Phase 3 · Groundwork · Task 04 of 10
Foundations and Footings for Your Extension
What type of foundations your extension needs, how deep they go, what they cost, and what to expect on dig day.
Foundations are where the money disappears into the ground. Literally. You'll spend thousands of pounds on a hole full of concrete that nobody will ever see again, and if it's wrong, you'll spend thousands more putting it right. The rest of your extension sits on this. Every wall, every roof timber, every kitchen unit. Get the foundations wrong and nothing above them is stable.
The good news: for a typical single-storey rear extension, foundations are straightforward. Your structural engineer designs them, your groundworker digs and pours them, and building control inspects them. Your job is to understand what's happening, know what to check, and have enough contingency in the budget for the surprises that are genuinely common at this stage.
Do this first
Before foundation work begins, several things must be in place. The footprint must be set out and checked before the dig, because the trench is excavated straight to those lines. Your builder should be appointed with a signed contract. Your structural engineer's foundation design must be completed and approved. Building control must be notified (formal notice of commencement, at least two working days before the dig). An LSBUD search must be done to map underground services. Your site should be set up with access for a digger and somewhere to pile spoil. If your extension is near a boundary, your party wall agreement must be completed first.
What this guide covers
- 01Foundation Types: What Your Structural Engineer Will Specify
- 02How Deep Do Foundations Go?
- 03The Structural Engineer's Role
- 04Before the Dig: Trial Holes, Made Ground and Soft Spots
- 05What Happens on Dig Day
- 06The Building Control Inspection
- 07Concrete: What You Need to Know
- 08What It Costs
- 09What Goes Wrong
- 10Curing: The Boring But Critical Bit
- 11What Happens Next
Free guide — just enter your email
This guide is free. Enter your email to unlock it instantly. You'll also get notified when new guides are published.
No spam. Unsubscribe anytime.