When you purchase a new build home, you expect everything to function properly, including elements that are largely invisible after construction, such as your below-ground drainage system. While out of sight, these systems are crucial to your home’s functionality and longevity. Problems with below-ground drainage can lead to serious issues, from unpleasant odours to structural damage if left unaddressed.

The National House Building Council (NHBC) provides comprehensive technical standards that builders must follow to ensure new homes are constructed properly. Chapter 5.3 of the 2025 NHBC Standards focuses specifically on “Drainage below ground” and outlines the requirements for these essential systems.

What is Below-Ground Drainage?

Below-ground drainage encompasses all the pipework and associated components that carry waste water and surface water away from your property. This includes:

• Foul water drainage: Carrying waste water from toilets, sinks, baths, showers, and appliances
• Surface water drainage: Managing rainwater from roofs, driveways, and other hard surfaces
• Groundwater drainage: Removing excess groundwater from around foundations

Properly designed and installed drainage systems prevent water damage, soil erosion, and potential health hazards associated with standing water or sewage backups.

Key Elements of the NHBC Standards for Below-Ground Drainage

The NHBC Standards emphasise that all drainage schemes require approval from the building control authority. Additionally, local sewerage undertakers may impose further requirements, especially if the drainage system is to be adopted under Section 104 of the Water Industry Act 1991 or the Sewerage (Scotland) Act 1968.

For septic tanks or equivalent sewage treatment plants, satisfactory outfall disposal is essential, and permissions from the Environment Agency (in England and Wales) or equivalent authorities in Scotland and Northern Ireland may be required.

Design and Documentation

Proper documentation is vital for drainage systems. The standards require detailed information including:

• Proposed drain layout
• Invert levels and locations of existing sewers
• Junction locations
• Ground floor and external finished levels
• Inspection and access points
• Methods of disposal for both foul and surface water
• Details of any septic tanks, cesspools, or soakaways

This documentation ensures that installers understand the design intent and that future maintenance can be carried out effectively.

Foul and Surface Water Disposal

A critical aspect of drainage design is ensuring foul effluents and surface water are conveyed satisfactorily to appropriate outfalls. The standards cover:

• Connections to public and private sewers
• Surface water disposal systems (typically kept separate from foul water)
• Rights of connection to disposal systems
• Compatibility with existing sewerage systems
• Capacity requirements for private sewers
• Treatment plants for multiple homes

Most importantly, surface water drainage is generally required to be separated from foul water drainage, preventing unnecessary burden on sewage treatment facilities.

System Performance and Safety

The standards address several performance and safety considerations:

• Ventilation of drainage systems to prevent trap seal siphonage
• Prevention of gases entering the home (particularly important in areas with radon or landfill gas)
• Proper siting of septic tanks and cesspools (minimum distances from homes and access requirements)
• Design requirements for pumped systems where gravity drainage isn’t possible

Avoiding Damage and Blockages

To minimise the risk of damage and blockages, the standards provide detailed guidance on:

• Accommodating ground movement with flexible pipes and joints
• Designing appropriate pipe runs and gradients
• Correct sizing of pipes
• Proper access points for maintenance and clearing blockages
• Appropriate drainage covers and gully grids
• Protection against groundwater ingress
• Measures to prevent flooding

Septic Tanks, Cesspools, and Soakaways

For properties not connected to public sewers, the standards provide extensive requirements for:

• Capacity calculations for septic tanks and cesspools
• Access and ventilation requirements
• Permeability standards to prevent leakage
• Proper drainage connections
• Soakaway design for septic tank outflows
• Field drain specifications
• Percolation testing procedures

Similarly, surface water soakaways are covered in detail, including location requirements, design calculations, and testing procedures.

