You can often build over a private drain, but a public sewer needs your water company's permission. Under Building Regulations Part H4, a build-over agreement is usually required when work sits within about 3m of a sewer or the pipe is above certain diameters. Before you commission any drawings, check the public sewer map and contact your water company.
TL;DR:
- Apply before construction because approvals are not retrospective; submit scaled plans, foundation sections, pipe details, and a CCTV report or written replacement commitment.
- A sewer map may omit private pipes; use CCTV and electronic location to verify an unclear route and its depth before fixing the extension layout.
- Thames Water’s fees range from £130 to £585, while Class 3 cases may add about £1,000 in legal costs and take months.
- Show the sewer, invert levels, and clearance line on foundation plans; agree manhole access, piling, surface loads, and pipe repairs with the water company.
Table of Contents
- When do you need a formal build-over agreement?
- How does the application work, and what must you supply?
- What technical requirements must your builder meet?
- What will a build-over agreement cost, and how long does it take?
- How do you find sewers and brief your design team?
- How should a managed design-and-build team handle sewer risk?
- Why the sewer check matters more than the form
- How we handle surveys, the application and the build
- FAQ
- Sources
When do you need a formal build-over agreement?
Water companies worry about two things: how close your work is to the pipe, and how big or deep that pipe is. Councils such as Manchester City Council apply a simple test. If work is within 3m of the sewer centreline and the sewer is over 225mm in diameter or deeper than 3m, you will normally need a formal agreement. Smaller, shallower pipes can sometimes be approved locally under building control inspection.
Size matters because a bigger, deeper pipe is harder to repair, and a failure under a house is expensive. Large or deep public sewers often carry strict no-build zones, and where a diversion under Section 185 is needed, the process is more complex and far more costly than a standard agreement.
Know which pipe you are dealing with:
- Private drain: serves only your property and is yours to maintain.
- Lateral drain: the stretch outside your boundary that connects your drain to the sewer.
- Public sewer: serves more than one property and belongs to the water company.
Since the 2011 Private Sewer Transfer, many shared pipes that were once private are now maintained by the local water company. Building regulations approval does not replace a separate water-company agreement. The Water UK homeowner guide explains these distinctions and advises checking the sewer map first.
How does the application work, and what must you supply?
Approvals cannot be given retrospectively, so apply before any work starts. Severn Trent sets out the steps below, and other water companies follow a similar pattern.
- Confirm from the sewer map that a public sewer or lateral drain is affected.
- Prepare a scaled site layout and a proposed layout showing the sewer line.
- Add sectional drawings showing foundations, the sewer and its invert levels.
- Gather the sewer details the form asks for: diameter, depth and length to be built over, as listed on the application form.
- Supply a CCTV survey report, or confirm in writing that you will replace the pipe.
- Submit the form with your contact details and pay the fee.
- The water company reviews the application and may request a pre-construction survey.
- If satisfied, it issues provisional approval with conditions.
- Build to those conditions, then arrange a post-construction CCTV survey for final approval.
Water companies aim to respond within around 10 working days, though complex cases take longer. A Full Plans building regulations application also triggers consultation with the sewerage undertaker, and the local authority decision period is normally five weeks unless extended by agreement.
What technical requirements must your builder meet?
The engineering rules shape your foundation design, and therefore your budget. The requirements come from Approved Document H, and your water company's conditions add to them. Your structural engineer should design around these points from day one:
- Bridging foundations: where a standard foundation would load the pipe, the foundation spans over it, or goes below the pipe invert so that loads pass beneath it. The 45° rule is the usual test, and our 45 degree check walkthrough explains it step by step.
- Access clearance: manholes and rodding eyes need enough clear space for crews to reach them. Ask the water company what access is required for your site. If an access point ends up under the new structure, the usual options are to relocate it or replace it with one outside the footprint.
- Piling: discuss any proposed piling near a sewer with your structural engineer and water company, and follow the site-specific conditions they set.
- Existing pipe condition: ask the water company or a qualified drainage professional whether the pipe needs repair or replacement before work starts.
- Surface loads: ask your engineer and water company how to manage construction materials and equipment near the sewer.
Pro Tip: Ask your engineer to mark the sewer, invert levels and 3m line on the foundation drawing itself, so the water company and building control review the same picture.
