Build over agreements and building over sewers

If you wish to build over a sewer, you need a build over agreement. This is needed if you are planning to construct a building, an extension, underpinning or undertaking similar construction work near to, or directly over an existing sewer. The distance away from the sewer applicable depends on several factors, including the depth of the sewer, how critical the sewer is etc, although it is typically 3m.

As sewers are frequently located running along the front and back of a houses, they are usually needed for otherwise simple house extensions or new conservatories.


Some companies disreputably attempt to sell  “Build Over Indemnity Insurance Products”. These financial products are frequently sold for a few hundred pounds, covering a minimal expenditure and with multiple exclusions. It is strongly suggested you steer clear of these morally questionable products. They do not secure any legal rights. The liability stated rarely covers realistic costs were trespass occur of the sewer. The policies have to be renewed every year, with premiums increasing.

The big cost to a client is not paying to secure a legal/engineering agreement, it is the cost if a sewer collapses or blocks, then tankers (which cost around £2000 per day) for which you would be liable and in some exceptional situations the building has to be demolished. The costs of the trespass frequently exceed £1 million. Its fairly obvious why indemnity polices for the failure to obtain the legal rights for work on our property are a scam.

Build over agreements are a complex field in both Engineering and Legal terms and those who are not professional developers should consider obtaining professional advice. A professional civil engineering consultant with particular experience of build over agreements, sewers and foundations should be employed. The costs of damage either to the sewer or ones foundations without consents and agreements can be prohibitive.

says Stephen Gibson – Wilsham Consulting’s Director and former Lead Design Engineer for Thames Water Engineering,

build over agreements



The Building Regulations part H4

The requirement for a build over agreement is set out in the Building Regulations Part H4

To simplify- the word sewer for the below shall mean sewer, drain, pumped rising main of sewerage or other sewerage infrastructure.

Firstly – That the building or extension or work involving underpinning-

  • is constructed or carried out in a manner which does not overload or otherwise cause damage to the sewer either during or after the construction
  • will not obstruct reasonable access to any manhole or inspection chamber on the sewer.

Secondly – In the event of the sewer requiring replacement there is

  • a satisfactory diversionary route or
  • the extension will not unduly obstruct work to replace the drain on its present alignment.

Thirdly – The risk of damage to the building as a result of failure of the sewer, is not excessive having regard to:

  • nature of the ground
  • condition, location or construction of the sewer
  • nature, volume and pressure of the sewer
  • design and construction of the foundations.

Building Control will consequently require both details of the proposal and copy of the build over agreement provided by the Sewerage Authority.

What will I need to try and obtain a build over agreement?

It depends on the individual situation, but for an application to build over a public sewer, you are likely to need the following:

  1. Employ a professional civil engineering consultancy (such as Wilsham Consulting) with particular experience of build over agreements, sewers and foundations.
  2. Drawings, including a plan and cross section showing the relationship between the foundations and drains.
  3. Copy of the public sewer records and details of the sewer confirmed.
  4. A CCTV survey, before and after.
  5. An easement plan.
  6. A solicitor who can provide you legal advice on the build over agreement.
  7. Payment for application fees associated with the build over agreement.

I have received a demand from Thame Water that a build over agreement is needed. Can you help?

Potentially yes. We provide a cost effective service providing the professional design, advice and calculation service for householders and developers within the following regions:

  • Thames Water build over agreements
  • Severn Trent build over agreements
  • Anglian Water build over agreements
  • Northumbrian Water build over agreements
  • Welsh Water build over agreements
  • Southern Water build over agreements
  • South West Water build over agreements
  • United Utilities build over agreements
  • Yorkshire Water build over agreements

What are the problems if I don’t get professional help?

Common problems with build over agreements which are not professionally designed (DIY):

  1. Expensive legal action by Sewerage Authority for “trespass”.
  2. Breach of Building Regulations – Building Control officers will not “sign off the work”.
  3. Planning Authority may refuse planning condition discharge.
  4. Works may be required to demolished ( which regularly happens !)
  5. Approvals could be delayed.
  6. Works are done which cause damage to the sewer or the foundations. Failure to obtain professional design/advice is considered by insurers to in validate household / business insurance policies claims. The extension could be uninsurable.

Can I not get my builder to apply for the sewer build over agreement?

Anyone can apply for the agreement, including a householder or a builder. More relevant is who is qualified, experienced or insured to produce the pack of information required for the build over agreement. Clearly a builder may have experience of the physical work involved, but that is entirely separate to having experience of the design and having relevant professional indemnity insurance. Builders in the UK do not have insurance for such work.

I want you to help me with a sewer build over agreement, how do I move it forward?

Please email with your full detailed include all relevant background information for a quote, including proposal plans, sewer plans, surveys, photos, planning conditions, or anything else relevant to the proposed build over agreement.

Fees range from £1200 to £15,000 depending on the complexity.