The short answer
Groundworks usually account for around 10–15% of a typical UK build budget, whether that is an extension or a new house on reasonable ground. On a difficult site — shrinkable clay, a steep slope, made-up ground or anywhere needing piling or retaining structures — that share can rise to 20–25% or more, while on a small, level, free-draining plot it can be nearer 8–10%. The percentage varies because groundworks cost depends almost entirely on conditions below ground, which are invisible from the surface. The foundations and floor must meet Building Regulations Part A, and on a new home, NHBC or equivalent warranty standards.
People often ask what slice of a build groundworks take, usually to sanity-check a quote or set a budget. The honest answer is a range rather than a single figure, because the same house can need very different foundations on different plots. The sections below explain the typical share, why it moves, and how to use it for budgeting.
At a glance
- Typical shareAround 10–15%
- Easy, level plotNearer 8–10%
- Difficult / piled site20–25%+
- Most variable elementFoundations
- Budget contingency10–15% on groundworks
The typical share, and why it is a range
For most projects, groundworks fall in the 10–15% band of the total build cost. The reason this is a range rather than a fixed number is that the visible part of a building — walls, roof, windows, kitchen — costs roughly the same regardless of the plot, but the foundations supporting it can cost wildly different amounts depending on the soil. Two identical houses, one on firm gravel and one on shrinkable clay near trees, can have groundworks bills that differ by a factor of two or three, which shifts the percentage substantially.
| Site difficulty | Indicative groundworks share | What drives it |
|---|---|---|
| Easy, level, firm ground | 8–10% | Shallow trench-fill, no retaining |
| Typical site | 10–15% | Standard foundations + drainage |
| Sloping plot | 15–20% | Cut and fill, retaining walls |
| Clay / poor ground | 18–22% | Deep or reinforced footings |
| Piled foundations | 20–25%+ | Engineered solution |
Indicative percentages for guidance only. Actual share depends on plot conditions and design.
What the groundworks share covers
When people quote a groundworks percentage, they usually mean the whole substructure package: site clearance, excavation, foundations, the ground-floor slab or beam-and-block floor, and below-ground drainage, up to the point the bricklayers start above damp-proof course. That bundle is fairly consistent in scope, which is why the percentage is a useful rule of thumb even though the cash figure varies.
Where the percentage misleads is when scope is defined differently. If a quote stops at oversite and excludes drainage or the slab, its share looks artificially low; if it includes external works, retaining walls or service connections, it looks high. So when comparing your project against the 10–15% benchmark, it is worth checking that you are comparing the same scope. A groundworks figure that looks unusually low may simply have less in it, with the missing items reappearing later as separate costs. The most reliable comparison is always a line-by-line breakdown rather than a single percentage.
Why the share rises on harder sites
The groundworks share climbs whenever the ground forces a more engineered solution. Shrinkable clay, especially within influencing distance of trees, demands deeper foundations under NHBC and Building Control rules — more dig, more concrete, more spoil to remove. Sloping plots need cut-and-fill to create a level building platform, often with retaining structures and far more muck away. Made-up ground, peat, old foundations or a high water table can rule out a conventional footing entirely and require piling or a raft, which can multiply the foundation cost.
Drainage also pushes the share up where the ground will not accept a soakaway and a SUDS-compliant attenuation system is needed, or where the connection point is distant. Spoil disposal rises with dig depth and any contamination. Because all of these are below-ground unknowns, they are precisely the costs that surface during groundworks and rarely later, which is why this stage carries the most budget risk. A plot that looks identical to its neighbour from the road can sit on completely different ground, so the percentage is best treated as a guide that a site investigation then confirms or revises.
The 10–15% rule holds for both extensions and new houses, but the reasons it lands where it does differ. On an extension, the groundworks are a smaller absolute figure but can be a slightly higher share, because the rest of the project — tying into an existing structure, matching brickwork, a relatively small roof — is comparatively cheap, so the foundations and floor take a bigger slice. Awkward access at the back of a house also pushes the groundworks share up, since spoil and concrete are often handled by hand.
On a new build, the groundworks are a larger absolute cost but sit against a much bigger total, so they tend toward the middle of the band on a normal plot. Where new builds diverge is on difficult ground: because the whole building's load bears on the foundations, a poor-ground new build needing piling or a raft sees its groundworks share climb faster than an equivalent extension would. The practical takeaway is that the percentage is a useful starting point for either, but the absolute figure should always come from a measured quote against a real design. A share that looks normal can still hide an expensive plot, and a share that looks high may simply reflect honest pricing of difficult ground rather than an inflated quote.
Using the percentage to budget
The practical value of the 10–15% figure is as an early sanity check before you have detailed quotes. If you are planning a £300,000 new build, groundworks of roughly £30,000–£45,000 are a sensible expectation on a normal plot; a quote far outside that band is worth questioning, in either direction. A figure well below it may have excluded drainage or the floor; a figure well above it usually signals difficult ground that a soil survey will explain.
Beyond the headline share, the wise move is to hold a contingency specifically against the groundworks stage, because it is the part of the project most likely to overrun. A foundation that has to go deeper after Building Control inspects the open trench, a failed percolation test that forces an attenuation system, or an unrecorded drain across the footprint are all common and rarely visible until work starts. Budgeting an extra 10–15% on top of the quoted groundworks figure absorbs most of these surprises. Once you have a structural engineer's foundation design and a soil report, the percentage stops being a guess and the quote becomes the figure to plan around — but until then, 10–15% of the total build is the most reliable rule of thumb.
Frequently asked questions
Are groundworks the biggest cost in a build?
No — the superstructure, roof and internal fit-out usually cost more. But groundworks are the least predictable element, typically 10–15% of the total, and the part most likely to overrun because the cost depends on what is below ground.
Can groundworks ever exceed 20% of a build?
Yes. On a difficult plot needing piling, retaining structures or deep foundations on shrinkable clay, groundworks can reach 20–25% or more of the total build cost, even though the finished building above ground looks no different.
How do I budget for groundworks before I have quotes?
Use 10–15% of your total build budget as an early estimate, then hold a contingency of around 10–15% on that figure for below-ground surprises. Replace the estimate with a measured quote once you have a soil survey and foundation design.
Sources & further reading
- Checkatrade — groundworks cost guide
- Planning Portal — Building Regulations
- NHBC — foundations and ground conditions
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.