A build-over agreement may be required if you’re planning to build something new – whether it’s a house, extension or garage – and there is a public sewer where you want to position your new building.

With almost 450,000km of sewer pipes in the UK, running under streets and gardens, as well as underneath fields and other land, chances are there will be a public sewer close to your site. Building over or close to a sewer pipe could mean the pipe itself gets damaged, or the property itself, while it’s also important that the water company is able to access and maintain their sewers over time.

What happens if I want to build over, or close to a public sewer?

If you want to build within 3m of a public sewer, or within 1m of a public lateral drain, you will need to get consent for your works from the water company that collects your wastewater, according to Part H4 of the Building Regulations.

Your architect, builder, water company or sewer specialist can help you locate any sewer pipes running underneath your property, so you know what impact they will have on your plans. It’s best to tackle this question during the ‘feasibility study’ part of your project, so that you can be sure that your plans won’t need to be altered at a later stage. This is especially important if your works require planning consent, as alterations to accommodate sewer pipes could mean re-submitting plans, which will cost you more time and money.

Why is it so important to get build-over consent?

Consent from your water authority is a seal of approval that your works will not affect the structural integrity of the sewer pipe, and vice versa. It also demonstrates they are happy that they will have suitable access for maintaining their sewer after the build has completed. If the necessary approval is not in place, the water authority has the right to remove any structures blocking their access, and they will not be liable for any damage they may cause in doing so.

Aside from the possible impact of your water authority exercising their right to remove anything that gets in the way of their sewer maintenance works, consent to build close to or over a public sewer or public lateral drain is an important document to have when you come to sell your property. Potential buyers will want to see that everything is in order, and conveyancing will certainly be held up without the right approval in place.

Do I need a formal build-over agreement?

You may be able to obtain build-over consent with self-certification, or you may need to apply for a formal build-over agreement. This will depend on the specifics of your plot and your plans.

Although water authority requirements differ, you can potentially apply for self-certified build-over consent if:

• Your works are to add a structure to an existing building – an extension, conservatory, garage or annex
• There are no obvious problems with the sewer (blockages, smells, flooding) and it is in good condition
• The sewer being built close to or over has a diameter of less than 160mm
• Access points (manholes, rodding chambers, inspection points) are outside of, and at least 500mm from, the building
• The sewer will not change direction, gradient, diameter or material underneath the new building
• The sewer depth is no greater than 2m
• The sewer is made of clay or plastic
• You can achieve specified minimum clearances (typically 500mm from parallel foundations, 150mm above the bottom of the foundations, 300mm below underside of floor slab)
• No loadings are to be placed upon the pipe (a lintel is often used)
• Your new building will have traditional strip or trench fill foundations
• There are no easements or covenants relating to the sewer
• If building across the entire width of the plot, the sewer must still be accessible from a neighbour’s garden

What if I can’t self-certify for build over consent?

If the criteria for self-certification aren’t met, and build-over agreement is deemed to be required, then a variety of restrictions are likely to apply. These will vary according to the water authority in question, but might typically include:

• All works must comply with the latest version of ‘Sewerage Sector Guidance’ and be done in like-for-like materials
• No additional loads must be transmitted to the sewer, and foundations must be taken below the invert level (the level of the internal pipe base)
• The public sewer’s invert levels must be checked and verified prior to works beginning, and this is the applicant’s responsibility
• Sewers that are up to 1.1m deep must be at least 150mm away from the foundations; over 1.1m deep, at least 500mm away; if over 2m then a minimum of 1m away
• Driven piles are not permitted within 15m of a public sewer
• Piled foundations must be at least 1.5m or twice the pile diameter (whichever is greater) from the outside of the pile to the outside of the sewer pipe

How do I apply for a build-over agreement?

You will usually need to submit drawings of your proposed works, showing the existing building and drainage layout, and also a site location plan. You will likely need to submit a cross-section drawing if you’re building over the sewer, or within 1.5m, as you will have to indicate how the foundations will be constructed.

As part of your build-over agreement application your water authority will survey the public sewer to ascertain its condition before the works have been started, and they will do the same after completion, too. Whether this is carried out by CCTV or by entering the sewer will likely depend on its diameter. Thames Water, for example, will use a camera for pipes with a diameter between 160mm and 375mm, but may perform their checks by entering the sewer if its diameter is more than 375mm.

How much does a build-over agreement cost?

There will of course be a fee for your build-over agreement application – though how much will depend on the water company in question and sometimes on the size of the sewer.

*There is also a legal fee for Class 3 sewer build-over agreements, usually around £550

Different water companies also have slightly different processes for applying for a build-over agreement. We work regularly with Thames Water, Southern Water and Anglian Water, and are well-used to making applications on behalf of our clients.

Do I have to apply for the build-over agreement myself?

In short, no. We can help you to navigate the complexities of build-over agreements, so you don’t have to deal with the nitty gritty. Our expertise means we get it right first time, which ultimately means lower costs and less time spent sorting out compliance issues.

To find out more, call us on 01268 950050 or email [email protected].