If your extension, garage conversion, or loft build happens to sit above, or close to, a public sewer, you cannot simply lay foundations and hope for the best. You will almost certainly need a build-over agreement from your sewerage undertaker before work begins. Getting this wrong can mean building regs sign-off is refused, or worse, the water company demands you dig everything up to access a blocked drain.

What Is a Build-Over Agreement?

A build-over agreement (sometimes called a “build near” agreement) is a formal consent issued by your regional sewerage undertaker, Thames Water, Severn Trent, United Utilities, and so on, confirming that your proposed structure will not damage or obstruct a public sewer and that maintenance access can be preserved.

Sewerage undertakers have statutory powers under the Water Industry Act 1991 to reach public sewers for inspection and repair.1 If your extension sits on one, reaching the sewer may mean breaking through the structure, and the cost can fall on you. A build-over agreement sets out in advance how access and repairs will work.

When Do You Need One?

The trigger is proximity, not just whether your build physically sits on the sewer:

  • Within 3 m of a public sewer, Building Regulations requirement H4 applies, and most undertakers require a formal agreement or at least a build-near notification.2
  • Directly over a sewer, a full build-over agreement is almost always mandatory.
  • Over or near a public lateral drain (the section between your property boundary and the public sewer, transferred to the sewerage undertakers in 2011), some undertakers also require one: Thames Water’s trigger is within 1 m of the point where the pipe crosses your boundary.3

Private drains serving only your property are not covered by this regime, but it is worth clarifying ownership with a drainage search before assuming.

Finding Out If a Sewer Is Affected

Before spending money on designs, instruct your solicitor (or do it yourself via your undertaker’s mapping portal) to obtain a CON29DW drainage search. This shows the recorded location of public sewers on or near the site, which is indicative rather than exact, and costs around £70–£90: Severn Trent Searches, for example, charges £76.20 including VAT for a residential CON29DW delivered electronically.4 Sewerage undertakers must let the public inspect their sewer maps free of charge at their offices.5 Copies are also held by local authorities.2 Thames Water, for example, lets you view its maps free of charge by appointment.3

If a sewer is shown within 3 m of your proposed works, treat a build-over application as a project dependency, not an afterthought.

Build-Over Agreement: The Application Process

1. Pre-application check

Most undertakers will review outline proposals informally. Send a location plan (1:1250), a floor plan, and a site section through the proposed building. You will get a steer on whether a full agreement, a build-near notification, or no action is required.

2. Structural design

Your structural engineer must design foundations that span the sewer and transfer loads to either side. Where the sewer is more than 2 m deep and passes beneath the foundations, Approved Document H says the foundation should be designed as a lintel extending at least 1.5 m either side of the pipe.2 Typical solutions include:

  • Reinforced concrete ring beam or raft, spreads load away from the sewer.
  • Pad and beam foundations, pads positioned outside the exclusion zone, with beams spanning over.
  • Piled foundations, used where ground conditions or proximity require it; Approved Document H says piling should not be carried out where the distance from the outside of the sewer to the outside of the pile is less than twice the pile’s diameter.2

The undertaker’s own structural requirements, usually published in their technical guidance, govern matters such as clearances, inspection chamber access and pipe protection. Approved Document H itself says a building should not go over or within 3 m of a sewer more than 3 m deep or larger than 225 mm in diameter without the undertaker’s permission.2

3. Submit the formal application

Submit to your undertaker’s build-over team with:

  • Completed application form
  • Site location plan
  • Foundation drawings (plan and section), usually drawn to 1:50 or 1:100
  • CCTV survey of the sewer in its existing condition (many undertakers require one; Thames Water includes pre- and post-construction CCTV surveys in its class 2 fee)6
  • Application fee

4. Assessment and approval

The undertaker reviews the structural approach, may ask for design amendments, and issues a conditional agreement. Conditions typically include a post-construction CCTV survey and a requirement to keep an inspection chamber accessible outside the building.3

5. LABC notification

Build-over agreements sit alongside, not instead of, Building Regulations approval. Your local authority building control (LABC) or a registered building control approver must also sign off on the foundations. Many building inspectors will ask to see the undertaker’s agreement before approving the relevant inspection stage.

