5 Drainage Approvals London Homeowners Must Get Before an Extension
Approved Document H is the main regulatory reference for drainage on any English house extension, and it works alongside national SuDS standards and your local water company’s rules. Before you finalise foundations or order groundworks, map your existing drains and sewers, commission a percolation test, and check whether your build sits close enough to a public sewer to need a build over agreement. Get these three steps right early and you avoid the delays, extra costs, and refused completion certificates that catch out so many extension projects.
TL;DR:
Correct siting and sizing of soakaways require early percolation testing, ideally in different seasons, to ensure ground absorption rates are accurate.
Separation of foul and surface water drainage systems is recommended for new extensions to prevent long-term flooding issues during heavy weather.
Building control inspections and approvals, including pipe gradient and joint checks, must be completed before backfilling to avoid delays and costly rework.
If a public sewer is within three meters of your extension, a build over agreement is mandatory before starting groundwork to prevent legal and sale complications.
Engaging a drainage surveyor, installer, and designer early helps optimize costs and reduces drainage-related surprises during project completion.
Table of Contents
Which regulations and standards apply to your extension
England follows a clear hierarchy for surface water disposal: a soakaway first, then a watercourse, and only then a public sewer as a last resort. Foul water follows a similar logic, running to the existing foul sewer or a private treatment system when no sewer connection is available. This order exists because sewers and watercourses have finite capacity, and pushing more water into them than necessary increases flood risk downstream.
Approved Document H is the statutory guidance that building control uses to assess compliance, covering both foul and surface water disposal along with the construction of underground drains. Alongside it, BRE Digest 365 gives the detailed method for designing and testing soakaways, while national SuDS standards set the design criteria for managing surface water and explain when discharge to a sewer or watercourse becomes acceptable rather than a default choice.
If you are in Scotland, Wales, or Northern Ireland, the underlying principles are similar, but the specific documents differ, so check with your local building control body rather than assuming Approved Document H applies directly. Whatever your location, your contractor should be referencing the current national guidance for your nation, not a generic rule of thumb.
Foul water versus surface water: what your extension adds
Foul water comes from anything connected to sanitary use: a new bathroom, an extended kitchen, or an added utility room. Surface water is rainfall runoff, and an extension usually adds some through a larger roof, a wider patio, or new hard landscaping around the build. Both systems need separate consideration because mixing them, or under sizing either, creates problems long after the plasterers have gone home.
A rough per appliance estimate gives you a starting point, but a drainage surveyor using CCTV and flow checks will confirm whether your existing foul drains have the capacity for the extra load.
Bathrooms and kitchens add to the foul system and may require an upsized drain run if you are adding more than one wet room.
Roofs and patios add to the surface water system, and larger extensions often need new soakaways or additional gullies.
Combined systems in older properties sometimes need separating as part of the extension works, particularly where local SuDS policy discourages further loading of combined sewers.
Separation is generally preferred wherever it is practical, since it keeps rainfall out of the foul network and reduces the chance of surcharging during heavy weather.
Soakaways and percolation testing: siting and sizing basics
A soakaway only works where the ground can absorb water at a reasonable rate, and the only way to know that is a percolation test. This involves digging a trial pit, filling it with water, and timing how quickly the level drops. Testing in winter, or repeating the test after heavy rain, gives a more honest picture, since groundwater levels shift through the year and a soakaway sized on a dry summer test can fail come November.
Siting rules matter as much as the test result. A soakaway typically needs to sit a set distance from buildings and boundaries, away from areas with high groundwater, and clear of any contaminated ground. BRE Digest 365 provides the standard method for sizing a soakaway once you have percolation results, balancing the volume of runoff against the rate the ground can absorb it.
Designers commonly recommend building in an inspection access point so the soakaway can be checked and cleared without digging it up, and rubble filled systems generally need inspection or renewal every 8 to 12 years. A soakaway that is never checked tends to silt up quietly until the garden starts flooding after rain, by which point the fix is far more disruptive than a routine inspection would have been.
Pipe sizes, gradients, and what building control checks before backfill
Building control will not sign off drainage they cannot see, which is why pipe bedding, jointing, and gradient all get inspected before the trench is backfilled. Getting these details agreed with your contractor before work starts saves a return visit and a delay while the inspector waits to see what is already buried.
Minimum pipe diameters and gradients for domestic foul and surface drains are set out in Approved Document H, with gradient tables that vary by pipe size and expected flow.
Inspection chambers need to be spaced at set intervals along a drain run, giving access for future rodding and maintenance.
Gullies, traps, and ventilating pipes each have their own placement rules, and anti flooding valves or pumped systems come into play where gravity drainage genuinely cannot work on a sloping or low lying site.
Where gravity drainage is impracticable, designers specify package pumping installations built to recognised British Standards, sized to cope with a full day’s inflow so the household is not left stranded if the pump needs attention.