What will a build-over agreement cost, and how long does it take?
Fees depend on the class of application, which reflects the size and depth of the sewer. Thames Water's published schedule is a useful guide.
Thames Water lists build-over fees from £130 for small sewers up to 175mm, rising to £585 for large sewers, with Class 3 cases often adding legal costs from about £1,000. Treat these as an example, since other water companies publish their own schedules.
Those application fees typically cover technical plan review and coordination with building control. They do not include CCTV surveys or private legal advice, and the property owner normally pays for the surveys. Budget too for any repairs or pipe replacement the survey uncovers, and for refundable deposits where a minor diversion is involved.
On timing, many Class 1 and Class 2 reviews come back within roughly 10 to 21 working days, but Class 3 can run to months because of the legal and engineering work. Build that delay into your programme before you promise a start date to anyone.
How do you find sewers and brief your design team?
Start by requesting a sewer map from your water company. It shows public sewers and lateral drains, though not every private pipe, so a drain survey fills the gaps. Where the map is vague, commission a CCTV survey and electro-location to confirm the line and depth of the pipe.
Then brief your architect and structural engineer:
- Show the sewer, its depth, invert levels and the 3m line on every relevant drawing.
- Include sectional details of foundations crossing or near the pipe.
- Ask the builder whether the quote includes CCTV, and who pays if it finds defects.
- Ask whether repairs will be protected in place or the pipe replaced.
- Confirm who submits the application and who chases the water company.
If you are still deciding what to commission, our guide to extension planning drawings explains how to budget for them.
How should a managed design-and-build team handle sewer risk?
The cheapest mistake is the one you avoid on paper. A sound process checks the sewer map and speaks to the water company before any site survey is paid for, so you do not fund drawings for a layout that cannot be built. It also puts CCTV and a repair contingency into the pre-contract budget, and submits the application on your behalf.
Foundation choice matters here too. On West Midlands clay, foundations already go deep, so our guide to clay soil foundations is worth reading alongside your sewer position. Building control should see the sewer details from the outset.
Pro Tip: Ask any contractor to show you where CCTV and sewer repairs appear in the quote, not buried in a provisional sum.
Why the sewer check matters more than the form
Most advice treats the build-over agreement as paperwork to tick off once the design is done. I think that gets the order wrong. The agreement is the last step of a decision that should have been made earlier: where the extension sits relative to the pipe. The application fee may be modest beside the cost of redesigning foundations after drawings are finished, or pausing a build because a manhole turns up under your new kitchen.
What the research supports is simple. Smaller, shallower pipes beyond the 3m line rarely cause drama, while large or deep sewers can rule out a footprint entirely. Many homeowners also underestimate how much of the cost sits outside the application fee, in surveys, repairs and delay.
So prioritise in this order. Find the sewer on the map. Confirm it with a survey before you fall in love with a layout. Then let the drawings follow what the ground allows. Moving an extension on paper can cost far less than changing its position on site. And retrospective approval is not available, so there is no safety net if you build first and ask later.
— Raja
How we handle surveys, the application and the build
We manage house extension design and construction for West Midlands homeowners, with one team. If you are weighing a single-storey, double-storey or kitchen extension, our team can discuss your project.
Get in touch via our website to discuss your project.
FAQ
Can you build over a sewer in the UK?
Often yes, but it depends on the pipe. A private drain is usually straightforward, while a public sewer needs your water company's permission, typically through a build-over agreement. Check the sewer map and contact the company before you finalise plans.
How do I find the sewer line on my property?
Request a sewer map from your water company to see public sewers and lateral drains. Where the map is unclear, a CCTV drain survey and electro-location can confirm the exact line and depth.
Do I need a build-over agreement for a small extension?
Not always. Work within 3m of a sewer larger than 225mm or deeper than 3m will normally need one, according to Manchester City Council. Smaller, shallower pipes may be handled through building control, but confirm with your water company first.
Who pays for the CCTV surveys?
The property owner normally pays for pre- and post-construction CCTV surveys. Thames Water explains the pre- and post-construction survey requirements for its higher classes, and application fees do not cover them.
Sources
- Build over agreements | Home improvements | Thames Water
- Building over or close to a public sewer or lateral drain
- B6600 BOS Notes