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Do you need a bigger septic tank or sewage treatment plant for your home extension?

By: Becky Belton on Dec 14, 2023

When planning a home extension, one critical aspect that often gets overlooked is its potential impact on your septic tank or small sewage treatment plant. Here’s our guide to meeting planning requirements while choosing the most sustainable and efficient solution for your growing home.

Will your existing septic system be up to the job?

The first step in determining whether you need a bigger system is to assess your current setup against your future needs. 

An extension to your home, especially one that includes additional bathrooms, kitchens, or bedrooms, will invariably increase the amount of wastewater your household produces. 

What happens when you overload a system?

Overloading a septic system or sewage treatment plant (STP) can lead to a range of problems, including slow drainage, sewage backups and internal system failures. It could also lead to environmental damage as contaminated water leaks into the ground before it is processed by an overloaded tank.

To accommodate increased wastewater production, you’ll need to increase the capacity of your system.

But how can you calculate what capacity tank or plant you will require?

How can you size a new septic tank or sewage treatment plant?

British Water’s Best Practice Guide to Flows and Loads provides a table of loadings which allows the total daily sewage load from properties to be calculated and it is recommended that all designers use this table when sizing and designing non-mains sewage treatment system. 

The flows and loads values given represent current best knowledge within the UK but may change in line with per capita water use. 

The suggested sizing for sewage treatment plant generally depends on the number of bedrooms in the property (as a proxy for occupancy) from which the recommended size can be calculated. 

Septic tank size is usually based on the predicted daily flow rate.

Below is a simplified table to guide you through the sizing process for either option.

Domestic Sewage Treatment Plant and Septic Tank Sizing


Key Considerations for Septic Tank and STP sizing:

  • Population Estimate: Based on the number of bedrooms, assuming an average of 2 persons per bedroom plus one additional person for the household.

  • STP Size: Sewage treatment plants should be sized at least 1.5 to 2 times the predicted total daily flow to handle peak flows and ensure efficient treatment.

  • Septic Tank Size: Generally, septic tanks are sized larger than a sewage treatment plant. A common rule is to have a tank capacity of at least 2.5 to 3 times the daily flow rate. This allows for adequate settlement and digestion of solids.

  • Consider unusual water uses: Check for unusual water uses such as spa baths, multiple bathrooms or utility rooms, as these can increase the biological load on the system.

  • Round up the calculated Size: The calculated size provides the minimum requirement, so it's advisable to round up to the nearest available septic tank or sewage treatment plant size to ensure adequate capacity.

Regulatory considerations

In England, any home installing a new system discharging to ground or surface water must comply with the ‘General binding rules for septic tanks and small sewage treatment plants’. These have been augmented with additional rules that came into force in October 2023; for more information about these: read our latest blog post

In addition, you’ll need to observe the usual building regulations and planning permission requirements. Many regions have specific rules regarding the size and type of septic systems or sewage treatment plants. Before starting your extension project, it's crucial to consult with your local authority to understand these regulations and ensure your upgraded system complies with them.


If your discharge causes pollution you may be committing an offence. The Environment Agency will give you advice to help you fix the problem. but they may take enforcement action against you if the issue is not rectified.

Where to site your new system

To comply with all these different regulations and avoid sanction, you’ll need to carefully consider the location of your system and drainage field in relation to the proposed extension.  Please check with your local authority for regulation relating to Scotland, Northern Ireland and Wales.

In England the following rules apply:

  • The treatment system should be at least 7 meters away from any habitable parts of your property.
  • The treatment system should also be located within 30 metres of an access point; this ensures the tank can be easily accessed for emptying, service and maintenance.

Drainage field requirements 

The siting of your drainage field also needs to be considered:

  • Drainage field metres away from any building
  • At least 50 meters from a water supply
  • At least 10m from any watercourse or permeable drain.

The new rules discussed earlier are applicable here. Your system cannot now share a discharge point with a neighbour or be sited within 50 meters of “any other exempt groundwater activity or water discharge activity.”

What if an existing septic tank does not discharge to a drainage field?

If you have a septic tank that does not currently discharge to a drainage field the Guidance on the General Binding Rules (which applies to England) outlines your options:

  1. Connect to mains sewer
  2. Install a drainage field (also known as an infiltration system) so the septic tank can discharge to ground instead
  3. Replace your septic tank with a small sewage treatment plant

But anyone about to replace a system to increase capacity should consider whether a sewage treatment plant could be a better option.

Choosing a septic tank vs a sewage treatment plant

Septic tanks offer basic treatment by separating solids and liquids, allowing natural decomposition. Naturally occurring bacteria helps the system break down the solids and sludge, with the remaining liquids flowing out of the tank into a drainage field. Septic tanks only provide partial treatment. They can be lower in cost. They may have less frequent maintenance needs but require more frequent emptying.

The modern package sewage treatment plant (STPs) are designed with an aerated process for todays’ increased volume of water usage in everyday life. They offer significant biological breakdown of the organic and chemical contaminants by using air to circulate and treat the contaminants to a much higher standard than a septic tank. Package sewage treatment systems “clean” the wastewater significantly, which permits effluent discharge into less porous soils, effluent can then be discharged into a water course or a drainage field. 

Some sewage treatment options have mechanical and electrical parts within the tank which are prone to breakdown and therefore increase maintenance costs.


If you’re planning an extension that will increase the stress on your existing septic tank or sewage treatment plant, you should look carefully at capacity and planning advice to select the right solution for you. 

But there are many good reasons to upgrade to a sewage treatment plant if you currently have a septic tank.  

It should be noted that correct installation, consistent influent conditions and regular plant maintenance as per the manufacturer’s instructions are essential to ensure your sewage treatment plant warranty remains valid. For this reason we recommend working with a specialist installation and maintenance company.

Now might be the right time to take the leap and optimise your home for efficiency and environmental impact. It’ll make your effluent disposal easier to manage and could add to the value of your house in the long term.

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Topics: Wastewater Treatment

Becky Belton

Written by Becky Belton

Becky is a highly experienced Senior Account Manager who over the past 20 years has built a large customer portfolio and trusted relationships with our independent network of expert drainage engineers in the UK and Europe. She works closely with clients to specify the Diamond and HiPAF sewage treatment plants for the off mains drainage market. In addition to nurturing existing customer relationships, Becky oversees new business opportunities for the market whilst delivering exceptional customer service.

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