say an infiltration system. Ask the contractor what was installed and whether it meets the current binding rules. The survey has not made that comparison.

Can you connect the tank to the mains instead?

Sometimes, if a sewer is available and the water company agrees. The survey does not know the sewer. Ask the solicitor what the drainage search shows.