Installation and Protection

The standards provide comprehensive guidance on:

• Excavation requirements for drainage trenches
• Protection of pipework passing through walls or under building elements
• Appropriate bedding of pipes
• Sidefill and backfill materials and methods
• Protection of work during construction
• Special considerations for drainage under buildings, including suspended drainage and specialist foundations

Common Drainage Problems in New Builds

Despite the comprehensive standards, drainage issues can still occur in new builds. Common problems include:

1. Slow drainage or blockages: Often caused by debris left in pipes during construction, inadequate gradients, or poor connections
2. Foul odours: May indicate failing traps, inadequate ventilation, or leaking seals
3. Gurgling sounds: Could suggest partial blockages or air pressure issues in the system
4. Subsidence around drainage pipes: Potentially indicating inadequate compaction of backfill material or leaking pipes causing soil erosion
5. Water pooling in gardens or driveways: May suggest poorly designed or installed surface water drainage

What to Do If You Experience Drainage Problems

If you’re experiencing drainage issues in your new build home:

1. Document the problem: Take photographs or videos of any visible issues.
2. Report promptly: Contact your builder or developer immediately, as drainage issues fall under the NHBC Buildmark warranty.
3. Request inspection: Ask for a CCTV survey of the drainage system to identify any defects.
4. Check compliance: Request evidence that the drainage was installed according to NHBC Standards and approved plans.
5. Escalate if necessary: If your builder is unresponsive, contact NHBC directly to report potential standards violations.

Even properly installed systems require maintenance. As a homeowner, you should:

• Avoid flushing inappropriate items (wet wipes, nappies, cooking fats, etc.)
• Have inspection chambers checked periodically
• Clear gullies and rainwater goods regularly
• Address any slow-draining fixtures promptly
• Consider periodic professional cleaning of the system

Further Details

5.3.1 Compliance

This subchapter outlines the regulatory compliance requirements for drainage systems. It emphasises that all drainage schemes must receive approval from the building control authority, with local sewerage undertakers potentially imposing additional requirements. For properties using septic tanks or equivalent sewage treatment plants, satisfactory outfall disposal is essential, requiring appropriate permissions from environmental agencies (Environment Agency in England and Wales, NIEA in Northern Ireland, or local authorities in Scotland). The section stresses that ground conditions may preclude septic tank use in certain locations, and evidence of satisfactory percolation tests is required by NHBC for infiltration drainage systems.

5.3.2 Provision of information

This section details the documentation requirements for drainage systems. It specifies that designs and specifications must be clear, comprehensive, and properly distributed to all relevant personnel. Required information includes the proposed drain layout, invert levels and locations of existing sewers, junction positions, ground floor and external finished levels, inspection and access points, disposal methods for both foul and surface water, and details of any septic tanks, cesspools, or soakaways. For systems using field drains or soakaways, percolation test results and dimensions of trenches must also be documented to ensure proper functioning.

5.3.3 Preliminary work

This brief but crucial subchapter emphasises the importance of on-site verification before drainage work begins. It requires checking that invert levels, locations of existing sewers, ground floor levels, and external finished levels all match the design specifications. Where treated effluent disposal occurs through field drains, percolation tests should be verified, and the design should ensure that field drains can be accommodated within site boundaries. This preliminary verification helps prevent costly adjustments later in the construction process.

5.3.4 Foul and surface water disposal

This comprehensive section addresses the design requirements for conveying foul effluents and surface water to appropriate outfalls. It covers connections to sewers (requiring agreement from responsible authorities), surface water disposal systems (generally kept separate from foul drainage), rights of connection to disposal systems, compatibility with existing sewerage infrastructure, capacity requirements for private sewers, and treatment plants serving multiple homes. The subchapter emphasises the separation of foul and surface water systems where possible, with specific guidance on interceptors and trapped gullies when systems must be combined.

5.3.5 Drainage system performance

This subchapter focuses on ensuring drainage systems are suitably located and prevent health hazards. It addresses four key areas: ventilation of drainage systems (typically achieved through discharge stacks, with air admittance valves permitted in certain circumstances), prevention of gases entering the home (particularly important in areas with radon or landfill gas), proper siting of septic tanks and cesspools (minimum distances from homes and access requirements), and design requirements for pumped systems when gravity drainage isn’t possible. For pumped systems, the standards require sufficient holding capacity, warning systems, and suitable equipment housing.

