Your options if your septic system needs expanding
Making changes to your home is exciting. But if you have an off-mains drainage system, those changes may have implications that are worth understanding early.
This guide explains how changes to your property can affect your septic system, what the outcome might be, and what to do next.
Whatever your situation, we can help you make sure your system is ready for what's ahead.
Why you might need to expand your off-mains system
Your septic system may need expanding if:
- you are planning a home extension or loft conversion that will add bedrooms
- you are converting an outbuilding, barn, or garage into living space
- you are creating a granny annexe or welcoming permanent new occupants
- your household has grown and your system has been struggling to keep up
- a Building Control Officer has already reviewed your plans and flagged your drainage system
If any of these apply to you, you're in the right place.
How property or household changes impact your septic system
Your off-mains system was designed for your property at a specific point in time.
The size of your system — the tank, the drainage field, and everything in between — was calculated based on the number of bedrooms, following the British Water Flows and Loads guidelines that govern how off-mains systems are sized in the UK.
When your property changes, the demand on your system changes too. A system that was once perfectly sized may struggle to cope with a conversion that adds another bedroom. A tank that handled three occupants may not be right for five.
That isn't a flaw in your system. It simply has a finite capacity, and that capacity was set at the time of installation.
Sort your off-mains system early in your planning
There's a lot to think about before your property or household changes, and it's easy to overlook your septic system. But it deserves attention early in the process — not at the end.
Contact a drainage specialist before your plans are finalised or submitted.
From there, the process works like this:
- The specialist visits your site and assesses whether your existing system can handle the increased demand.
- They produce a report confirming whether your system is adequate or whether an upgrade is needed.
- You include the specialist's findings in your Building Regulations application.
- The Building Control Officer reviews the drainage findings as part of their approval of your application.
Leaving these steps until late in the process is a common mistake. It can cause lengthy delays, unexpected costs, and complications that may be difficult to resolve mid-project.
The most common ways your system might change
When you change your property or household, your off-mains system may need to change too. How much depends on your site, your existing system, and the nature of your plans.
In most cases, there are three possible outcomes:
- Your existing system is adequate
If your existing system has enough capacity to handle the change, no upgrade is needed. Get confirmation of that fact and your plans are good to go. - Your existing system needs upgrading
If your system cannot handle the increased demand, it will need to be upgraded. That could mean a larger tank, an additional treatment stage, an extended drainage field, or a combination of these. The exact upgrade depends on the characteristics of your site. - You need a separate system
If you are converting an outbuilding or creating a new dwelling far from your house, it may not be practical to connect to your existing system. In that case, a separate system is likely the right solution.
To know which outcome applies to your situation, a drainage specialist needs to visit your site. That's exactly what our free site assessment is for.
Get your drainage sorted before your project begins
Our free site assessment tells you whether your system needs an upgrade. If it does, we'll walk you through your options so there are no mid-project surprises.
