Ordering ready mix concrete for a home extension is one of those jobs where a small mistake costs real money. Order too little and you have a half filled trench and a truck driving away. Order too much and you pay for concrete that ends up as a lump at the end of the drive.
This guide shows how to work out the volume for foundations and a floor slab, how much extra to allow, and when to book so the delivery fits around your builder and building control.
What decides how much concrete an extension needs
Three things set the volume: the foundation design, the floor slab and the ground itself.
Your structural engineer or the approved drawings will state the foundation type, width and depth. Most extensions use strip or trench fill foundations, and the depth depends on the soil, nearby trees and drains. Do not guess these figures from a neighbour’s job. Use the drawings.
The floor slab is the second part. Its thickness is also on the drawings, and it is often somewhere around 100 to 150 mm over a compacted sub base. If the plans include a ground beam or a thickened edge, add those separately.
How to calculate the volume
The basic sum is length multiplied by width multiplied by depth, all in metres. The answer is in cubic metres, which is how ready mix concrete is sold.
Here is an example for a single storey extension that is 4 metres wide and projects 5 metres from the house. Treat the numbers as an illustration only, because your own depths will come from your design.
| Part | Working | Volume |
| Trench fill foundation on three new sides | 14 m long by 0.6 m wide by 1.0 m deep | 8.4 cubic metres |
| Floor slab at 100 mm | 4 m by 5 m by 0.1 m | 2.0 cubic metres |
| Total before allowance | Both parts combined | 10.4 cubic metres |
With a sensible allowance added, an order of around 11.5 cubic metres would be reasonable for this example.
Why you should order a little extra
Trenches are never perfectly neat. Digging goes slightly wider in places, the base is uneven, and some concrete is lost to spillage and the last of the load stuck in the drum. An allowance of roughly 5 to 10 percent is a common rule of thumb, and many builders lean towards the higher end for foundations.
Running short is usually worse than having a barrow load spare. A second delivery for a tiny amount can mean a short load charge, a wait, and a join in the concrete you did not plan for. If you do end up with leftovers, a path base or a hardstanding patch is a sensible place to use them.
Large orders and multiple trucks
A mixer truck carries a limited load, often in the region of 6 to 8 cubic metres depending on the vehicle, so confirm this with your supplier. In the example above, the foundations alone would need more than one truck.
When more than one load is needed, the trucks should arrive close together. The first load must not begin to set before the second goes in, or you risk a weak join. Ask for a delivery rhythm when you book, and have enough people on site to place each load as it arrives.
When to book ready mix concrete for an extension
Foundations are the pour most people book first, and the timing has a few moving parts.
The trench has to be dug and, in most cases, inspected by building control before any concrete goes in. Ask your inspector how much notice they need, because it is usually short but not zero. Book the concrete only once the inspection date is firm, then confirm the order the day before.
A reasonable sequence looks like this:
- Final drawings and volume agreed with your builder.
- Concrete supplier contacted several days ahead with a provisional date.
- Building control inspection arranged for the same morning, or the day before.
- Order confirmed the day before, with a named contact on site.
- Pump booked as well if the truck cannot get close to the trench.
The reason to pour soon after inspection is simple. Open trenches can collapse, flood or turn to mud after rain, and any of those means more digging and a fresh inspection.
Same day and urgent concrete delivery
Sometimes plans change. A trench passes inspection earlier than expected, or a pour comes up short and you need a top up. That is when people look for same day concrete delivery.
It can work, but it depends on plant capacity, truck availability and how early you call. Standard mixes in modest quantities have the best chance. A large foundation pour for an extension is a poor candidate for a rushed booking, because several trucks need to be lined up.
Urgent ready mix concrete and emergency concrete delivery are worth asking about if something has gone wrong on site, but no supplier can fairly promise a time before they know your postcode, quantity, mix and access. If you want to book ready mix concrete same day, ring first thing in the morning and have your volume ready.
My honest advice is to plan for next day and treat same day as a bonus. A foundation pour is the wrong job to leave to chance.
What to sort out before the truck arrives
Ready mix concrete delivery goes much more smoothly when the site is prepared. Check these before the driver turns up.
Start with access. Look at gate width, overhead cables, soft ground and where the truck will stand. A fully loaded mixer is heavy and can damage a drive or a verge, so ask your supplier about its weight.
Next, think about how the concrete will reach the trench. If it is far from the road, a pump or a barrow run needs organising, and barrowing a large volume over a long distance takes more labour than most people expect.
Finally, name one person on site to direct the driver, agree where to discharge and accept the delivery ticket. Have shovels, rakes and a level ready, and make sure the crew is in place before the truck arrives.
Conclusion
Getting the right amount of ready mix concrete for a home extension starts with the drawings, a simple volume calculation and a sensible allowance for waste. Booking is easier if you line it up with your building control inspection, confirm the day before and plan for more than one truck on bigger pours.
If you have your measurements and a provisional date, contact RMS Concrete and ask what delivery options are realistic for your job.