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Buying a home with septic tank

(Last Updated: 05/08/2019)
7 min read
Buying a home with a septic tank brings in an added dimension where your pre-buying checks are critical because of the necessity to understand the complications of the workings of the equipment itself but also the potential for a mismanaged tank to pollute - illegally - and devastate a local ecosystem.

How many septic tanks are in the UK?

It's difficult to find an accurate answer to this question with many experts estimating that septic tanks are attached to perhaps 3% - 10% of housing in England & Wales. They have a long history and have developed over time as a solution to a dwelling being unable to be linked up to the main sewage/drainage system. They are more prevalent in many other countries because of the highly urbanised nature of the UK's housing, where most properties are built such that they are linked up to main sewerage pipes for efficient disposal.

This article examines septic tanks and gives concise information on:

Buying a house with a septic tank?

You are always advised to get a home buyers survey for your own peace of mind and a house with a septic tank, by nature of having one, is likely to be of non-standard construction and so requires a full Building Survey with an experienced RICS surveyor: our surveyor network covers England and Wales.

Steel frame building specialists - RICS regulated - same week availability


    What is a septic tank?

A septic tank is an underground chamber made of concrete, fibreglass, or plastic through which domestic wastewater (sewage) flows for basic treatment.

There might be one or more tanks of between 4000 and 7500 litres with one end connected to a an inlet wastewater pipe and the other to a septic drain field. 

We use the term basic because septic tank efficiency in relative terms is only moderate, although settling and anaerobic processes reduce solids and organics.

The term "septic" refers to the anaerobic bacterial environment that develops in the tank that decomposes or mineralizes the waste discharged into the tank.

Septic tanks can be coupled with other onsite wastewater treatment units such as biofilters or aerobic systems involving artificially forced aeration.

Generally these pipe connections are made with a T pipe allowing liquid to enter and exit without disturbing any crust on the surface with modern usually incorporating two chambers, each with their own manhole cover and separated by a dividing wall with openings located about midway between the floor and roof of the tank.

Two stage system

The treated liquid effluent, which is relatively clear having allowed solids to settle and be digested anaerobically and for scum to settle in the second chamber, is then commonly disposed in a septic drain field, which provides further treatment.


    How are septic tanks maintained?

Waste that is not decomposed by the anaerobic digestion must eventually be removed from the septic tank.

This is done by emptying the contents of the tank - the accumulated sludge (septage, also known as fecal sludge) is pumped out of the tank by a vacuum truck.

How often the septic tank must be emptied depends on the volume of the tank relative to the input of solids, the amount of indigestible solids, and the ambient temperature (because anaerobic digestion occurs more efficiently at higher temperatures), as well as usage, system characteristics and the requirements of the relevant authority.

Some health authorities require tanks to be emptied at prescribed intervals, while others leave it up to the decision of an inspector. Some systems require pumping every few years or sooner, while others may be able to go 10–20 years between pumpings. An older system with an undersize tank that is being used by a large family will require much more frequent pumping than a new system used by only a few people. Anaerobic decomposition is rapidly restarted when the tank is refilled.


    How can septic tanks malfunction?

The basic issue is that the rate of accumulation of sludge—also called septage or fecal sludge—is faster than the rate of decomposition. Therefore, the accumulated fecal sludge must be periodically removed.

To complicate matters further, various other issues can occur:

  • the soakaway drainage field - the most important part of the system - often requires replacement and the frequency depends often on the type of soil involved, sandy soils lasting perhaps four times as long as clay. There were no Building Regulations concerning soakaways before 2000, this is no longer the case.
  • the tank discharges into a ditch or stream - this is completely illegal and always has been and all such systems have to have been replaced by 1 July 2020
  • groundwater pollution can occur - this is why there are now rules that the water table is not allowed to get to within 2 metres of ground level
  • odour, gas emissions and even undecomposed sewage can be discharged directly into the drainage field if the tank isn't maintained properly - this can even be down to cooking oils, grease, cigarettes, water softeners attacking the useful microbes
  • excess nitrogen and phosphorus from the sewage can poison wild fish and other water life.


    How do you know if you are buying a home with a septic tank?

Generally speaking, the more rurally remote a property is, the more likely that a septic tank will be used at least in part to dispose of sewage. It should be something declared in the Seller's Property Information Form (TA6).

If you buy the Water and Drainage Search (click to find out more - all mortgage buyers have to do this), it will be apparent that the property is not joined up to the main sewage system, therefor some sort of septic tank must be involved.


    What is the regulatory position of septic tanks within England & Wales?

Building Regulations 2000 were the first Building Regulations which imposed clear construction etc. rules on septic tanks.

Since 2015, only certain property owners in England and Wales with septic tanks or small packaged sewage treatment systems need to register their systems, and either apply for a permit or qualify for an exemption with the Environment Agency.

Permits need to be granted to systems that discharge more than a certain volume of effluent in a given time or that discharge effluent directly into sensitive areas (e.g., some groundwater protection zones). In general, permits are not granted for new septic tanks that discharge directly into surface waters.

the Environment Agency has set up Groundwater Source Protection Zones to protect drinking water abstracted from boreholes. If you are in Zone 1, then you cannot have a new soakaway field there.

All new foul water soakaways must be in a new area of the garden away from any rainwater soakways, not under drives, parking areas or paths and at least 15 metres from any building and 2 metres from a boundary. As the average soakaway drainfield takes up at least 100M², you can see that you need a very big lawn to site a new one.

Finally, regarding soil types and in particular the composition ratios between sand and clay, your soil type for a soakaway cannot be either 'too sandy or too heavy': 60% of new sites apparently fail these tests.


    What information can you get which best informs you whether to proceed with a purchase or not?

An experienced solicitor will scrutinise property searches - particularly the drainage search - heavily if a septic tank is involved. Additionally they will raise enquiries if a septic tank is mentioned in form TA6.

Finally, when the report from an experienced RICS surveyor comes back, your solicitor will particularly examine the findings, in particular the legal information and any insurance issues

Buying a house with a septic tank?

You are always advised to get a home buyers survey for your own peace of mind and a house with a septic tank, by nature of having one, is likely to be of non-standard construction and so requires a full Building Survey with an experienced RICS surveyor: we cover England and Wales.

* Steel frame building specialists - RICS regulated - same week availability

Andrew Boast of Sam Conveyancing
Written by:
Andrew started his career in 2000 working within conveyancing solicitor firms and grew hands on knowledge of a wide variety of conveyancing challenges and solutions. After helping in excess of 50,000 clients in his career, he uses all this experience within his article writing for SAM, mainstream media and his self published book How to Buy a House Without Killing Anyone.
Caragh Bailey, Digital Marketing Manager
Reviewed by:

Caragh is an excellent writer in her own right as well as an accomplished copy editor for both fiction and non-fiction books, news articles and editorials. She has written extensively for SAM for a variety of conveyancing, survey and mortgage related articles.

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Water and Drainage Search explained by SAM Conveyancing

Water and Drainage Search

Building Regulations

Building Regulations

Property Information Form TA6 explained by SAM Conveyancing

Property Information Form TA6 explained

What are legal enquiries - Solicitor with a spy glass

What are legal enquiries


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