Garden drainage and sloping gardens in Kent

Updated 9 November 2026 · 11 minute read · General guidance only: check your own project with your council, Building Control and a qualified professional.

Why Kent gardens get waterlogged: clay, chalk and compaction

The RHS identifies the usual causes of a waterlogged garden: poorly drained or compacted soil, a high water table, and hard surfaces that stop rain soaking in. All of them are common in Kent.

• Clay: the Weald Clay of the Low Weald, around Headcorn, Marden, Tonbridge and Paddock Wood, and the London Clay along the north Kent coast, including Sheppey, Whitstable and Herne Bay, drain slowly. The RHS describes clay as wet in winter and baked dry in summer.
• Chalk: much of Thanet, the Dover and Deal area and the Kent Downs sit on chalk, which usually drains freely. Water can still pond where chalk is capped by clay-with-flints or the ground has been made up.
• Low-lying land: Romney Marsh and parts of the river valleys of the Medway, Stour and Darent have high water tables, so water may have nowhere to soak into.
• Compaction: new-build gardens, ground crossed by diggers during building work, and lawns used as paths all compact the soil, which then stays wet after rain.
• Run-off: water from patios, roofs and neighbouring higher ground can all end up on the lowest part of a lawn.

The British Geological Survey's free geology viewer shows the rocks under your area, though a landscaper digging a trial hole will tell you more about your own garden.

First checks: where the water comes from and where it can go

Before anyone digs a drain, it pays to understand the problem. The RHS's advice on waterlogged lawns makes the key point plainly: drainage is only an option if there is somewhere for the water to go, such as a ditch, drain or soakaway.

1. Watch the garden in heavy rainNote where water comes from: a downpipe, the patio, a neighbour's slope or the soil itself. Take photos.
2. Dig a test holeDig a hole about a spade deep in the wettest spot after rain. Water sitting near the surface suggests a high water table or very heavy clay. Grey or mottled soil is a sign of periodic waterlogging.
3. Try simple fixes firstFor a compacted lawn, the RHS suggests aerating with a hollow-tined aerator, ideally 10 to 15 cm deep, then brushing in a free-draining top dressing such as horticultural sand. On clay, hollow tines work better than solid spikes.
4. Find an outletIdentify where water could legally go: a soakaway on suitable ground, a ditch you own, or, with consent, a surface water sewer.

Soakaways and percolation testing

A soakaway is a pit or trench, filled with rubble or lined with perforated units or plastic crates, that stores water and lets it soak into the ground. For roof and paved areas that form part of building work, Part H of the Building Regulations sets an order of priority for rainwater: first a soakaway or other infiltration system, then a watercourse, and only then a sewer.

Approved Document H gives guidance that is a sensible benchmark for garden soakaways too:

• Percolation tests should be carried out to find out how quickly the soil absorbs water. The test in Approved Document H uses a 300 mm square hole, filled and left overnight, then refilled and timed as it drains, repeated at least three times in at least two holes.
• The test shouldn't be done in abnormal weather such as heavy rain, severe frost or drought.
• Soakaways shouldn't be built within 5 metres of a building or road, in unstable ground, or where the water table reaches the bottom of the soakaway at any time of year.
• Soakaways should be designed for a storm with a return period of once in ten years. Approved Document H says soakaways serving larger areas should be designed to BS EN 752 or BRE Digest 365, the Building Research Establishment's soakaway design method.

On heavy Kent clay, percolation tests often show that a soakaway won't work, or would need to be very large. That isn't a failure of the test; it is the answer. Approved Document H notes that where the ground is marginal, an overflow can be acceptable. On chalk, soakaways often work well, but made-up ground and nearby buildings still need care, so take local advice.

French drains, land drains and swales

Where water needs moving rather than storing, a drain carries it from where it collects to somewhere it can be dealt with.

• French drain: a gravel-filled trench, usually with a perforated pipe at the bottom wrapped in or lined with geotextile, that intercepts water and carries it away. It suits the low edge of a lawn or the foot of a slope.
• Herringbone land drains: a pattern of perforated pipes under a lawn, feeding a main drain. They are a bigger job, usually only needed for large or very wet lawns.
• Channel or slot drains: a grated channel along the edge of a patio or path that catches surface water.
• Swales: the RHS describes swales as shallow, planted or grassed channels that collect water and drain it slowly. They suit gentle slopes in larger gardens and can be attractive features.

Every one of these needs an outlet. Ask the landscaper where the pipe will discharge, how it will be accessed for cleaning, and what happens in a prolonged wet spell when the ground is already saturated.

Permeable paving and rain gardens

Sustainable drainage, often called SuDS, means dealing with rain close to where it falls, slowing it down and letting it soak in, rather than piping it away as fast as possible. In a garden, two of the most useful techniques are permeable paving and rain gardens.

Permeable paving, such as permeable block paving, gravel or porous surfaces on a free-draining base, lets rain soak through. Approved Document H describes the stone layer underneath as a storage reservoir that holds peak flows while water soaks into the subsoil, and says it can be used with an impermeable lining and outlet where the ground won't take water. For front gardens, the 5 square metre rule on hard surfaces is covered by our sister site Kent Resin Services.

A rain garden is a shallow, planted dip that takes water from a downpipe or a patio and lets it soak away over a day or so. The RHS gives practical guidance:

• Site it in a reasonably well-drained area on a very gentle incline, of 10 per cent or less.
• Test the ground by digging a hole about 25 cm deep, filling it, refilling it once it drains and timing it. The RHS gives an ideal drainage rate of at least 1.25 cm per hour.
• As a guide, make the rain garden about 20 per cent of the roof area that drains into it.
• Make sure overflow water at the lowest point can reach a drain or another part of the system.
• If it would be closer than 5 metres to the house, the RHS suggests getting advice from a geotechnical specialist first.

