Why does my septic tank smell, and what should I do?

Smell is a symptom, not a diagnosis. Where you notice it tells you most of what you need. Work from the cheapest cause to the dearest.

First: where is the smell?

Indoors: the cheap fixes

Dry traps. Every sink, bath, shower and floor drain has a U-bend that holds water. That water stops drain air entering the room. In a spare bathroom or utility room it can evaporate. Run the tap for a minute. If the smell fades, that was it.

Vent pipes. Your drains need air. Approved Document H expects ventilating pipes open to outside air to finish at least 900 mm above any opening into the building within 3 m. A pipe that’s blocked (nests, leaves) or cut too short can push smells back indoors or near windows.

WC seals. A loose toilet pan or perished seal lets smells out at floor level.

At the tank

A strong smell at the tank usually means it’s due for emptying, or the lid doesn’t seal. The general binding rules require desludging at least once a year, or as the manufacturer says. If you can’t remember the last visit, book one. See how often to empty.

Over the garden: the serious one

A sewage smell over the drainage field, especially with soggy ground, slow drains or gurgling, points to a field that isn’t coping. Checkatrade lists those exact signs of failure. Don’t ignore it. Read our drainage field failure guide.

What the law says

In England, gov.uk says you must have your system repaired or replaced if it’s not in good working order. It names sewage smells as one example, alongside leaks, cracks, blocked pipes, pools of water around the drainage point and failed pumps or motors.

Anyone who maintains your system must be competent. Gov.uk suggests engineers on British Water’s accredited list.

If sewage is reaching a ditch, stream or pond, it may be pollution. In England you can report incidents to the Environment Agency.

Things not to do

• Don’t climb into a tank or chamber. Sewage systems can hold hazardous gases.
• Don’t pour bleach or strong chemicals down in bulk to “kill the smell”.
• Don’t cover the field or tank with patios, sheds or parked cars.
• Don’t assume an additive will fix it. We found no official guidance recommending one.

FAQ: septic tank smells

Is it normal for a septic tank to smell?

A faint smell right at the lid when it is opened is common. A persistent sewage smell in the house or across the garden is not normal. Gov.uk lists sewage smells as one sign a system is not in good working order and must be repaired.

Why does my house smell of sewage when I have a septic tank?

Start with the simple causes inside: a dried-out trap in a sink, shower or floor drain that is rarely used, or a blocked or damaged vent pipe. If those are fine, the tank may be full or the drainage field failing.

Will emptying the septic tank stop the smell?

If the tank is overdue, yes, it often helps. If the smell is over the drainage field and the ground is wet, emptying is only a short-term fix and the field may need investigating.

Do septic tank additives stop smells?

We have not found official guidance recommending additives. The regulators' advice is regular emptying and maintenance in line with the manufacturer's instructions.

Who do I call about a septic tank smell?

A septic tank maintenance company or emptying firm. Gov.uk says anyone maintaining your system must be competent, such as engineers on British Water's accredited list. If sewage is polluting a watercourse, report it to the Environment Agency.

Sources

All links checked October 2026.

1. GOV.UK: General binding rules, small sewage discharge to the ground
2. GOV.UK: Approved Document H, drainage and waste disposal
3. Checkatrade: Drainage field cost (signs of failure)
4. Call2Drain: Cesspit emptying cost UK 2026 (warning signs)
5. GOV.UK: Report an environmental incident