Groundworks are the part of a building project most homeowners never see once it's finished, but they decide how well everything above them performs. In Cornwall, the ground can change from rock to clay to made-up ground within a single plot, and there are local issues such as old mine workings and radon to consider. This guide explains what groundworks involve, the questions to ask early, and how foundation choices are made for extensions and new builds.
What groundworks include
Groundworks cover everything needed to prepare a site before the building goes up. Depending on the project, that can include:
Site clearance, demolition and removing topsoil
Excavation and levelling, including cutting into sloping sites
Foundations, from strip footings to rafts and piles
Drainage for foul and surface water, including connections, soakaways or treatment plants
Service trenches for water, electricity and data
Ground floor slabs, retaining walls and hardstanding for driveways and paths
Getting this stage right avoids cracking, damp and drainage problems later, and it's often where unexpected costs appear if the ground hasn't been investigated properly.
Ground conditions in Cornwall
Cornwall's geology is varied. Much of the county sits on slates and shales known locally as killas, with granite areas around Bodmin Moor, St Austell, Carnmenellis and West Penwith. Some sites have clay, weathered rock or loose fill, and valley bottoms can be soft or wet. Hard rock can be a good bearing layer but slow and costly to dig through, while softer ground may need deeper or wider foundations.
On many projects a site investigation, such as trial pits or boreholes, is worth doing before the design is finalised. It gives the structural engineer the information to design the right foundations and helps avoid surprises once the diggers arrive.
Mining searches
Cornwall has a long history of tin and copper mining, and old shafts and workings aren't always recorded on modern maps. For many sites, particularly in historic mining areas, a mining search is a sensible early step. If it shows a risk, a mining engineer may recommend further investigation before building. Mortgage lenders and Building Control may ask about this too, so it's best dealt with at the start.
Radon protection
Large parts of Cornwall are in areas where radon, a naturally occurring radioactive gas from the ground, can be present at higher levels. Depending on the location, new homes and extensions may need radon protection, such as a gas-resistant membrane and sometimes a sump or ventilation measures. UK Health Security Agency maps and a radon report can show whether your site is affected, and Building Control will confirm what's required for your project.
Choosing the right foundations
The structural engineer will recommend a foundation type based on the ground, the building's loads and nearby features such as trees and drains. Common options include:
Strip foundations, a continuous concrete strip under the walls, used on firm ground.
Trench fill foundations, where the trench is filled with concrete almost to ground level. These are quicker to build and often used for extensions.
Raft foundations, a reinforced slab spreading the load over a wider area, useful on weaker or variable ground.
Piled foundations, used where suitable bearing ground is deep, on very soft ground or near large trees.
Building Control inspects foundation excavations before concrete is poured, so the programme needs to allow for that.
Drainage and sloping sites
Drainage is a big part of Cornish groundworks. Surface water may go to a soakaway, which needs a percolation test to show the ground can drain, or to a sustainable drainage solution. Where there's no mains sewer, which is common in rural Cornwall, foul water may go to a septic tank or package treatment plant, and these need to meet the Environment Agency's rules.
Sloping plots are common too. They can give great views but usually need careful excavation, retaining walls and drainage to stop water running towards the building. It's worth factoring this in when comparing plots for a new build.
What affects groundworks costs?
Groundworks costs depend on access for machinery, how much soil needs removing, the ground conditions, the foundation type, drainage requirements and whether rock is found. Muck-away and tipping charges can add up on larger sites. Because so much depends on what's underground, a good quote will explain its assumptions and how any unforeseen ground conditions would be handled.
Groundworks FAQs
Do I need a site investigation for an extension?
Not always, but it can be worthwhile where ground conditions are unknown, near trees or on sloping sites. Your engineer can advise.
How long do groundworks take?
For a typical extension, groundworks may take a few weeks. New builds and difficult sites take longer, and wet weather can slow progress.
Will I need a mining search in Cornwall?
It's often recommended, especially in historic mining areas. It's a relatively simple check that can save problems later.
Get your project off to a solid start
Cornish Building Company carries out groundworks and foundations for home extensions, new builds and outbuildings across Cornwall. Call us on 01208 816991 or contact us to talk about your site.
