Field guide
What Are Groundworks?
See what groundworks include, from excavation to foundations and drainage. Six stages, real project photos and the questions to ask before accepting…
Read articleGroundworks & foundations · Specialist guide
Surrey Clay & Extension Foundations Guide
Start with the ground, not a standard depth
A set trench depth is only a starting point. The approved design must suit the ground, clay that shrinks and swells, the type and distance of trees, groundwater, the existing house and the weight of the extension.
Trees can reach further than you think
On clay, trees that are there now, were removed or will be planted may affect foundation depth and the need to allow for ground swelling. Note the type, rough height and distance early so your designer and Building Control see the same facts.
Old fill and past changes matter
Filled-in ponds, old buildings, old drains and terracing can leave mixed ground. Trial holes or a ground survey can show what you cannot see from the surface.
Know your existing foundations
Where new work meets the house, digging a small hole can show how deep the old foundations are and what state they are in. The joint should allow for old and new moving differently.
Plan drains and digging together
Deep trenches beside working drains or next door’s building raise questions about order and support. Plan drain routes, inspection points and safe digging before machines arrive.
Keep a record
Photos, sizes, notes on the ground, concrete delivery tickets and Building Control records give you a useful record before the foundation is buried.
Before a site visit
Make the survey useful
Five things that let a contractor give you a clearer answer on the first visit, and a more accurate price after it.
Build a site brief- Property postcode
- Photos during the problem
- Approximate measurements
- Access width and route
- Drawings or drain records
Official sources
Check the official guidance
Rules change. Before you decide, check what applies to your property with the source named here, your local council or Building Control.
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Surrey Clay & Extension Foundations Guide
Questions about surrey clay & extension foundations
Straight answers before you commit. If yours is not here, ask us directly.
01Are all Surrey homes built on clay?
No. The ground varies a lot, sometimes within one garden. Geology maps can help early on, but the design should use what is found at the property.
02Will a nearby tree always mean piled foundations?
No. It depends on how much the soil shrinks and swells, the tree, the loads, nearby buildings and your designer or Building Control.
03Who decides the final foundation design?
Your designer and Building Control decide what is acceptable. The builder builds to that design and records the work.
Your own site
Turn the guide into a plan for your property
Use the guide to work out your first questions. Then check them against your own ground, levels, access and drainage.
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Get advice for your own site
Share what you have: photos, drawings and rough sizes. We will explain what should be surveyed, designed or approved before work begins.