Summary
- Part H covers foul drainage (H1), wastewater treatment (H2), rainwater drainage (H3), building over sewers (H4), separate systems (H5) and solid waste storage (H6) — extensions routinely trigger H1, H3 and H4.
- Building within 3 metres of a public sewer (or over it) needs a build-over agreement with the water undertaker before work starts — check the asset map at Stage 1, not when the digger finds the pipe.
- Surface water must go to soakaway, watercourse or sewer in that order of preference; soakaways need percolation testing to BRE Digest 365 and 5 m separation from buildings.
What the Building Safety Act requires for Part H on non-HRB domestic jobs
Most domestic extensions touch drainage three ways: new sanitary connections into the foul system (H1), new roof area needing surface water disposal (H3), and — since private sewers transferred to the water companies in 2011 — a surprising number of shared drains now classed as public sewers running through back gardens (H4).
The build-over question is the one that stops projects. If any part of the works is within 3 m of a public sewer, the water undertaker's agreement is needed: self-certified for straightforward cases over smaller pipes, a formal agreement otherwise. As Principal Designer (BSA), obtaining the asset location search at Stage 1 and logging the agreement reference is cheap insurance against a redesign at commencement.
For surface water, the hierarchy is soakaway first, then watercourse, then sewer — and Building Control will ask for the justification if you land on the sewer. A percolation test and a BRE 365 soakaway calculation is usually a half-day exercise that keeps the decision defensible.
Key design risks and evidence needed (RIBA Stages 2–4)
- Stage 1–2: water company asset search; identify shared/public sewers, existing manholes and invert levels within the site; flag septic tanks or treatment plants (H2) on rural jobs, including the 2020 general binding rules for discharges.
- Stage 3: drainage strategy drawing — foul runs, surface water disposal route, soakaway location at least 5 m from buildings, build-over implications for foundations (typically concrete-encased pipe or lintelled openings over the sewer).
- Stage 4: percolation test results and soakaway sizing to BRE Digest 365; build-over agreement reference; access points positioned per AD H (every change of direction/gradient accessible for rodding).
Standards and references you should be citing
- BS EN 752 — drain and sewer systems outside buildings (design framework).
- BS EN 12056-2 — gravity drainage inside buildings; pipe sizing and trap seals.
- BRE Digest 365 — soakaway design, including the percolation test method and storage sizing.
- Sewerage Sector Guidance / water undertaker build-over procedures for H4 agreements.
- General binding rules: small sewage discharges (Environment Agency) for septic tanks and package treatment plants.
Surface water disposal hierarchy
| Priority | Route | Evidence to log |
|---|---|---|
| 1 | Soakaway / infiltration | Percolation test results, BRE 365 sizing calc, location plan (≥5 m from buildings) |
| 2 | Watercourse | Riparian owner/EA consent where needed, outfall detail |
| 3 | Surface water sewer | Justification why 1 and 2 are impracticable, water company consent |
| Never | Foul sewer (unless combined, with consent) | Written confirmation the sewer is combined |
Common pitfalls that catch out sole practitioners
- Discovering a public sewer under the extension footprint after tender — the asset search costs little at Stage 1 and reprices everything if skipped.
- Soakaways drawn 2 m from the new wall to make the plan work; the 5 m rule exists to protect the foundations you just designed.
- New WCs added on long, flat drain runs without checking gradients (1:40 to 1:110 for 100 mm foul, depending on flow) — persistent blockages become your latent defect.
- Rural jobs where the existing septic tank fails the general binding rules and nobody prices the replacement until the client is committed.
Frequently asked questions
How do I find out if there is a public sewer in a back garden?
Order an asset location search (sewer map) from the water undertaker at feasibility stage, but treat it as indicative — many transferred private sewers are unmapped. Lifting manhole covers and checking flow directions on site remains essential, and any shared drain serving more than one property built before July 2011 is likely to be a public sewer.
When can surface water from an extension go into the existing combined drain?
Only when infiltration (soakaway) and discharge to a watercourse are both shown to be impracticable — for example, failed percolation tests on clay. Keep the test results as evidence; connecting new roof area to a combined sewer without justification is a common Building Control query.
Track this natively instead of in Word templates
PD Pathway gives small practices an automated Parts A–T relevant-requirements tracker, stage-by-stage Principal Designer checklists and one-click compliance documents — built strictly for non-HRB projects. First project free, then from £10 per project or £99/yr unlimited.