What Is a Build Over Agreement? A Complete Guide

If you're planning a home extension, conservatory, or garage, there's one thing that catches a lot of people off guard: public sewers running underneath their property. Build over one without permission, and you could end up with a serious headache down the line — including having to demolish what you've built.

That's where a build over agreement comes in. In this guide, we'll explain what it is, when you need one, and how to get the process right from the start.

What Is a Build Over Agreement?

A build over agreement is a formal approval from your water company that allows you to build a structure over or close to a public sewer. It's there to make sure the water company can still access, maintain, and repair the sewer in the future — and that your building work won't damage it.

Without this agreement, you're building at your own risk. If the sewer needs repairing in 10 years' time and your extension is sitting on top of it, the water company has the legal right to require you to remove the structure at your expense.

When Do You Need One?

You'll typically need a build over agreement if you're planning to build within 3 metres of a public sewer. Common scenarios include:

• House extensions (single or double storey)
• Conservatories with foundations
• Garages and outbuildings
• Porches and entrance canopies with footings
• Garden walls with deep foundations

You don't need a build over agreement for private drains that only serve your property. This only applies to adopted (public) sewers. If you're not sure whether the drain under your property is public or private, check your water company's sewer map or give them a ring.

How to Apply

1. Check the Sewer Map

Find out exactly where the public sewer runs. Your water company will have a sewer map — most are available online.

2. Get a CCTV Drain Survey

Most water companies require a pre-build CCTV survey as part of the application. This records the condition of the sewer before any building work starts. You'll also need a post-build CCTV survey once the work is complete.

We carry out CCTV drain surveys for build over agreements regularly. If you need one arranging, give us a call on 0333 577 4242.

3. Submit Your Application

You'll need to send the water company your building plans, foundation design details, the pre-build CCTV survey report, and an application form.

4. Wait for Approval

Processing times vary. Some water companies turn applications around in a couple of weeks; others can take 6–8 weeks. It's worth factoring this into your project timeline.

What Happens If You Don't Get One?

• Legal action from the water company — They can require you to remove the structure at your own cost.
• Problems selling your home — Conveyancing solicitors check for build over agreements. If one is missing, it can delay or block a sale.
• Insurance issues — Some insurers won't cover damage related to structures built over sewers without proper agreements.
• Building control complications — Your local authority building inspector may flag the issue.

If you've already built over a sewer without an agreement, some water companies offer a retrospective build over agreement. For more on the costs involved, see our guide on CCTV drain survey costs.

What If the Drain Needs Diverting?

Sometimes the water company will decide the sewer needs to be diverted rather than built over. This usually happens when the sewer is in poor condition, the foundations would be too close, or the sewer is particularly large. Diverting a sewer costs several thousand pounds and the water company will carry out the work at your expense.

Typical Costs and Timeline

• Pre-build CCTV survey: £150–£400
• Water company application fee: £0–£350 (varies by company)
• Post-build CCTV survey: £150–£400
• Sewer diversion (if required): £3,000–£15,000+

Allow 8–12 weeks from your initial CCTV survey to getting the green light.

Need a CCTV Survey for a Build Over Agreement?

We carry out pre-build and post-build CCTV drain surveys across the UK, specifically tailored for build over agreement applications. Call us on 0333 577 4242 or visit our CCTV drain surveys page to find out more.