Extension foundations are designed to suit the ground conditions and the structure, and are checked by building control before concrete is poured. This guide covers the practical side of ordering concrete for the footings and the floor slab that often follows.
Follow the design and building control
Foundation width, depth and concrete grade are set by the engineer, architect or building control. Book the building control inspection of the excavated trenches before you book the concrete, and have the specified grade to hand when you enquire.
Working out the volume
Trench length × width × concrete depth in metres. For example, 18 m of trench, 0.6 m wide, filled to 1.0 m deep is 10.8 m³. Trench sides are rarely perfectly straight, so allow extra – or consider volumetric concrete, which is mixed on site.
Access on residential streets
Extensions are often at the back of occupied homes. Measure the route from the road to the trenches and tell us about parked cars, narrow side passages, and anything the vehicle must not drive over. A concrete pump can place concrete into rear trenches without barrowing.
Staged pours
Footings, ground floor slab and screed are usually poured on different days. List each stage and its approximate volume so a supply plan can be agreed. For floor finishes see screed, liquid screed and underfloor heating screed.
Trench-fill concrete
Where trench-fill foundations are specified, a flowing concrete such as Payneflow may be worth discussing, as it requires less compaction. Use it only where it meets the specification.
On the day
- Trenches inspected and approved
- Water pumped out of trenches
- Level pegs set
- Labour ready to place the concrete
See also domestic concrete and our concrete cost guide.
Frequently asked questions
What concrete is used for extension foundations?
The grade is set by the design or building control. Send it with your enquiry.
Can you pump concrete to rear-garden footings?
Yes, pumping can be arranged for difficult-access pours, subject to the site.
Should I order extra concrete for footings?
Trenches are rarely exact, so an allowance is sensible. Volumetric supply avoids paying for unused concrete.