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Before ordering an extension ensure your property does not need a build over agreement


If you are thinking about extending your home, or building a conservatory, you need to be mindful that any extension might/might not impact on underground sewer pipes that lie within your property’s boundary.

So before ordering an extension it is important to ensure that your property does not need a build over agreement putting in place first. Fortunately, to help you understand your obligations to protect any sewer pipes on your property, Water UK has produced a helpful customer guide in conjunction with each of the ten water companies in England and Wales called “Building over or close to a public sewer or lateral drain” 

In simple terms, building over or near water mains i.e. the ones that supply your clean drinking water, is generally strictly taboo, so unfortunately you will need to contact the water company that supplies your drinking water if your plans might affect water mains. The two main things to look out for are a) lateral drains, which carry wastewater from a single dwelling and meet the public sewer network outside the boundary of that house, and b) sewers, which carry wastewater from two or more houses and which are within your property’s boundary.

The main advice given by Water UK in their “Guide for homeowners” is that “building over or near a sewer pipe could lead to the pipe itself, or your home, being damaged. It is also important to avoid undertaking any building works which make it harder for water companies to access and maintain their sewers in the long term. This would result in increased costs to customers and may also cause significant disruption at your home. So, if there are sewer pipes on your land, you may need to consider the position, size and design of your building work before you start. Your work may need Planning Permission or Building Regulations approval from the local authority. Also, some work falls under homeowners’ permitted development rights, or doesn’t need Building Regulations approval (things like conservatories, car ports and porches sometimes fall into this category). However, none of these permissions give you automatic consent to build over, or close to, a water company’s sewers.”

It is not possible to give you an exact rule of thumb as the procedures that followed by each water company can differ, although in principle (though don’t take it as gospel) they have all agreed that if you can meet certain conditions, your extension project will be approved. There are sadly exceptions to this, such as where a Water Company has site-specific knowledge about problems with the sewer network which are not more widely known about. Just so you know, if water companies want to make use of this exception, they must let you know within 14 days of your application being received.

Even where your project does not fulfil these conditions, some water companies may offer a service where you can discuss problem areas with them before submitting a revised application which successfully meets all the conditions. Where an extension or conservatory project does not meet the conditions, it will need to be dealt with on a case by case basis. This will normally require (at the property owner’s expense) a formal build-over agreement to be entered into between the homeowner and your water company. There are strict rules as to what you/your builder/conservatory installer can do in terms of building over both the existing sewer/drain cover, and existing sewer/drain pipes, so make sure you speak to experienced experts before ordering anything. 

In respect of Build Over Agreements, not a lot has changed during 2019, other than (at time of going “to press”) Severn Trent are the only Local Water Authority that will allow inspection chambers to be raised and sealed. All other Authorities insist that the chamber is re-sited. Sadly, in dealing with all Water Authorities, the fees, the application forms and the timescales are not consistent at all. The fees chargeable and timescales for these different services and further information can be found by contacting your water company. However, Thames Water have now introduced a build-over self-certification scheme for building near domestic sewers with a diameter of 160mm or less. Click here for details


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Credit is subject to status and affordability. Terms and conditions apply. Finance on windows & doors is available to a maximum loan value of £50,000 with zero deposit. Finance on conservatories is available to a maximum loan value of £30,000 with a 15% deposit. £1,000 minimum order value applies to all finance options. Authorised and regulated by the Financial Conduct Authority. Hazlemere Window Company Limited at Cressex Business Park, Wellington Road, High Wycombe, Buckinghamshire HP12 3PR (Financial Services Registered No. 796507) is a credit broker and is authorised and regulated by the Financial Conduct Authority. Credit provided by Hitachi Personal Finance, a trading style of Hitachi Capital (UK) PLC, authorised and regulated by the Financial Conduct Authority (Registered No. 704348). 12/2019