What’s Causing This?

Poor sub-base compaction (most common): When a driveway is installed without adequate compaction of the hardcore or MOT Type 1 sub-base, the material gradually settles under the weight of vehicles. This creates depressions that worsen over time. See our guide to Driveways & External work for how a correctly built driveway should be constructed.

Drainage failure and water erosion: If water cannot escape from beneath the driveway surface — due to blocked drainage channels, missing edge restraints, or an impermeable surface laid without adequate falls — it saturates the sub-base and washes fine particles away. This leaves voids that cause the surface above to drop. A sunken driveway near a lawn or border may also be linked to waterlogged ground conditions affecting the whole area.

Tree root movement: Roots from nearby trees can lift sections of a driveway, but as roots die back or are cut, they decompose and leave voids. The surface then drops into those gaps. This is especially common with block paving or concrete slabs close to established trees.

Underground pipe failure or void: A collapsed or leaking drain, old culvert, or disused inspection chamber beneath the driveway can create a void large enough to cause significant sinking. This is a structural issue requiring investigation before any surface repair. See our guide on repairing a cracked driveway, which often accompanies subsidence from voids.

Clay soil shrinkage or heave: In areas with heavy clay subsoil, prolonged dry spells cause the ground to shrink, removing support from the sub-base above. Conversely, wet winters can cause heave. This is a longer-term cause that tends to produce gradual, widespread settlement rather than a single localised dip.

How to Diagnose the Exact Cause

1. Locate all sunken areas precisely. Use a long spirit level or straight timber batten across the driveway surface to identify every dip. Mark the edges with chalk. Note whether depressions are isolated spots, run along a line, or form a broad shallow bowl.
2. Check for surface cracking patterns. Radial or spiderweb cracks suggest a void or point load collapse beneath. Parallel cracks running along the driveway suggest differential settlement — typically sub-base failure on one side. Fine map-cracking across the whole surface is more likely surface wear than structural sinking.
3. Inspect drainage channels, gullies, and edges. Clear any debris from drainage channels and pour water along the driveway to check it flows freely to an outlet. If water pools in the sunken area and does not drain, blocked or absent drainage is likely contributing.
4. Check for nearby trees or stumps. Look for tree roots within 3–5 metres. Probe the sunken area with a metal rod — if you hit a void at shallow depth (under 150 mm), decomposing root matter or a collapsed pipe is probable.
5. Locate underground services and drains. If the sunken area follows a straight line and is near a manhole cover, a drain run beneath may be the cause. Lift the nearest inspection chamber lid and check for evidence of collapse or root ingress inside. If the chamber is structurally damaged or the issue is near shared drainage, stop and contact a drainage specialist.
6. Assess the scale of settlement. A dip of under 25 mm over a small area (under 1 m²) on block paving or gravel is almost always a DIY repair. A dip of 50 mm or more, a large affected area, or any sign of a void deeper than 200 mm warrants professional investigation before you attempt any repair.

How to Fix It

Poor sub-base compaction: Excavate the sunken area, remove the existing surface material, add and compact fresh MOT Type 1 hardcore in 75–100 mm layers using a vibrating plate compactor, then re-lay the surface. For block paving, individual blocks can often be lifted, the sub-base repaired beneath, and blocks re-laid. See our guide on pointing a block paved driveway once blocks are re-laid.

Drainage failure: Re-establish falls on the surface (minimum 1:60 gradient), clear or install drainage channels at low points, and ensure water discharges to a suitable soakaway or drain. On tarmac, small sunken areas caused by drainage issues can sometimes be cold-patched once drainage is resolved — see Repair Tarmac Cracks for surface patching technique.

Decomposed tree roots: Excavate the affected area, remove all root material, backfill with compacted hardcore and sharp sand, then re-lay the surface. If the tree is still living, consider installing a root barrier before reinstating the driveway.

Underground pipe void: A drain specialist must inspect and repair the pipe before you reinstate the surface. Do not fill a void caused by a collapsed drain without repairing the drain first — the void will return. Once drainage is repaired, backfill and compact in layers before re-laying the surface finish.

Gravel driveways: Sunken areas in gravel are the most straightforward fix — rake existing gravel aside, top up the sub-base if needed, and add fresh angular gravel. Our Lay a Gravel Driveway guide covers correct installation depth and materials.

When to Call a Tradesman

Call a qualified drainage contractor if you suspect a collapsed or leaking underground drain is causing the sinking. Attempting to fill the area without repairing the drain is dangerous and ineffective. Under the Water Industry Act 1991 and subsequent Private Sewer Transfer regulations, responsibility for drains beneath driveways can sit with the householder or the water company depending on whether they serve one property or multiple — a drainage surveyor can clarify this before you spend money on repairs.

Call a structural engineer or specialist ground investigation company if the sunken area is large (over 3 m²), if the dip is deeper than 50 mm and continuing to worsen, or if there is any evidence of voids deeper than 200 mm when probed. Significant subsidence in clay soils near trees may also require a specialist report, particularly if you intend to make an insurance claim.

Note that replacing or substantially altering a driveway with an impermeable surface (concrete, tarmac, or sealed paving) on the front of a house in England requires planning permission unless adequate drainage is provided — check with your local planning authority first. Permeable surfaces such as gravel or permeable block paving are generally permitted development. See guidance on Driveways & External planning requirements before starting work.

How to Prevent It Happening Again

• Specify and compact a proper sub-base: A minimum 100 mm depth of well-compacted MOT Type 1 hardcore is required for standard domestic driveways; 150 mm for anything carrying heavier vehicles. Never skip the plate compactor — hand tamping is not sufficient for vehicle loads.
• Ensure surface water drainage from the start: Build in a minimum 1:60 fall across the driveway surface and install edge channels at low points. Check that gullies and soakaways are clear and functioning before laying any new surface.
• Keep drainage channels and gullies clear: Inspect and clear driveway drainage channels at least once a year — autumn leaf fall is the main cause of blockages. A blocked outlet means standing water, which accelerates sub-base erosion.
• Manage nearby trees carefully: Avoid planting large-canopy trees within 5 metres of a driveway. If removing an established tree near a driveway, have a drainage inspection carried out before reinstating any surface, as root channels can create direct paths for water ingress beneath the sub-base.