Permeable Paving and SuDS: Sub-Base Design, Planning Rules and Drainage

Quick Answer: Permeable paving allows surface water to drain through the surface to a sub-base reservoir, where it either infiltrates the ground or attenuates before piped discharge. Domestic front driveways over 5m² in England must be permeable or drain to a soakaway/landscaped area to avoid planning permission (since October 2008). BS 7533-13 and Approved Document H cover SuDS-compliant construction. Sub-base depth is typically 250–350mm of 4/20mm clean stone for domestic, with a permeable jointing aggregate (2–6mm).

Summary

Sustainable Drainage Systems (SuDS) are now a planning requirement for most new development in England, Wales and Scotland. The principle is to mimic natural drainage by capturing rainfall close to where it falls, attenuating peak flows, and improving water quality before discharge to ground or watercourse. Permeable paving is one of the most useful SuDS components for domestic and commercial driveways because it combines a hard wearing surface with sub-surface attenuation.

For domestic front driveways, the 2008 amendment to the General Permitted Development Order changed the rules: any new or replacement hard surface over 5m² to the front of a house requires planning permission unless it is permeable, or drains to a soakaway, lawn or border within the curtilage. The intent was to slow the loss of urban green space to impermeable parking and reduce flood risk in established areas. The rule remains in force in 2026.

For commercial and larger schemes, the planning authority and Lead Local Flood Authority (LLFA) impose SuDS requirements via planning conditions. The standard reference is the CIRIA SuDS Manual (C753) and BS 7533-13:2009 for the pavement construction. A typical scheme combines permeable paving with sub-base attenuation, optional geotextile, and an outflow control (orifice or perforated pipe to soakaway) to limit discharge to greenfield runoff rates. See block paving for standard block paving construction and soakaway sizing for design of the sub-surface reservoir.

Key Facts

• Permitted Development rule — domestic front driveways >5m² must be permeable or drain to landscaped area; The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class F
• CIRIA C753 — The SuDS Manual; primary design guidance for permeable paving and SuDS
• BS 7533-13:2009 — Code of practice for the structural design of permeable concrete block paving
• Approved Document H — Drainage and waste disposal; SuDS hierarchy requires infiltration first, then watercourse, then sewer
• Permeable block types — concrete block paving (CBP) with widened joints; porous concrete block (open structure); porous concrete or asphalt; gravel grids
• Sub-base material — 4/20mm clean angular stone with no fines; voids ~30% provide attenuation volume; typical depth 250–350mm domestic, 450mm+ commercial
• Jointing aggregate — 2/6mm clean angular grit; sweeps into the joints and laying course; never use kiln-dried sand
• Laying course — 50mm of 2/6mm clean grit (not 0–6mm building sand) — must be permeable
• Geotextile separator — non-woven geotextile (Terram 1000 grade) below sub-base if subgrade is fine grained; prevents migration of fines
• Infiltration rate — measured per BRE Digest 365; clay soils may require partial or no infiltration with piped overflow
• Greenfield runoff rate — typical 1.4 L/s per hectare for design return period; LLFA dictates rate
• Attenuation volume — sized to store the 1-in-100 year storm plus climate change allowance (20–40% depending on planning authority)
• Edge restraint — concrete edging or kerb required to retain blocks; concrete haunch behind kerb prevents lateral spread
• Maintenance — annual sweep to remove organic debris from joints; periodic deep vacuum extraction if surface clogs (every 5–10 years)
• Loading classes — BS EN 1338 markings; domestic driveway typically Class T3, light commercial T4

Quick Reference Table

Detailed Guidance

Step 1: Site survey and infiltration test

Before specifying permeable paving, check the subsoil infiltration capacity. BRE Digest 365 sets out the standard test: dig a trial pit 0.3–1.0m deep, fill with water to a known depth, measure drop over 1 hour after presoak. Infiltration rate is calculated in m/s. Typical values:

• Sand/gravel: 1 × 10⁻³ m/s or better — full infiltration suitable
• Sandy loam: 1 × 10⁻⁴ to 1 × 10⁻³ m/s — full infiltration possible
• Clay loam: 1 × 10⁻⁵ to 1 × 10⁻⁴ m/s — partial infiltration, may need piped overflow
• Heavy clay: < 1 × 10⁻⁶ m/s — tanked construction with piped outflow to soakaway

Check water table. SuDS designs need ≥1m clear between the base of the sub-base reservoir and the seasonal high water table.

Step 2: Hydrological design

Calculate runoff for the design storm. For domestic schemes, the LLFA may accept a simplified calculation; for commercial, a hydrological model (Microdrainage, InfoDrainage) is standard.

Runoff calculation (simplified, 100m² driveway, 1:100 year + 30% climate change):
  Rainfall intensity (1h, 1:100): ~50 mm/h
  Adjusted for climate change:    65 mm/h
  Runoff over 1 hour:             100 × 0.065 = 6.5 m³
  Attenuation volume needed:      6.5 m³ minimum
  Sub-base voids @ 30%:           6.5 / 0.30 = 21.6 m³ of sub-base
  Over 100 m² at 250mm depth:     25 m³ ✓ adequate

For partially infiltrating designs, model the time-volume curve and check storage adequacy.

Step 3: Excavate to formation

Strip topsoil and excavate to formation level. Inspect subgrade — soft spots reworked or replaced with compacted Type 1 (only for tanked designs). For full infiltration, the subgrade must not be over-compacted (max 3–5 passes of light roller) — heavy compaction destroys infiltration capacity.

Provide a 1:60 fall to the formation if any piped overflow is included.