Terracing a sloping garden

Kent has plenty of sloping gardens, from the hillsides around Folkestone, Chatham and Rochester to plots on the scarp of the North Downs. Terracing turns a slope into usable level areas, linked by steps or paths. It is one of the most rewarding garden projects, but also one of the most involved.

• Cut and fill: most terraces cut into the higher ground and use the spoil to build up the lower ground. Balancing the two reduces muck-away, but filled ground needs proper compaction.
• Drainage: water runs down a slope and collects behind walls and on terraces. Each level needs a fall and an outlet, and walls need drainage behind them.
• Access: steps should have even risers and treads, and handrails where the drop is significant. Think about how a mower, wheelbarrow or wheelchair will move between levels.
• Planning: the Planning Portal says raising a garden level by a small amount is unlikely to need planning permission, but increases of over 30 cm may, because they can affect drainage and neighbours. Decking or platforms more than 30 cm above the ground also need permission under permitted development rules.
• Neighbours: raising ground near a boundary can cause overlooking and push water towards the lower property.

A scaled plan that shows existing and proposed levels is worth paying for on a sloping site. It lets landscapers price the same scheme and shows where walls, steps and drains go.

Retaining walls in the garden

A retaining wall holds back soil, and the soil pushes against it all the time, more so when it is wet. Garden retaining walls are built in brick, block, stone, timber sleepers, gabions or concrete, and the right choice depends on height, soil, loads above and appearance.

• Ask for a structural engineer's design for anything other than a low wall, and for any wall that retains a drive, a path near a building, a neighbour's land, or soil on a steep slope.
• Expect drainage behind the wall, such as free-draining backfill, a perforated pipe and weep holes, so water doesn't build up behind it.
• Timber sleeper walls are popular for low raised beds and small level changes, but they rot over time and have limited strength.
• A retaining wall close to a road that holds back a significant height of ground can need the highway authority's approval under section 167 of the Highways Act 1980.

Our sister site Local Builders Kent covers garden walls, boundaries and the structural side of retaining walls in more detail. If an existing retaining wall is leaning, bulging or cracking, have it looked at promptly rather than adding more soil or load behind it.

Surface water and your neighbours

Water flowing between gardens is a common source of disputes. The law in this area is complex and depends on the facts, so treat what follows as general background rather than legal advice.

• Broadly, land lower down a slope is expected to receive water that flows naturally from higher land.
• Problems tend to arise when someone changes things, for example by paving a large area, raising ground levels, building a terrace or piping water so that more of it, or more concentrated flows, reach a neighbour's land. In that situation the person who made the change may be liable for the damage it causes.
• If a ditch or stream runs through or alongside your garden, GOV.UK guidance says you must let water flow naturally and may have to pay damages if you obstruct it and cause flooding to other people's property. Works on watercourses can need consent from the Environment Agency, the lead local flood authority, which is Kent County Council or Medway Council, or an internal drainage board.

The practical approach is to deal with water within your own garden, keep it away from the boundary, and talk to neighbours before changing levels near a shared fence. If a dispute develops, take advice from a solicitor or chartered surveyor.

Checking flood risk before changing levels

Before raising ground, building a terrace on low land, filling a ditch or adding large areas of paving, check the flood risk for your address. The GOV.UK long-term flood risk service shows the risk from rivers and the sea, surface water, reservoirs and, where data is available, groundwater.

• Surface water flood risk is often the most relevant for gardens, because it shows where rainwater collects in heavy storms.
• Romney Marsh, the Hoo Peninsula, Sheppey and parts of the Medway, Stour and Darent valleys include low-lying land where flood risk needs particular care.
• Raising levels on land at risk of flooding can push water elsewhere, including onto neighbours' property, so ask your council before doing it.
• If your home is in a flood risk area, think about keeping air bricks and door thresholds clear of new paving, and about where water would flow in an extreme storm.

Landscapers who work locally often know which streets suffer from standing water. When you ask for quotes, mention any flooding or drainage history, and ask each landscaper how their design deals with it.

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Sources

• GOV.UK: Approved Document H, drainage and waste disposal (Sections H2 and H3): www.gov.uk/government/publications/drainage-and-waste-disposal-approved-document-h
• RHS: Waterlogging and flooding solutions: www.rhs.org.uk/soil-composts-mulches/waterlogging-flooding
• RHS: Waterlogged lawns: www.rhs.org.uk/lawns/waterlogging
• RHS: Rain gardens: www.rhs.org.uk/garden-features/rain-gardens
• RHS: Swales: www.rhs.org.uk/gardening-for-the-environment/swales
• Planning Portal: Do I need planning permission to raise my garden level?: interactive.planningportal.co.uk/faqs/do-i-need-planning-permission-to-raise-garden-level
• GOV.UK: Guidance on the permeable surfacing of front gardens: www.gov.uk/government/publications/permeable-surfacing-of-front-gardens-guidance
• GOV.UK: Owning a watercourse: www.gov.uk/guidance/owning-a-watercourse
• GOV.UK: Check your long term flood risk: www.gov.uk/check-long-term-flood-risk
• legislation.gov.uk: Highways Act 1980, section 167: www.legislation.gov.uk/ukpga/1980/66/section/167
• British Geological Survey: Geology Viewer: www.bgs.ac.uk/map-viewers/bgs-geology-viewer/