Building over sewers or public drains

There are three classes of build over agreement, depending on the size of the public sewer or lateral drain you wish to build over - see the FAQ section below for more details.

You’ll need to supply essential documents with your application. The plans supplied by your architect will likely be different to those supplied for building regulations purposes. Please share the below examples with your architect or structural engineer to ensure they provide what we need.

For all three classes, you’ll need to upload the following:

Location plan

• Site map showing the boundary lines between yours, and any neighbouring properties
• View a site plan example

Existing and proposed building and drainage layout plan

• Ground floor plan showing existing and proposed sewer pipes, manholes, sewer flow direction, including any private connections onto the sewer
• View a drainage plan example

Sectional drawing showing proposed foundations and sewer(s)

• A plan showing the foundation design, sizing and depths in relation to the public sewer pipe
• If using trench/trip foundations, you’ll need to include bridging details

• Where applicable, include sewer and manhole invert levels on the sectional drawing

• View a sectional drawing example

Other documents may be needed, in addition to those listed above:

• If you’re entitled to claim disability benefit, please provide proof of this to receive a discount (for example a photocopy of your blue badge, or official letter from DWP)
• If you’re planning to use piling foundations, you’ll also need to submit a piling method statement
• If you’re planning to use raft foundations, you’ll also need to submit structural calculations

Once you’ve gathered everything you need, please complete your build over application. You can then pay for your build over application. 

You’ll need to know:

• The applicant’s name and contact details
• The address of the site requiring a build over agreement
• The property owner’s name and contact details
• Invoice recipient details
• Property details:
  • What you’re constructing
  • The local authority for the site
  • When the property was built
• The internal diameter of your sewer

It’s vital that you upload the correct documents and information, otherwise your application will be put on hold, which may result in delays.

Once you’ve applied you’ll be asked to make your payment.

Our technical team will review your application within 21 days of submitting your application and making your payment.

If we need any further information, your application could be delayed. To avoid this, please make sure you submit everything that we’ve asked for in the ‘application preparation’ section above.

Class 1 applications only (for sewers up to 175mm)

If your application is approved, we’ll issue your build over agreement and your construction can commence. This is the end of your application process. If we couldn’t approve, we’ll be in touch with more information.

For class 2 applications (for sewers from 176mm to 375mm)

We’ll be in touch to arrange a CCTV survey, which usually takes place within 10 days. It will assess the condition of the sewer to find out if any repair work is needed before you start construction.

For class 3 applications (for sewers over 375mm)

We’ll be in touch to arrange an engineer to assess the condition of the sewer, which usually takes place within 10 days. We’ll check if any repair work is needed before you start construction. At this stage, other legal agreements will be put into place.

For both application types, providing our survey doesn’t show any issues with the sewer, we’ll send you a formal notice allowing you to start construction works.

You can undertake your construction work as per your agreement with Thames Water.

For class 2 applications (for sewers from 176mm to 375mm)

You’ll get in touch with us to arrange a CCTV survey, which usually takes place within 10 days of your call. We’ll assess the condition of the sewer to make sure no damage occurred during construction.

For class 3 applications (for sewers over 375mm)

You’ll get in touch with us to arrange an engineer to assess the condition of the sewer to make sure no damage occurred during construction. This usually takes 10 days from your call.

Once we’ve approved the post-construction survey, we’ll issue your build over agreement.

• Class 1 build over agreements are usually issued within 21 days of receipt of all required information and documents
• Class 2 build over agreements are usually issued within 21 days of receipt of all required information and documents, however this may vary if any repair work is required on the sewer
• Class 3 build over agreements are complex and involve legal contracts to be created. The full application process can take more than 3 to 6 months

Once you’ve received your build over agreement, keep this for your records as it may be needed when you sell the property.

• Sewer - a pipe serving two or more properties
• Lateral drain - a pipe serving one property but located outside that property boundary
• Drain - a pipe serving one property and located within the boundary of that property
• Public sewer/lateral drain - an asset which is maintained by us (funded by bill payers)
• Private sewer/lateral drain/drain - an asset which is maintained by someone else (householders, management company, developer, etc)
• Manhole - an access point which allows for man-entry into the chamber (only by qualified workers using strict confined space working set-up)
• Inspection chamber - an access point not suitable for man-entry but enables access for rodding and camera survey equipment
• Sewerage - the pipework conveying sewage
• Sewage - the effluent being conveyed by sewerage

• Class 1: £130 - the internal diameter of the sewer is 175mm or less
• Class 2: £490 - the internal diameter of the sewer is 176mm to 350mm
• Class 3: £585 - the internal diameter of the sewer is over 350mm + (additional legal costs starting at £1,000)

These charges are updated on 1 April each year.

The application fee covers:

• Processing your application
• Technical reviews of your plans
• Discussing and agreeing any design change proposals
• Liaising with your building control officer (where required)
• Issuing your agreement
• Registering the agreement to build over or near the public sewer on our records

Class 2 applications also include pre and post-construction CCTV surveys.

Class 3 applications also include two pre and post-construction surveys conducted by one of our approved contractors, impact assessments (where required) and additional legal work.

Applicants with a disability receive a £130 discount on their build over agreement application by submitting proof of entitlement to disability benefit. For example, a letter from the Department of Works and Pensions or a Blue Badge.

View our build over technical specifications.

Complete the build over agreement application and tick the box for a retrospective application. For all classes of application, you’ll need a CCTV or on-site survey to check that no damage has occurred to the sewer during construction.

There are two ways to apply for a sewer diversion, depending on the complexity of your requirements.

Build over: minor sewer diversions

For short diversions to smaller sewers required for domestic extensions or home improvements, you may be able to apply for a minor diversion. Your proposal must meet the following criteria: 

• The internal diameter of the sewer to be diverted is 150mm or less 
• The total length of the sewer diversion is 20m or less 

A CCTV survey will be required before construction begins, to assess the condition of the sewer, and after it’s been completed, to ensure it’s been constructed to the agreed specifications and no damage has occurred. A refundable cash deposit of £2,000 is required and will be refunded once the completed diversion has been inspected by Thames Water and found to be in a satisfactory condition. 

To apply for a minor sewer diversion, complete the build over application online. 

Sewer diversions

If you do not meet the minor sewer diversion, you’ll need to complete a full sewer diversion application. 

Read more about the process and apply on our diverting a sewer page.