Moving a drain for an extension is more common than you might think, yet it is important to understand the regulations surrounding the job if you want to ensure not only that you won't be breaking any rules by carrying out the work, but also that the structural integrity of your new structure is not negatively affected.
While building an extension won't always affect your drains, there are instances where building out from the existing house will mean the new structure sits over a drain or manhole cover. In this case, it will be necessary to notify Building Control or your local water authority, depending on the type of drain you are dealing with.
Are you allowed to move a drain for an extension or renovation?
If you have found that your extension ideas will require a drain to be moved, the first thing you need to look into is whether or not you will need to apply for permission to do so. In many cases, rather than moving a drain, it will be a case of building over them instead.
The drains located beneath your property will be either private or public. Drains that pass through your property at the same time as serving another property nearby are classed as public, while drains that serve only your property are classed as private.
If your drain is private you can go ahead and build an extension over it – although Building Control will want to come out and approve the works as part of the extension.
'If it is private, then your contractor can relocate it without a submission by following standard technical guidance,' confirms Zak Gilby, managing director and co-founder of DeVis Architecture.
Zak Gilby is managing director and co-founder of DeVis Architecture. By trade he is an architectural technologist with 5+ years in the industry. He works on small-medium scale, residential projects across London and the South of England, and has expertise on all things home extensions, new builds, interior design and sustainability.
How do you get permission to move or build over a public drain?
If there is a drain getting in the way of your extension plans, you may need to seek permission in order to move it
In terms of moving or building over public drains things are definitely more complicated than with private – this will add more to your extension costs too.
Public drains are the responsibility of the local water authority and every public drain will have an 'easement' – a protected area on either side. For pipes measuring more than 300mm in diameter this will commonly be 5m while those that are smaller than this will usually have an easement of 3m. To find out whether or not an application is required, speak to your local water authority.
If you find that the pipes are either 100mm or 150mm in diameter (which they commonly will be) you can submit an Application to Build Over or Close to a Public Sewer. This kind of application will include a CCTV survey both before and after the work is carried out.
What is a Build Over Agreement?
In short, a Build Over Agreement is a contract between a homeowner and the local water authority that gives the homeowner permission to build over a public drain or sewer in order to extend their property. It is often a requirement for both rear extensions as well as side extensions.
Providing all criteria are met, such as the minimum distances mentioned above, it should not be too difficult to secure a Build Over Agreement, although there are instances where things can get a little more complex.
It is important to note that Build Over Agreements won't usually be approved for detached buildings, such as garages or annexes. For this, you would need to apply for a diversion.
How much does a Build Over Agreement cost?
The cost of a Build Over Agreement depends on the size of the sewer in question.
According to Thames Water, prices start at £130 for a sewer with an internal diameter of 150mm or less, rising to £520 for a sewer with an internal diameter more than 375mm – plus there will be additional legal costs for this size which are likely to start at £1,000.
Is it possible to move a public drain?
In some cases, it will be necessary to divert a drain to build an extension – so be prepared for even seemingly simple kitchen extension ideas to become more complex if you think this might be the case for your own project.
Adam Boucher, network operations lead from Severn Trent has some useful advice on what to do if you find your extension will require drains to be moved.
'If you have identified a public sewer on your land which is owned by Severn Trent and your new development affects it by having properties proposed over them or within 2.5m (for sewers less than 300mm in diameter) or 5m (for 300mm diameter sewers and above) you will need to apply for it to be diverted,' explains Adam Boucher.
Applying to divert public drains is a quite an involved process, involving detailed design drawings, CCTV drain surveys and even Habitat Surveys.
Adam Boucher was the Operational Lead for Severn Trent’s Mansfield Sustainable Flooding Resilience Project, the largest SuDS retrofit project of its kind in the UK. Adam has over 30 years’ experience in wastewater management, including capital delivery and operations, and now leads the Green Solutions team on the WINEP programme for Severn Trent for AMP8.
How much does it cost to move a drain?
The cost of moving drains varies hugely, depending on their size, whether they are public or private and the distance they are being moved. If you are hoping to add value to your home with an extension, bear these costs in mind.
You will need to factor in materials, labour, design drawings and permission from the water authority if the drain in question is public.
According to Checkatrade, you should expect average costs of between £4,500 - £6,000 to move a drain for an extension. They estimate that to relocate a manhole cover, you should budget around £1,000.
FAQs
How much does it cost to apply to move a drain for an extension?
Along with the materials and labour required to move a drain, don't forget to factor in the cost of an application.
According to Checkatrade, the cost of getting permission from the local water authority to move the drain costs an average of £750.
Extensions come in all shapes and sizes. If you are struggling to find space in your existing home, take a look at our round up of extension ideas for small houses to see whether there is an option you haven't yet considered.