5.3.6 Groundwater drainage

This subchapter addresses the requirements for groundwater drainage systems, which are designed to convey excess groundwater to suitable outfalls. It covers two main aspects: layout of pipes and pipe construction. The section outlines various design approaches based on site contours and ground conditions, including natural, herringbone, grid, fan-shaped, and moat systems. Pipe perforations should be appropriate holes or slots to suit the ground conditions. When groundwater drains connect to foul, surface water, or combined systems, they must discharge through a catchpit. Alternatively, groundwater drainage may discharge into soakaways (preferably through a catchpit) or directly into watercourses where suitable.

5.3.7 Design to avoid damage and blockages

This comprehensive section focuses on minimising the risk of damage and blockages in drainage systems. It covers eight critical areas: ground stability (accommodating movement with flexible pipes and joints), pipe runs (straight with minimal direction changes), appropriate pipe sizes, correct gradients (maintaining self-cleansing velocity), access and connections (providing rodding points and inspection chambers), drainage covers and gully grids, groundwater protection, and flood prevention. The subchapter provides detailed specifications for various access points, including dimensions for rodding points, access fittings, inspection chambers, and manholes, ensuring every length of drain can be accessed for maintenance.

5.3.8 Durability

This subchapter emphasises the need for drainage systems to be adequately durable and protected against damage. It addresses four key considerations: loads from foundations (ensuring foundation loads aren’t transmitted to pipes), proper bedding of pipes, protection against loads from overlying fill and traffic, and protection against chemicals in ground and groundwater. Where drainage trenches are near foundations, specific precautions are required, such as ensuring foundation bottoms are lower than adjacent trenches or filling trenches with concrete to suitable levels. Special protection may be needed where pipes are near the surface or subject to heavy loads, with appropriate bedding class and backfill grading conforming to relevant standards.

5.3.9 Septic tanks and cesspools

This subchapter details the requirements for correctly installing septic tanks and cesspools. It covers capacity calculations (based on the number of people served), access and ventilation requirements, permeability standards (tanks must be impermeable to contents and subsoil water), and proper connections. The section differentiates between septic tanks (treatment plants requiring suitable outfalls for treated effluent) and cesspools (storage tanks requiring periodic emptying). Specific construction materials and dimensions are prescribed, including minimum thicknesses for brickwork or concrete construction, and access points must have lockable covers with no dimension less than 600mm.

5.3.10 Septic tank outputs

This section addresses the drainage connections for septic tanks, focusing on five key areas: outfall requirements, flow velocity controls, soakaways for septic tanks, field drains, and underdrains. It emphasises the importance of securing discharge consent early in the design process, as certain locations and ground conditions may preclude proprietary septic tank use. For less porous subsoils, the subchapter provides detailed guidance on percolation testing and sub-surface irrigation systems, including charts for determining field drain trench area and pipe length based on percolation values. Where the water table might rise close to field drains, subsurface irrigation is not acceptable, and alternative solutions must be implemented.

5.3.11 Surface water soakaways

This subchapter covers the requirements for soakaway drainage systems designed to handle surface water. It focuses on two main aspects: soakaway location and design. Soakaways must be built on land lower than or sloping away from buildings, sited at least 5m from foundations, and positioned to drain water away from the building. The section emphasises the importance of percolation testing, especially where there are doubts about ground conditions or where large quantities of runoff are expected. It distinguishes between large soakaways (lined pits or precast concrete rings) and small soakaways (holes filled with granular material), providing detailed testing procedures and design calculations for determining appropriate dimensions based on soil permeability and collection area.

5.3.12 Component requirements

This brief but essential subchapter specifies that drainage systems must be constructed with materials that ensure satisfactory service throughout the system’s life. It presents a comprehensive table of British Standards and relevant codes of practice that components should comply with, covering everything from vitrified clay pipes, cast iron drain pipes, and thermoplastics to concrete pipes, gully tops, and manholes. The standards address various aspects of drainage components, including pipes, fittings, joints, inspection chambers, and septic tanks, ensuring that all materials used are appropriate for their intended function and will provide long-term performance.

