Artificial Grass Ground Preparation
The base is the part of the job you never see and the part that decides everything. Get the excavation, aggregate and levelling right and the lawn stays flat and free-draining for years.
Ground preparation is the unglamorous half of every artificial grass project, and the half that separates a lawn that still looks crisp in a decade from one that dips, ripples and holds water within a season. Before a single roll of grass is unpacked, the ground beneath it has to be excavated, built back up in compacted layers and levelled to a precise, free-draining finish.
This page explains how a proper sub-base is built and why each layer is there. If you want the full fitting service from first dig to final brush-up, see our installation page; for how water is managed through and around the base, read our guide to drainage.
Built in layers
MOT Type 1 compacted in shallow lifts, not dumped in one go, so the whole depth consolidates and will not settle later.
The right depth
Around 75 to 100mm of excavation for a domestic lawn, deeper where heavier loads or soft, peaty ground demand it.
Falls for drainage
A gentle, consistent fall screeded into the levelling layer so surface water runs away rather than pooling.
How a sub-base is built, layer by layer
A well-made base is a sequence of layers, each doing a specific job. From the bottom up, a typical domestic build-up looks like this:
- Excavation: the existing turf, topsoil or surface is dug out to roughly 75 to 100mm for a standard garden, with more for driveways, play areas or soft ground that needs a deeper, stronger base.
- Sub-base (MOT Type 1): crushed graded aggregate laid and compacted in layers of around 75mm. Compacting in shallow lifts with a vibrating plate is what gives the base its strength; a thick layer rammed once simply traps loose stone underneath.
- Levelling (laying) course: a thin blanket of sharp sand or granite dust, usually 10 to 25mm, screeded smooth and to the correct falls. This is the surface the grass actually sits on, so it must be flat and true.
- Weed membrane: a geotextile layer, often laid on top of the levelling course (and sometimes at the base of the excavation too), with joints overlapped by about 100mm so nothing pushes through the seams.
Edgings such as timber, composite, treated battens or a concrete haunch are fixed around the perimeter so the grass has something firm to be pinned or glued to and the base cannot spread.
Why compaction and depth matter
Nearly every base failure comes down to one of two shortcuts: not enough depth, or not enough compaction. A base that is too shallow has no structural mass and moves under foot traffic; one that is laid in a single deep layer never fully consolidates, so it settles unevenly and the lawn develops dips and hollows.
Compacting MOT Type 1 in thin lifts, wetting it lightly to help the fines bind, and finishing with a flat, firm levelling course is slow work, and it is exactly where a cut-price quote tends to save time. A base you cannot see is easy to skimp on and impossible to put right later without lifting the whole lawn.
MOT Type 1, sharp sand or granite dust?
These materials are not interchangeable, and each has a role:
- MOT Type 1 is the structural sub-base. It is a crushed, graded stone that locks together and compacts to a hard, stable platform while still letting water through.
- Sharp sand makes a good levelling course on most domestic jobs, screeded over the Type 1 to take out the last imperfections.
- Granite dust (grano) is a popular laying-course alternative that screeds to a very smooth, firm finish and is often preferred for a crisp, flat result.
What you should never use is soft builder's sand, which holds water and will not hold a level, or soil left in place as a shortcut. For a deeper look at materials, see our article on the best base for artificial grass.
Preparing different ground types
The ideal build-up changes with what you are starting from:
- Clay or waterlogged ground: often needs a deeper excavation, a more generous aggregate layer and drainage designed in from the outset, covered in detail on our drainage page.
- Existing concrete or paving: if sound and free-draining, it can serve as the base with a shockpad over it rather than being dug out. If water pools on it, that has to be resolved first.
- Soft, sandy or made-up ground: may need extra depth or a geogrid to stop the aggregate sinking into it over time.
Benefits and considerations
Benefits
- A flat, true surface with no dips, hollows or ripples
- A base that drains freely rather than holding puddles
- No settlement later, because the aggregate is compacted in layers
- Weed growth suppressed from below by an overlapped membrane
- Firm perimeter edging the grass can be fixed to securely
- A foundation sized to how the lawn will actually be used
Worth considering
- Groundwork is labour-intensive and the single biggest reason a good quote costs more than a cheap one
- Waterlogged or frozen ground can delay digging and prevent proper compaction
- Excavated spoil has to be removed and disposed of, which adds skip or haulage cost
- Narrow or stepped access means materials are barrowed by hand, increasing time and labour
How we prepare the ground
Each site is different, but the order of a sound groundworks job rarely changes.
Assess & set out
We check levels, access and how the ground drains, then mark out the area and the depth to dig.
Excavate & clear
The old surface is dug out to the required depth and the spoil is removed from site.
Edgings fixed
Perimeter edging is installed to contain the base and give the grass a firm fixing line.
Lay & compact the sub-base
MOT Type 1 goes down in shallow layers, each one compacted with a plate before the next.
Screed the levelling course
Sharp sand or granite dust is screeded smooth and to the correct falls for drainage.
Lay the membrane
A weed-control geotextile is laid with overlapped joints, ready for the grass to go on top.
What affects groundwork cost
Ground preparation is usually the largest labour element of an installation, which is why fully fitted lawns in the UK commonly land between around £50 and £90 per square metre, rising to roughly £65 to £110 in London and the South East. The groundwork share depends heavily on the factors below.
Key cost factors
- The depth of excavation needed and the amount of spoil to remove
- Ground condition, since clay, soft or waterlogged ground needs more work
- The volume of MOT Type 1 and levelling material required
- Any drainage work designed into the base
- Site access for machinery, aggregate deliveries and skips
- The size and shape of the area, as small or intricate spaces cost more per m²
Frequently asked questions
For a standard domestic lawn, excavating around 75 to 100mm leaves room for a compacted MOT Type 1 sub-base and a levelling course. Driveways, play areas or soft, peaty ground need to go deeper so the base can carry more load without moving.
It is not advisable. Soil holds water, settles unevenly and lets weeds through, so grass laid directly onto it soon dips and ripples. A compacted aggregate base is what keeps the surface flat and free-draining.
Compacting MOT Type 1 in shallow lifts lets the whole depth consolidate properly. Tip in one thick layer and only the top compacts, leaving loose stone beneath that settles later and causes dips in the finished lawn.
Ground preparation is building the structural base: excavation, aggregate and levelling. Drainage is how water is moved through and away from that base, using permeable layers, falls and features such as soakaways. They are planned together; see our drainage page for the detail.
Yes. A geotextile membrane laid over the levelling course, with joints overlapped by around 100mm, suppresses weeds from below while still letting water drain through. Most quality installations also benefit from a membrane at the base of the excavation.
Related services and guides
Want the base done properly?
Tell us about your site and we will quote the groundwork and installation in full, with the sub-base specified clearly and in writing.