Public sewers and when you need a build over agreement
If a public sewer or lateral drain runs on or near your proposed footprint, you need more than a nod from your planning officer. A build over agreement is a legally binding contract with your sewerage undertaker, not a formality that can be sorted out after the trench is dug. Thames Water’s guidance sets out trigger distances, commonly within 3 metres of a public sewer, where consent is required before work can proceed, and the undertaker can require protective measures or a diversion as a condition of approval. Our own guide to the Thames Water build over process walks through what London homeowners can expect from the application.
Start by locating any sewers through a sewer map search or a CCTV survey, then contact your sewerage undertaker as early as possible since lead times for approval can run to several weeks. Minor diversions may carry a fee on top of the application cost. Proceeding without the required consent risks a refused completion certificate and can complicate a future sale, since conveyancing solicitors routinely check for unresolved build over issues.
Approvals, inspections, and the paperwork you will need
Building control follows a set sequence of checks, and having the right paperwork ready at each stage keeps the process moving.
Pre-backfill inspection, confirming pipe gradient, jointing, and bedding before the trench is covered.
Percolation test report, required if a soakaway forms part of the design.
CCTV survey, often requested before and after any connection to a public sewer.
Build over agreement or diversion consent, where the sewerage undertaker’s trigger distance applies.
Final inspection, confirming the completed system before a Building Regulations completion certificate is issued.
Sewerage undertakers usually expect at least three weeks’ notice before any connection to a public sewer, so factor that into your programme. Budget for the test and application costs as a separate line item rather than folding them into a general contingency, since they are easy to underestimate.
Practical pre-build checklist and common mistakes to avoid
Most drainage problems on extension projects trace back to one thing: leaving it too late. A short checklist at tender stage catches the issues before they become site problems.
Map existing drainage on site before finalising the extension layout.
Commission a CCTV survey to confirm the condition and route of existing pipes.
Run a percolation test early enough to influence soakaway siting, not after the foundations are poured.
Check public sewer maps for anything running near your footprint.
Apply for a build over or diversion consent as soon as you know you are within the trigger distance.
Design in inspection chambers rather than treating them as an afterthought.
Book building control visits at the pre-backfill and final stages before the programme is set.
Common mistakes include testing percolation too late to influence the design, assuming a shared drain is not your responsibility, and underestimating the cost of relocating a manhole that turns out to sit under the new extension.
Pro Tip: Bring in a drainage surveyor for the CCTV and percolation work, a certified installer for the pipework itself, and, on anything with a soakaway or a nearby sewer, a drainage designer to check the sizing before you dig.
Why early drainage design saves London homeowners time and money
Most drainage surprises we see on London extensions come from a survey that happened too late, after the layout was already fixed. Once we know where the sewers and existing drains sit, we can design around them rather than reacting to them. When you review a contractor’s quote, check that it names the percolation test, the CCTV survey, and any build over application as separate, costed items rather than a vague allowance buried in the groundworks figure.
— Mateja
How Tenen Ltd supports your extension’s drainage from day one
Getting drainage right on a London extension means coordinating a survey, a percolation test, and often a build over application before the first trench is dug, and that coordination is exactly where a managed service pays for itself. Drainage surveys, CCTV mapping, soakaway commissioning, and build over applications can be coordinated as part of a wider extension project, ideally with clear, itemised quotes to avoid surprise line items once work starts.
If you are planning a rear, side, or wrap around extension in West or Central London, get in touch to arrange a site visit through our extension services page and we will walk you through what your drainage survey and approvals will involve.
Sources
Approved Document H sets the statutory baseline, BRE Digest 365 covers soakaway sizing, national SuDS standards guide surface water strategy, and Thames Water’s build over guidance explains sewer proximity rules.
FAQ
What are the drainage rules in the UK?
English extensions must comply with Approved Document H, which covers foul and surface water disposal and the construction of underground drains. Alongside it, national SuDS standards set out how surface water should be managed on site before it reaches a sewer or watercourse.
Do all extensions need a soakaway?
Not every extension needs one, but soakaways are the preferred first option in the surface water disposal hierarchy, ahead of watercourses and sewers. Whether one is suitable depends on a percolation test confirming the ground can absorb water at a workable rate, as set out in BRE Digest 365.
What are the rules for extensions in the UK?
Extension drainage must follow Approved Document H for both foul and surface water, and any work near a public sewer may require a build over agreement from the local sewerage undertaker. Building control inspects pipe gradient, jointing, and bedding before backfill, and issues a completion certificate once all required tests and consents are in place.
What is the British standard for drainage systems?
There is no single British Standard that covers every aspect of domestic drainage. Approved Document H references the relevant standards for pipe sizing and gradients, while sewage treatment plants discharging to ground must meet the British Standards referenced in current guidance.
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