If you are planning an extension in Hampshire and there is a sewer anywhere near the proposed foundations, one of the first questions you need answered is whether a build over agreement is required.
This catches far more homeowners than you might expect. You can have planning permission in place, Building Control lined up, and a builder ready to start, only to discover that a public sewer runs directly through the build area or sits too close to the foundations. At that point, the project can slow down quickly unless the issue is dealt with properly.
At Dorrington Groundworks, we regularly help clients across Southampton, Winchester, Fareham, Eastleigh, and the wider Hampshire area identify drainage constraints before they become expensive problems. Here is what you need to know.
What Is a Build Over Agreement?
A build over agreement is formal permission from the water company to build over, or close to, a public sewer.
In most of Hampshire, that water company is Southern Water.
The purpose of the agreement is straightforward. Public sewers need to remain protected, structurally sound, and accessible for maintenance. If foundations are placed too close, or if the extension sits directly above the sewer without approval, there is a risk of damaging the pipe, restricting access, or creating expensive problems later on.
A build over agreement is not the same thing as planning permission. It is also separate from Building Regulations approval. You may need all three depending on the project.
When Do You Need a Build Over Agreement?
In practical terms, you should assume you need to investigate this issue whenever your extension foundations are within 3 metres of a public sewer or manhole.
Southern Water’s guidance says approval is required if you are building within that zone. They also explain that the measurement is taken from the outside of the sewer pipe or manhole to the outside face of the foundations.
That means even if the sewer is not directly under the proposed extension, it can still affect the design if it sits nearby.
Typical situations where this becomes an issue include:
- rear extensions where an existing sewer crosses the garden
- side extensions built close to shared drainage runs
- projects where an old manhole sits in the footprint of the new room
- properties on estates where several houses share a sewer connection
- extensions on older homes where drainage records are incomplete or unclear
If there is any doubt, check before excavation starts. Digging first and asking later is the wrong order.
How Do You Know If the Sewer Is Public?
This is where many homeowners get caught out.
People often assume that if a pipe is inside their boundary, it must be private. That is not always true. Some sewers serving more than one property are public, even if they run through your garden.
The safest route is to obtain the drainage records and compare them against the proposed extension footprint. Southern Water’s maps are an important starting point, but records are not always perfect, particularly on older sites. In some cases, the exact line of the sewer is only confirmed when the site is opened up.
That is why we always recommend checking drainage early, especially if:
- the extension is close to an existing inspection chamber
- the property is older
- neighbouring houses appear to share drainage
- the site has already had drainage alterations over the years
If necessary, a drainage survey or trial excavation can remove uncertainty before the project reaches site.
What Happens If the Sewer Runs Through the Extension Footprint?
There are usually three possible outcomes.
1. Build Over Approval Is Possible
If the sewer meets the water company’s criteria, the project may be approved subject to conditions.
Southern Water states that smaller sewers can sometimes remain in place, particularly where they are relatively shallow and modest in size. In some cases, small diversions can also be dealt with as part of the same process.
2. The Foundations Need to Change
Where the extension can proceed but the sewer must be protected, the foundation design may need adjustment. Southern Water’s guidance says foundations in the affected zone need to extend below the sewer invert by a specified margin. In plain English, this usually means deeper excavation and more careful structural detailing.
That can affect programme, cost, and concrete quantities.
3. The Sewer Must Be Diverted
If the sewer is too large, too deep, badly positioned, or the proposed structure would create access and maintenance issues, a diversion may be required instead.
This is usually more expensive and takes longer, but sometimes it is the only viable route.
Does Building Control Still Matter?
Yes. Absolutely.
A build over agreement does not replace Building Control. Foundations and drainage still need to comply with Building Regulations, and Building Control still needs to inspect the relevant stages.
In practice, the two processes overlap. The water company is focused on protecting the public sewer. Building Control is focused on structural compliance and drainage standards for the building itself.
If your contractor is experienced, these issues can be coordinated properly from the outset. If they are not, you can end up with conflicting drawings, delayed inspections, or expensive redesigns.
What Information Does Southern Water Usually Need?
The exact requirements vary by project, but in general you should expect to provide:
- a site plan showing the proposed extension
- the position of the sewer and any manholes
- foundation details
- drainage layout drawings
- dimensions showing distances between the sewer and the proposed works
- in some cases, structural details or further technical information
This is another reason to deal with the issue early. If the extension drawings are vague or the drainage route is uncertain, approval becomes slower and harder.
What About Manholes?
Manholes often cause as many problems as the sewer itself.
If an existing manhole sits inside the footprint of the extension, it usually cannot just be buried and ignored. It may need to be relocated, rebuilt externally, or incorporated in a way that still preserves access, depending on the circumstances and the water company’s requirements.
This is a common issue on rear extensions and kitchen enlargements. It is also one of the easiest things to miss if the site has been paved over or altered in the past.
How This Affects Groundworks Costs
A public sewer near the extension can increase groundworks costs in several ways:
- additional surveys or drainage investigations
- revised foundation depths
- slower excavation around live drainage
- sewer protection measures
- manhole relocation
- potential sewer diversion works
- added time before work can begin on site
We cover broader pricing in our guide to groundworks costs in Hampshire, but this is one of the clearest examples of why two extensions of a similar size can have very different foundation and drainage costs.
Common Mistakes That Delay Extensions
These are the mistakes we see most often:
Assuming planning permission covers everything
It does not. Planning permission, Building Regulations, and build over approval are separate issues.
Starting excavation before drainage checks are complete
This is how projects get paused halfway through site set-up.
Assuming a sewer inside the boundary is private
That assumption is often wrong.
Forgetting about manholes
A single manhole in the wrong place can force a redesign of the extension footprint or drainage layout.
Treating drainage as an afterthought
Drainage should be checked before the groundworks start, not once the digger is already on site.
What Should You Do Before Work Starts?
If you are planning an extension in Hampshire, the sensible sequence is:
- Check whether there are any sewers or manholes near the proposed extension.
- Confirm whether the sewer is public.
- Compare the drainage position against the foundation layout.
- Apply for build over approval if required.
- Finalise the foundation and drainage design before excavation begins.
That process avoids unnecessary delays and gives the groundworks team a clear plan to work from.
We Can Help You Identify the Problem Early
At Dorrington Groundworks, we have nearly 20 years of experience dealing with foundations, drainage, and site preparation across Hampshire. We understand the practical issues that slow extension projects down, and public sewers are one of the most common.
If you are planning an extension in Southampton, Winchester, Fareham, Eastleigh, or anywhere nearby, we can assess the site, review the drainage constraints, and help you plan the groundworks properly from the start.
Call us on 01489 539197 or get in touch through our contact page to arrange a site visit.
Frequently Asked Questions
Do I need a build over agreement for a small extension?
If the extension is within 3 metres of a public sewer or manhole, you need to investigate it, regardless of whether the extension itself is small.
Can I build over a private drain?
Possibly, but the first step is confirming that it is actually private and not part of a public sewer serving more than one property.
Can a sewer be diverted instead of building over it?
Yes. In some cases diversion is the better option, especially where the sewer is badly positioned, too large, or access would be compromised.
Does a build over agreement replace Building Control?
No. You still need Building Regulations approval and inspections for foundations and drainage.
For related guidance, you can also read our article on whether groundworks need planning permission.
