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A house that has an extension built over a manhole sewer drain

Can you build over a manhole with a Build Over Agreement?

(Last Updated: 28/02/2024)
5 min read
Key Takeaways
  • You need a Build Over Agreement if you’re building within 3 metres of a public sewer or 1 metre of a public lateral drain.
  • If you're buying a property that doesn't have a build-over agreement, you can get indemnity insurance.
  • Your Water and Drainage Search tells you where the sewers and drains are.
  • The Water Authority has the right to enter your property, damage or demolish the structure over the sewer with no obligation to pay for repair.
  • If you don't have a Build Over Agreement, you can obtain indemnity insurance to protect the lender if the Water Authority devalue the property when they undertake works.
  • You can't use indemnity insurance if the water authority knows about the buildover.
  • Your building insurance may not cover repairs to the property.

Before October 2011, you could build over a manhole once you've obtained permission from the Waterboard and the local authority and have a Build Over Agreement. A Water Authority has a statutory right to enter your grounds to access a sewer. If you don't have a Build Over Agreement, they can damage or demolish the structure built over the sewer without obligation to repair or pay for any damage caused.

Since 2011, if a Build Over Agreement is allowed, you cannot build over a manhole and should extend the piping, cover and reconstruct a manhole outside the property. This was required due to the risk of flooding and odours when a manhole was inside a property. The Water Authority will refuse any application for an internal manhole cover.

How do I know if there’s a sewer or drain in my property boundary?

When you purchase a property, you'll obtain a Water and Drainage Search. If your property has been extended over on top of a public sewer, you'll be able to tell by:

  • Comparring the floor plan and see if the extension is within 3 meters of a public sewer; or
  • There is a manhole within the property.

Get a Water and Drainage Search

Find out where your sewer line runs to avoid building within 3 metres. Ordered immediately, and the water board will confirm delivery time. Includes:

  • Mains Water
  • Foul Water
  • Surface Water Sewer
  • Sewer Pipe Within Boundary
  • Water Pipe Within Boundary

Drainage and Water Search

£106 INC VAT

Order Search

Can you build a conservatory over a manhole?

If this is a new conservatory, you must:

  • obtain a Build Over Agreement with the Water Authority; and
  • move the manhole cover outside of the conservatory.

Since 2011, you can't build a conservatory over a manhole cover, leaving it inside the conservatory. If the manhole is in the conservatory, find out when the conservatory was built over the manhole. If it was before 2011 then if there is a Build Over Agreement, you are fine from a water authority point, but you're at risk if there is flooding or foul odours in your property.

Why can't you just build over a manhole?

The waterboard needs access to the public sewer if it needs to be repaired. If you build over a manhole cover and don't replace the access outside of the property , then the water board won't be able to readily fix an issue if it arises.

What is a build-over agreement?

A Build Over Agreement is drafted by the Water Authority, for example, Thames Water, to allow you to build over or near a sewer. The agreement protects you and your neighbours by allowing access to repair the communal sewer pipes in an emergency.

You must obtain the agreement before the work is started as there may need to be changes to the design, costing you money and delaying your build. You cannot obtain planning permission to build over a sewer or a drain.

The water authority will charge a fee for the agreement and often charge for additional visits to inspect the works once completed..

How long does it take to get a Build Over Agreement?

Depending on how busy the Water Board is, expect to wait 15 working days (3 weeks) from submission to approval. The process can be delayed if you fail to submit all the correct paperwork.

How much does a build over agreement cost?

The cost for a Build Over Agreement varies depending on who your water company is. Thames Water charges £299, and Southern Water charges £112. Here is a list for 2024 prices:

Water Authority
What is the cost for a Build Over Agreement?

Thames Water


Southern Water


Affinity Water


You should also budget for a CCTV Drain Survey, costing £99 to £399 INC VAT. You should also budget for the actual building work in moving the manhole cover so it is outside of the property and adding a concrete lintel for support.

What if there is no build-over agreement?

If you are buying a property and the seller doesn't have a Build Over Agreement, then you can ask the Water Authority for a retrospective agreement. The water supplier attends the property and inspects the work. Where the manhole is situated outside, and the work is in good order, a retrospective agreement could be granted.

If access to the sewer and/or the weight of the extension can cause damage, you may have to demolish the structure if there is a major issue - at your own cost to repair.

Should I get Build Over or Near Indemnity Insurance?

Buyers often opt for this choice as it avoids the risk of retrospective and expensive work. The indemnity insurance protects the mortgage lender from the property being undervalued because of the damage caused by removing the extension/conservatory. The insurance does not cover the rebuilding of the property, and it is unlikely your building insurance will cover this.

Frequently Asked Questions
Andrew Boast of Sam Conveyancing
Written by:
Andrew started his career in 2000 working within conveyancing solicitor firms and grew hands on knowledge of a wide variety of conveyancing challenges and solutions. After helping in excess of 50,000 clients in his career, he uses all this experience within his article writing for SAM, mainstream media and his self published book How to Buy a House Without Killing Anyone.
Caragh Bailey, Digital Marketing Manager
Reviewed by:

Caragh is an excellent writer in her own right as well as an accomplished copy editor for both fiction and non-fiction books, news articles and editorials. She has written extensively for SAM for a variety of conveyancing, survey and mortgage related articles.

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