If you're planning an extension, new build, or major renovation, you'll hear the term "groundworks" a lot. It's the essential first stage of construction — but what does it actually include?
The Short Answer
Groundworks are all the work that happens below ground level before the main construction begins. This includes:
- Site preparation: Clearing vegetation, levelling the ground, removing existing structures
- Foundations: Strip foundations, trench fill, piled foundations
- Drainage: Foul water, surface water, soakaways, septic tanks
- Slabs and floors: Ground-bearing concrete slabs
- Services: Trenching for water, gas, electric, and communications
Why Groundworks Matter
Everything built above ground depends on what's below it. Poor groundworks lead to:
- Settlement: Cracks in walls and floors as the ground compacts unevenly
- Damp: Water pooling around foundations, rising through walls
- Drainage failures: Blocked or collapsed drains, flooding
- Structural issues: Walls shifting, doors sticking, roofs sagging
Groundworks aren't visible in the finished project, but they determine whether your extension lasts 5 years or 50.
Site Preparation
Before any construction starts, the site needs to be ready:
- Vegetation removal: Trees, shrubs, grass, roots
- Topsoil stripping: The organic layer (topsoil) can't support foundations, so it's removed and stored for later landscaping
- Leveling: Creating a flat working surface
- Demolition: Removing existing structures (conservatories, garages, sheds)
Foundations
The most critical part of groundworks. Foundations transfer the weight of your building to stable ground.
Types of Foundations
| Type | Best For | Cost | Depth |
|---|---|---|---|
| Strip foundations | Standard houses, good ground | £100–£150/m | 0.9–1.2m |
| Trench fill | Poor ground, heavy loads | £150–£200/m | 0.9–1.2m |
| Piled foundations | Vey poor ground, near trees | £300–£500/m | Variable |
| Raft foundations | Low bearing capacity | £200–£300/m² | Shallow |
What Affects Foundation Choice
- Soil type: Clay expands and contracts; sand drains well; rock is stable but hard to excavate
- Trees: Clay soil near trees needs deeper foundations (2m+) to prevent movement
- Water table: High water tables require drainage before foundations are poured
- Nearby structures: Party walls, neighbouring foundations, services
Drainage
Managing water is essential. There are two main types:
Foul Water Drainage
Water from toilets, sinks, showers, and washing machines. Must connect to the main sewer or a septic tank.
Surface Water Drainage
Rainwater from roofs and hard surfaces. Can discharge to:
- Main sewer (if capacity allows)
- Soakaway (a pit filled with rubble that allows water to drain into the ground)
- Watercourse (with permission)
Common Drainage Issues
- Clay soil: Doesn't drain well — soakaways need to be larger or use crate systems
- Combined sewers: Older systems carry both foul and surface water; adding surface water can overload them
- Existing drains: Building over or near existing drains requires permission from the water company
Ground Slabs
After foundations and drainage, the ground floor slab is poured. This is typically:
- 100mm concrete on
- 150mm hardcore on
- 50mm sand blinding on
- Damp-proof membrane
For extensions, the slab must connect to the existing house properly — same level, properly sealed, insulated to current standards.
Services Trenching
Before the slab is poured, trenches are dug for incoming services:
- Water supply pipe
- Gas pipe (if required)
- Electricity cable
- Communications (broadband/phone)
These need to be at specific depths and may need ducting for future access.
How Long Do Groundworks Take?
| Project Type | Typical Duration |
|---|---|
| Small extension (20m²) | 1–2 weeks |
| Large extension (40m²) | 2–3 weeks |
| New build house | 4–8 weeks |
Groundworks can take longer if:
- Ground conditions are poor (clay, high water table)
- Existing structures need demolition
- Services need to be diverted
- Weather delays excavation
How Much Do Groundworks Cost?
Groundworks typically account for 15–25% of the total build cost. For an extension:
- Site preparation: £500–£2,000
- Foundations: £3,000–£8,000
- Drainage: £1,000–£3,000
- Slab: £2,000–£4,000
Total for a 25m² extension: £6,000–£17,000 depending on ground conditions.
Why Choose DNDH for Groundworks
DNDH Construction has completed groundworks across Cambridgeshire — from clay-heavy areas in Cherry Hinton to the chalk soils of Trumpington. We understand local ground conditions and can anticipate issues before they become expensive surprises.