Are you planning on adding an extension over where a drain sits? Don't, in all the excitement of planning your new space, neglect to take into account the less glamourous aspects of the project – including how it will affect existing services, like your drains.
Building an extension can throw up all kinds of unexpected issues, be that a planning constraint you weren't previously aware of, unsuitable ground conditions or the need for a Party Wall Agreement. Being aware of these 'hidden' complications early on means no pesky delays or the need to unexpectedly spend more than you had budgeted for.
1. Are you dealing with a public or private drain?
First things first, what type of drain are you dealing with here – public or private? The answer will very much affect how you should proceed.
'When building an extension, be that a side extension or a rear design, the first step is to understand if it's a public or private drain,' picks up architectural technologist Zak Gilby, co-founder of DeVis Architecture. 'If the sewer connects to any neighbouring properties, it's classed as public and maintained by the water board. In this case, a Build Over Agreement would be needed to bridge over, relocate or divert the sewer/inspection chamber. 'If it is private, then your contractor can relocate it without a submission by following standard technical guidance.'
You should bear in mind that you will not be allowed to have a junction of a public drain beneath your new floor. This is because there is a risk of a blockage occurring and access would be needed to solve this. Instead, all drains from the extension should pass to an external space where they enter a manhole.
Zak Gilby is managing director and co-founder of DeVis Architecture. By trade he is an architectural technologist with 5+ years in the industry. DeVis Architecture work on small-medium scale, residential projects across London and the South of England, and have expertise on all things home extensions, new builds, interior design and sustainability.
2. Understand how close you can build to a drain
You need to know where the drains for your property run before designing your extension ideas – your architect should be well aware of this. Locating drains, however, is not always as simple as you might think. If you are lucky, a drainage plan may be included with your deeds, but this is not always the case.
If you are struggling, you might need a drain survey which will use CCTV to determine the positioning of drains. Once located, you need to bear in mind the minimum distances you are allowed to build your extension from the drains.
'As a rule of thumb, any new foundation or footing should be at least 500mm away from the centre of the inspection chamber/manhole,' says Zak Gilby. 'The pipe should be a minimum of 150mm from any foundations where running parallel if less than 1.1m deep, or 1.2m away if deeper.
'If either of these limits is breached, then the a Build Over Agreement is required, and a submission to your local water authority. Typically this would include existing and proposed drainage drawings, scaled detail drawings, and, occasionally, a CCTV drain survey from a professional.'
3. Be aware you might need a new manhole
In some cases, your new addition might mean that you need a new manhole to be installed – you should factor this into your overall extension costs, or at the very least into your contingency fund.
'In most cases the main shared sewer pipe stays in its existing position, with the new foundation bridging over it at either side to avoid loading pressure on the pipes (usually clay or PVC),' explains Zak Gilby. 'A new, offset manhole would then be installed outside the extension footprint for access and maintenance.
'Internal manholes are not permitted, so these are then blocked up and the pipe is replaced with a continuous section.'
4. Expect the unexpected
Even once you know where the drains are, there are other factors that can, in some cases, step in and further complicate things, meaning the rerouting of drains or the positioning of your new extension may need to be reconsidered.
'While navigating drains is a common scenario for most residential projects, certain factors can complicate things further, for example, large trees, which can affect diversion routes, or ground conditions such as clay soil or a high water table,' points our Zak Gilby. 'The unfortunate thing is that these don't always become clear until after works have begun. Working with a professional design team and contractor helps measure risk and costs at each stage.'
The key is to prepare for the unexpected – failure to do this is one of the biggest extension mistakes out there. Having a contingency fund for these kinds of issues is very wise – 10-15% of the overall costs is recommended, although with fluctuating costs, 15-20% could be a safer bet.
5. Understand who is responsible for damaged drains
However careful your groundworkers and build team might be, during extension work – namely at the stage where foundations are being excavated – drains can get damaged. Whether you are building a single storey extension or something larger, if this happens, you need to be aware who is responsible for their repair as well as the costs that will be involved.
'If there's any damage to the sewer during or after works, it is usually the responsibility of the person carrying out the works or the homeowner to rectify the damage,' reveals Zak Gilby. 'Whether this is due to a design fault or build error would likely be investigated, but good craftmanship and professional input goes a long way in reducing potential issues.'
In short, if the damaged drains are within the boundaries of your property, you will be responsible for their repair. If you are unsure, you can request a sewer map from your local water authority which should help determine what kind of drain you are dealing with.
6. Be prepared to make compromises
It is a really good idea to be prepared for the fact that you are likely to need to make some compromises during the course of your extension – and your drains could well be one of the main reasons for these adjustments.
'One issue which we previously encountered was an extension planned to run directly above the existing sewer line, which was public,' shares Zak Gilby. 'This would have exceeded the 150mm distance allowed, and instead we had to make a decision. We had three choices. We could reduce depth of the extension, apply for planning permission to extend out further, or apply to divert the sewer. The last option can be costly and slow-moving, as the work has to be undertaken by the water authority's own contractors.'
The message? Even if you find drains are in the way of your plans, there will be a solution. Just be ready to make changes to your design, your schedule and your budget.
FAQs
How close to a drain can you build an extension?
This depends on the drain in question. If it is a public sewer and you plan on building over or within 3m of it, you will require a 'Build Over Agreement'.
You should also remember that new foundations or footings for your extensions should be a minimum of 500mm away from the centre of any inspection chamber or manhole,'
Who is responsible for damaged drains?
If the drain in question is within the boundaries of your property, it is highly likely that you will be responsible for its repair. Damaged public drains are the responsibility of the water authority.
Check to see what your home insurance covers, depending on your policy, you might find the costs are covered.
Keen to add space to your home next year? Check out our guide to what size extension you can build without applying for planning permission – the process could be simpler than you imagined.