Costs and Timescales

ItemTypical cost (2026)
CON29DW drainage search£70–£90
Pre-construction CCTV survey (arranged by Thames Water for a class 2 sewer; otherwise commissioned privately)£0–£400
Build-over application fee (Thames Water 2026: £130, £490 or £585 by sewer size)£130–£585
Structural engineer fee (foundation redesign)£800–£2,0007
Post-construction CCTV survey (as above)£0–£400
Typical total add-on to project cost£1,000–£3,500 (plus legal costs of £1,000 or more for a class 3 sewer)

Thames Water’s application fees are £130 for a class 1 sewer of up to 175 mm, £490 for class 2 (176 mm to 350 mm) and £585 for class 3 (over 350 mm), with legal costs starting at £1,000 added for class 3.3 For a class 2 sewer Thames Water arranges the pre- and post-construction CCTV surveys itself.6

Timescales vary by undertaker: Thames Water, for example, says it responds to a class 3 (large sewer) application within 21 days, but that such a case can take 3 to 9 months or longer from application to construction.8 For a straightforward domestic case, factor 6–10 weeks into your programme to be safe.

What Happens If You Don’t Apply?

A missing build-over agreement is not in itself a criminal offence, although building over a sewer without meeting the Building Regulations is a separate breach, and the consequences are serious:

  • Building regs sign-off refused, inspectors can and do check drainage compliance, and Thames Water warns that without an agreement you may not be granted a completion certificate.3
  • Retrospective agreement, many undertakers will grant these, but they can cost more and require a CCTV survey that may reveal damage already caused.6
  • Indemnity insurance, increasingly hard to obtain for undisclosed build-over situations, and mortgage lenders commonly ask specifically about sewer proximity.
  • Demolition risk, if the building limits the undertaker’s ability to maintain the sewer, it can seek a court injunction to have the structure removed and may claim damages.3

Build-Near vs Build-Over: Key Difference

If your proposed structure is within 3 m but not directly over the sewer, your undertaker may require only a build-near notification rather than a full agreement. The notification process is lighter-touch, you submit a plan and a covering letter confirming the sewer will not be affected, and any fee is usually lower than for a full agreement. Check with your specific undertaker, as thresholds and processes differ between the water and sewerage companies in England and Wales.

Practical Tips

Commission the drainage search early. Sewer positions discovered mid-design can force expensive foundation redesigns or even a change to the building footprint.

Use a structural engineer experienced in drainage proximity work. The undertaker’s technical guidance documents (all published online) contain specific requirements for concrete mixes, pipe surround materials, and minimum cover that a generalist may overlook.

Keep access open. A common reason an agreement is refused is that the proposed layout removes practical access to an inspection chamber. If your design covers an existing manhole or inspection chamber on a shared sewer, it will normally have to be relocated outside the building: Approved Document H says extensions should not be constructed over a manhole or inspection chamber on a sewer unless alternative access acceptable to the undertaker is provided.2

Check whether a lateral drain is involved. Since the 2011 transfer of private sewers to water company ownership, many drains that homeowners assumed were private are now public.9 The CON29DW search will confirm this, but it catches many applicants off guard.

Check the sewer map before you design

Sewerage undertakers must make their sewer maps available to inspect free of charge, and many offer online map services. A five-minute check before commissioning drawings can save thousands in redesign fees.

Build-over agreements add cost and time to a project, but they are straightforward when planned early. Treat the drainage search as step one of any ground-floor extension project and the rest of the process follows logically.

References (9)