Step 4: Geotextile separator

Lay a non-woven geotextile (Terram 1000 or equivalent) on the subgrade. The geotextile prevents fine soil particles migrating up into the sub-base voids, which would clog the system over time. Overlap joints 300mm minimum.

For tanked designs, an impermeable membrane (1mm HDPE or similar) replaces the geotextile and lines the excavation with welded joints, forming a watertight tank.

Step 5: Sub-base

Place 4/20mm clean angular stone in 150mm layers, compact each layer with a light vibrating plate (not a heavy roller — over-compaction reduces voids). Avoid material with fines (Type 3 modified, MOT Type 3) — only clean stone provides reliable voids.

Compaction target: 95% Maximum Dry Density at OMC. Test with sand replacement or nuclear density gauge for commercial schemes.

Step 6: Laying course

50mm of 2/6mm clean grit, screeded level. Do not over-compact — the laying course must remain permeable.

Critical: never substitute building sand or 0–6mm grit. The fines block the joints and the system stops working within 12 months.

Step 7: Lay blocks

Permeable CBP has either chamfered/widened joints or distance pieces that create wider gaps than standard paving (typically 6–10mm joint width vs 3mm for standard). Lay to manufacturer pattern (typically stretcher bond or herringbone) on the laying course, with the wider joints aligned.

Step 8: Joint filling

Sweep 2/6mm jointing aggregate into joints. Re-sweep multiple times — the joints must be fully filled to the top, with the aggregate locked in. Test by plate vibration — re-sweep again.

Step 9: Edge restraint

Concrete kerb or edging set in C20 concrete bed and haunched to 100mm height behind the kerb on the soil side. The edge restraint must extend below the sub-base level to confine the entire pavement.

Step 10: Commissioning and handover

Test infiltration by simulating rainfall (10 L over 1m² takes <5 minutes to disappear). Provide the homeowner with maintenance guidance:

• Sweep autumn leaves promptly (organic debris is the main joint clogger)
• Avoid kiln-dried sand or other fine material near the surface
• Inspect joints annually; re-sweep with 2/6mm grit if low
• Vacuum-extract joints every 5–10 years if surface infiltration noticeably reduces

Frequently Asked Questions

Do I need planning permission to repave a front driveway?

Not if the new surface is permeable, or if water runs off into a permeable landscaped area within the curtilage (lawn, planting bed, gravel border). If the surface is impermeable (standard block paving, asphalt, concrete) and is over 5m², and there is no on-plot drainage to a soakaway or permeable area, planning permission is required.

What's the difference between permeable and porous?

Permeable paving allows water through the joints — the blocks themselves are solid. Porous paving (porous concrete, porous asphalt) allows water through the material itself. Both are SuDS-compliant if specified correctly. Permeable CBP is more common and easier to install reliably; porous concrete needs careful mix design and surface texture.

Can I install permeable paving on clay?

Yes — but the design must include piped overflow to a soakaway or surface water sewer because the clay subgrade cannot infiltrate at a useful rate. The sub-base still provides attenuation, slowing the discharge. Lined "tanked" designs with controlled outflow are common for clay sites.

Will permeable paving clog up?

Joints can clog with organic debris (leaves, moss, soil tracked from gardens) over 5–10 years, reducing surface infiltration. Annual sweeping prevents most clogging. If the surface does clog, vacuum extraction with a road-sweeping vacuum truck can restore infiltration without lifting the blocks. The sub-base reservoir itself almost never clogs if the geotextile and laying course were correctly specified.

Can heavy vehicles drive on permeable paving?

Standard 80mm permeable CBP supports cars, vans and light commercial vehicles. For HGV traffic (loading bays, commercial yards), structural design per BS 7533-13 is essential, with a thicker sub-base (450mm+) and confirmed bearing capacity. Heavy point loading (skip wagons, fire tenders) may require an alternative pavement type.

What about EV charging cable across the paving?

Permeable surfaces are compatible with surface-laid EV charging cables run to a cable cover or shallow conduit. The cable does not affect drainage. Note that EV charging point installation may also need separate Part P notification — see domestic installation.

Regulations & Standards

• BS 7533-13:2009 — Code of practice for the structural design of permeable concrete block paving

• BS 7533-1 to 12 — Pavements constructed with clay, natural stone or concrete pavers

• BS EN 1338 — Concrete paving blocks; class T markings for traffic loading

• Building Regulations Approved Document H — Drainage and waste disposal; SuDS hierarchy

• Approved Document Part R — Physical infrastructure for high-speed electronic communications networks (where chambers cross paving)

• CIRIA C753 — The SuDS Manual (2015 revision); primary design reference

• BRE Digest 365 — Soakaway design and percolation testing

• Town and Country Planning (General Permitted Development) (England) Order 2015 — Schedule 2 Part 1 Class F (front driveways)

• Flood and Water Management Act 2010 — Schedule 3 (SuDS); enacted in Wales; partial enactment in England

• Sewers for Adoption (Water UK) — adoption requirements where SuDS connects to public sewer

• National Planning Policy Framework — Paragraph 165: major development to include SuDS unless impractical

• CIRIA SuDS Manual C753

• Approved Document Part H drainage

• Planning Portal — paving over front gardens

• Interpave permeable paving guidance

• BRE Digest 365 Soakaway design

• block paving — standard block paving construction

• soakaway sizing — soakaway design per BRE Digest 365

• resin bound gravel — alternative permeable surface

• driveway planning permission — when planning is needed

• suds sustainable drainage — SuDS overview

• domestic installation — EV charging point installation