5.3.13 Excavation

This subchapter outlines the requirements for excavation work to ensure that the invert levels and gradients specified in the design are achieved. It covers three key areas: setting out dimensions (verifying benchmarks and reporting discrepancies), depth of trenches (excavating to design depths with appropriate filling where necessary), and width of trenches (keeping them as narrow as practicable while allowing minimum 150mm working space on each side of the pipe). The section emphasises the importance of removing hard spots and filling soft spots to avoid stress points under pipes, ensuring a stable and properly graded foundation for the drainage system.

5.3.14 Protection of pipework

This section addresses the requirements for protecting drainage pipework against damage. It covers three main areas: pipes passing through substructure walls (accommodating movement with clearance or flexible joints), pipework under finishes (ensuring adequate protection and compaction), and movement joints (providing for thermal expansion and contraction). For pipes passing through walls, the standards require either 50mm clearance all round, a sleeved opening, or bedded pipes with flexible joints near the wall faces. When drains pass under roads or drives, specific protection measures are required depending on pipe type and depth, including concrete encasement for rigid pipes and bridging slabs for flexible pipes.

5.3.15 Laying pipework

This subchapter details the requirements for laying pipework to the designed lines and gradients, focusing on bedding and backfill procedures. Pipes must be firmly supported throughout their length and bedded as specified to resist loads. The section prohibits using bricks or hard materials as temporary supports and specifies appropriate bedding materials and depths for different pipe sizes. For backfilling, the standards require placement as soon as pipes are bedded and inspected, using either granular material or selected backfill free from large stones, clay lumps, timber, or vegetable matter. Backfill should be well-compacted in layers no deeper than 300mm, with mechanical compaction only used when compacted backfill exceeds 450mm above the pipe crown.

5.3.16 Protection of work

This brief but crucial subchapter emphasises the importance of protecting drainage systems from damage during construction activities. It explicitly states that damaged drainage will not be accepted, highlighting the serious consequences of inadequate protection. The section recommends preventing heavy loading or underground work above or near unprotected drainage and prohibits driving dumpers, trucks, fork lifts, or other heavy vehicles along or near pipe runs. The accompanying illustration clearly shows crossed-out heavy equipment over drainage trenches, visually reinforcing the message that construction traffic must be kept away from installed drainage systems to maintain their integrity.

5.3.17 Drainage under buildings

This comprehensive subchapter addresses the specific requirements for drainage systems installed beneath buildings. It covers four key areas: ground supported pipework, suspended drainage, examples of site installations, and drainage beneath specialist foundations. The section emphasises that support systems must account for ground conditions and potential movement. For ground at risk of settlement, drains may be laid at steeper gradients than minimum requirements or fully suspended. Suspended drainage must use adjustable, rigid brackets (not light pre-galvanised strapping) with support at every socket and at appropriate centres. The subchapter includes photographic examples of both unacceptable installations (using flexible strapping) and proper installations (using rigid proprietary brackets), providing clear visual guidance for practitioners.

5.3.18 Testing

This concise subchapter states the fundamental requirement that all foul and surface water drainage systems must be adequately watertight and tested where appropriate. It specifies that inspection and testing should be arranged when required by the local authority, sewerage undertaker, or NHBC. The section advises builders to test systems before backfilling, when visual inspections are still possible. Most importantly, it emphasises that when the home is handed over to the owner, the drainage system must be in full working order and completely free from any obstruction, establishing a clear standard for final acceptance.

5.3.19 Further information

This final subchapter provides a list of reference documents for additional guidance on drainage design and installation. It includes key documents such as BS 6297 (design and installation of drainage fields for wastewater treatment), BS EN 752 (drains and sewer systems under buildings), BRE Reports 211 and 212 (radon protection and construction on gas-contaminated land respectively), BRE Digest 365 (soakaway design), and SFA 7 (Sewer for Adoption guide for developers). These references direct professionals to authoritative sources for more detailed information on specific aspects of drainage systems, complementing the standards contained within the chapter itself.