Trades master copy
Last reviewed: March 2026
What Part H is
Part H is the drainage bit of Building Regulations. It covers foul water (toilets, sinks, showers), rainwater, septic tanks and treatment plants, building over sewers, and bin storage for dwellings.
The aim is: get foul and surface water away from buildings safely, without blockages, flooding or pollution, and don't wreck public sewers or neighbour's drains while you're at it.
This guide is a summary to make Part H easier to use on site. It does NOT replace Approved Document H: Drainage and waste disposal (current edition).
You must read and follow the full Approved Document H and any water company/septic manufacturer guidance for your job.
This guide is written for England. Wales, Scotland and Northern Ireland have their own versions of building regulations - the principles are similar but the documents and approval routes differ, so check local requirements if you're working outside England.
Where it applies on your jobs
Part H crops up on:
- New builds and extensions - internal soil/vent pipework, below-ground foul drains, manholes, rainwater pipes and soakaways/gullies.
- Moving bathrooms/kitchens - new or altered drainage runs inside and outside.
- Septic tanks and package treatment plants - including new drainage fields.
- Building near/over existing public sewers - especially in back-land and infill developments.
- Bin storage for new dwellings - location and access.
Key "trigger points" - what matters on site
Foul water drainage (H1)
Every building using water must have an adequate foul drainage system taking waste to, in order of priority:
- A public sewer,
- A private sewer connected to a public sewer,
- A treatment plant or septic tank,
- As a last resort, a cesspool.
Internal pipework (WCs, basins, showers, appliances) and external drains must be sized, laid to correct falls and ventilated to avoid smells and blockages.
Wastewater treatment, septic tanks and cesspools (H2)
Septic tanks/treatment plants must be:
- Of adequate capacity, watertight and ventilated.
- Connected to a properly designed drainage field, not a crude "soakaway pit" for foul effluent.
Cesspools are a last resort and must be watertight, with level alarms and proper emptying access.
Rainwater drainage (H3)
Surface water should, in order of priority, discharge to:
- Soakaway or infiltration system,
- Watercourse,
- Surface water sewer.
You should not mix foul and surface water in the same pipe where separate systems are available (see H5).
Building over and near sewers (H4)
- You must not build over or too close to public sewers without the sewer owner's permission.
- Typically, you can't build within about 3 m of a public sewer over 225 mm diameter or deeper than 3 m without special measures and approval.
- Foundations, manholes and structures must not compromise access to, or structural integrity of, drains/sewers.
Separate systems of drainage (H5)
- Where there are separate foul and surface water sewers, you must keep systems separate - foul to foul, surface to surface.
- You can't just tie roofs into the foul for convenience because the rainwater run is "right there".
Solid waste storage (H6)
- New dwellings must have suitable space for storing bins and recycling, with reasonable access for residents and collection.
Quick reference table - common jobs
New extension with relocated kitchen and WC
- Internal soil and waste must be correctly trapped, vented and sized.
- External foul drains must fall correctly, with suitable access (rodding eyes/manholes) and connection to existing drains/public sewer as agreed.
- Don't mix rainwater and foul if there's a separate surface water system.
New build in the countryside with no mains sewer
- Foul drainage priority: treatment plant / septic + drainage field, only using cesspool as last resort.
- Drainage field must meet siting rules (distances from buildings, boundaries, watercourses; depth above water table).
- Rainwater should go to soakaways/infiltration if ground allows.
Driveway extension across a public sewer easement
- Check with the water company before building over/near sewers; they may require build-over agreements or changed layouts.
- Don't put foundations, walls or heavy loads over sewers without approval and suitable protection.
New soakaways for roof water
- Soakaways for rainwater only, at correct distance from buildings, boundaries and other drainage fields, and above the water table.
- Size and construction must suit the roof area and soil percolation.
New houses on a small site
- Separate foul and surface systems if the street has separate sewers.
- Provide bin storage for each dwelling with decent access and no long, awkward drag routes to the collection point.
Routes to compliance for trades
Follow Approved Document H patterns
Use AD H for pipe sizes, gradients, manhole spacing, access requirements, soakaway/drainage field design principles and separation distances.
Follow H1-H6 structure so you don't miss drainage, rainwater or bin store requirements.
Coordinate with the water company and Building Control
Always check with the sewer owner before connecting to or building near public sewers; get any required build-over agreements.
Agree foul and surface water strategies on the drawings, not after trenches are dug.
Don't cut corners on gradients, access and venting
Follow the minimum gradient and diameter tables - too flat or too small is how you get persistent blockages.
Provide manholes/inspection chambers and rodding points where AD H expects them.
Treat septic/treatment systems and drainage fields as engineered
Follow manufacturer's installation instructions and Part H; don't guess drainage field size and layout.
Observe all distance rules from buildings, boundaries, watercourses and other soakaways; check the water table.
Who is responsible for what
On a typical domestic job:
- The designer/engineer is responsible for setting the drainage strategy (where foul goes, where rainwater goes, whether you need treatment plants or drainage fields, any build-over issues).
- The groundworker/builder is responsible for installing drains, manholes, soakaways, drainage fields and bin stores to the right levels, sizes and positions.
- The plumber is responsible for internal soil/waste layouts, venting and traps.
- The client/owner is responsible for ongoing maintenance of private treatment systems and drainage fields and for getting any required permissions from the sewer owner.
Blunt version:
If you "just connect it to the nearest pipe" without checking what that pipe is, or bury a manhole under an extension, you're buying someone a permanent drainage problem. That someone will remember your name.
Simple rule to drum into your team
If you're moving bathrooms/kitchens, digging new foundations, or touching drains, treat it as a Part H job. Work out where foul and rainwater are going, how they're getting there, and how anyone can get back in to clear a blockage.
On-site checklist (Part H)
Before you start
- Get the drainage plan: foul and surface runs, manholes, connection points, any septic/treatment plant or drainage field.
- Check for public sewers within 3 m of the works and contact the water company if you're anywhere near them.
- Confirm whether the site is on separate or combined sewers.
While you're working
- Lay pipes to the correct gradients and depths, with proper bedding and surround.
- Install manholes/inspection chambers and rodding points as per the plan and AD H.
- Keep foul and surface water systems separate where required; don't improvise connections.
When you finish
- Water-test or air-test new drains as required.
- Confirm all access points are visible and usable (not under patios or kitchens).
- Check bin store locations are actually usable and match the approved drawings.
Sources
Based on:
- Approved Document H: Drainage and waste disposal (2015 edition and later guidance).
- Planning Portal and LABC summaries of Part H (foul and surface drainage, wastewater treatment, build-over, separate systems, solid waste storage).
- Industry guidance on septic tanks/treatment plants, drainage fields/soakaways, and building near/over sewers.
This guide was last reviewed March 2026. SiteKiln does not provide legal, financial or tax advice. All content is for general information purposes only. Always seek professional advice for your specific situation.
Know someone who needs this?
Working in Wales? The building rules are different. See our Working in Wales guides.
Working in Scotland? Building standards work differently. See our Working in Scotland guides.
Working in Northern Ireland? The system uses Technical Booklets. See our Working in Northern Ireland guides.
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