Figures in this note were checked against the sources below on 2 October 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalvery strongThe National Archives (legislation.gov.uk), guidance.RN-062XEMSection 159 gives sewerage and water undertakers power to lay, inspect, maintain, repair or alter their pipes, including sewers, in private land.
  2. 2technicalvery strongMinistry of Housing, Communities and Local Government.RN-SM6TRWThe statutory source for soakaway design and the discharge hierarchy every garden drainage note should state. Requirement H3(3) lists the order of priority for rainwater discharge: (a) an adequate soakaway or other adequate infiltration system, or where not reasonably practicable (b) a watercourse, or where not reasonably practicable (c) a sewer. Paragraph 3.25 bars infiltration devices within 5 metres of a building or road, in ground where the water table reaches the bottom of the device at any time of year, too close to other drainage fields or soakaways, or where contamination could pollute groundwater. Paragraph 3.26: soakaways serving less than 100 square metres are generally square or circular pits filled with rubble or lined with dry-jointed masonry or perforated ring units. Paragraph 3.27: design to a return period of once in ten years, and for small soakaways serving 25 square metres or less a design rainfall of 10mm in 5 minutes may be assumed as the worst case. Paragraph 3.28 requires percolation tests to derive the infiltration rate, and paragraph 3.30 sends larger soakaways to BS EN 752-4 or BRE Digest 365 Soakaway design. Paragraphs 2.10 to 2.12 cover pervious paving over a granular storage reservoir and warn against its use where sediment or oil spillage would block the pores. For foul drainage, paragraph 2.33 sets a minimum 75mm internal diameter, or 100mm for a WC, and Table 6 gives minimum gradients of 1 in 40 for 75mm and 100mm at peak flows under 1 litre per second.
  3. 3technicalfairThames Water, guidance.RN-VI6AGOSets the trigger distances: an application is likely to be needed if you are building within 3 metres of a public sewer, or within 1 metre of the point where the pipe serving only your property crosses the boundary and becomes a public lateral drain. States plainly that planning permission does not grant permission to build over a sewer or drain, and that work should not start before the agreement is in place because the design may have to change. For a wholly new property, including a car port or shed not attached to the house, Thames Water requires the sewer to be diverted rather than built over. Liability for damage caused to sewers or drains during construction rests with the property owner. Asset maps can be viewed free of charge by appointment at Clearwater Court, Vastern Road, Reading RG1 8DB, or bought from utility search companies or the Property Searches department.
  4. 4costsupplier figureSevern Trent Searches.RN-LH2X7NA price, true at 2026-09. The product page of Severn Trent's official search provider for the residential CON29DW drainage and water search. It costs £63.50 plus VAT (£76.20) delivered electronically, or £89.40 including VAT with printing and delivery.
  5. 5technicalvery strongThe National Archives (legislation.gov.uk), guidance.RN-TQHA0PSection 199 requires every sewerage undertaker to keep records of public sewers and lateral drains and make them available for free public inspection as a map.
  6. 6technicalfairThames Water, guidance.RN-ITUBEVThames Water's homeowner build-over process, with its published 2026 fee scale and timescales. Three classes by sewer internal diameter: Class 1 £130 (175mm or less), Class 2 £490 (176mm to 350mm), Class 3 £585 (over 350mm) plus additional legal costs starting at £1,000. Class 1 and Class 2 agreements are usually issued within 21 days of receipt of all required information; Class 3 involves legal contracts and the full process can take more than 3 to 6 months. Class 2 fees include pre- and post-construction CCTV surveys; Class 3 adds two surveys by an approved contractor plus impact assessments. Applicants on disability benefit get a £130 discount on proof of entitlement (DWP letter or Blue Badge). Retrospective applications are accepted but need a CCTV or on-site survey to prove no damage, and may be refused if the pipework was not protected to specification. Self-service sewer diversion is offered where the internal diameter is 150mm or less, with a refundable £2,000 cash deposit returned after satisfactory post-construction inspection. Every application needs a location plan, an existing and proposed drainage layout showing manholes and flow direction, and a sectional drawing of the foundations against the sewer with invert levels; piling foundations need a piling method statement and raft foundations need structural calculations.
  7. 7costsupplier figureBeam Designs.RN-CUW5XPA price, true at 2026-06. Beam Designs's published price for the item the cost register files as "structural engineer fixed fee, loft conversion", which is the figure a cost footnote on this page points at.
  8. 8technicalfairThames Water, guidance.RN-5879YYThames Water developer guidance on class 3 build-over applications: a 21-day initial response, then surveys and easement, with 3 to 9 months or more overall.
  9. 9technicalvery strongThe National Archives (legislation.gov.uk), guidance.RN-8C16FLThe 2011 regulations under which private sewers and lateral drains connected to the public network vested in the sewerage undertakers on 1 October 2011.

  1. Water Industry Act 1991, section 159: Power to lay pipes in other land, legislation.gov.uk, accessed 29 September 2026. legislation.gov.uk

  2. Approved Document H, Drainage and waste disposal, 2015 edition, HM Government (GOV.UK), accessed 29 September 2026. assets.publishing.service.gov.uk

  3. Build over agreements, Thames Water, accessed 29 September 2026. thameswater.co.uk

  4. CON29DW Drainage & Water Enquiry, Severn Trent Searches, accessed 2 October 2026. Priced at 2026-09 and left there. severntrentsearches.com

  5. Water Industry Act 1991, section 199: Sewer maps, legislation.gov.uk, accessed 29 September 2026. legislation.gov.uk

  6. Building over sewers or public drains, Thames Water, accessed 29 September 2026. thameswater.co.uk

  7. Fast & Affordable Online Structural Calculations, UK Wide, Beam Designs, accessed 2 October 2026. Priced at 2026-06, so no calibration is needed. beam-designs.co.uk

  8. Developing over or near sewers, Thames Water, accessed 29 September 2026. thameswater.co.uk

  9. The Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011, legislation.gov.uk, accessed 29 September 2026. legislation.gov.uk