Ordering concrete for a home extension can feel like the trickiest part of the whole project, mainly because there’s so much to get right before the first lorry turns up. Get the concrete for house foundations wrong, whether that’s the mix, the volume or how it’s delivered to site, and you’re looking at delays, extra cost or a foundation that isn’t up to the job.
The good news is that none of this is complicated once you know what questions to ask. Before you can think about mix ratios or delivery logistics, though, there’s one decision that shapes everything else: what type of foundation your extension needs.
What Type of Foundation Does Your Extension Need?
Most home extensions in the UK use one of two foundation types:
- Strip Foundations
- Raft Foundations
Strip foundations are the most common choice for straightforward extensions on stable ground, formed by digging a trench around the perimeter and filling it with concrete to create a continuous strip the walls sit on.
Raft foundations spread the load across the whole footprint using a flat, reinforced slab, and they’re typically used where ground conditions are poor or a trench dig isn’t practical. Deciding between the two depends on your soil, your extension’s size, and what your engineer or building control surveyor recommends when specifying concrete for home extensions:
- Strip Foundations: Cost-effective, quicker to install, suited to stable ground.
- Raft Foundations: Better for poor ground, spreads the load evenly.
Ultimately, ground conditions and extension size will decide which type you need.
Getting Your Concrete Specification Right
Once you know your foundation type, the next job is nailing down the extension foundation concrete specification, meaning the exact mix and strength your engineer has designed for the job. Here’s how that breaks down for each type.
Strip Foundation Concrete Mix
For most domestic strip foundations, a standard mix such as C20 or C25 is specified, giving enough compressive strength for typical extension loads without over-paying for the concrete. Your engineer’s drawings will state the exact mix, but it’s worth knowing this yourself so you can check the supplier is delivering what’s been designed. Trench depth and width matter just as much, and building control will usually inspect the trench before any concrete goes in.
Raft Foundation Mix Requirements
Raft foundations generally call for a stronger mix, often C30 or above, because the concrete slab is doing more structural work and usually includes reinforcing steel mesh throughout. Any home extension concrete foundation here needs careful placement in one continuous pour wherever possible, since cold joints (where one pour meets a later one) can weaken the raft. This is one area where planning your delivery matters just as much as the mix itself.
Working Out Your Concrete Volume
To work out your concrete volume for extensions, multiply the length, width and depth of the foundation trench or slab in metres, which gives you a figure in cubic metres. And always add a small margin, typically five to ten per cent, to allow for uneven ground or trench sides that aren’t perfectly straight.
Your builder or engineer should confirm the final figure, since getting this wrong by even a small amount can mean an awkward second delivery.
How Site Access Affects Your Delivery Plan
Concrete for house foundations only does its job if it can get from the lorry to the trench or slab in good time, and this is where a lot of extension projects hit an unexpected snag. A standard concrete lorry needs reasonable width, a firm surface and enough turning space to get close to the pour, which many rear and side extensions simply don’t have.
Narrow side passages, gravel drives that can’t take the weight, low-hanging trees or overhead cables can all rule out a straightforward tip-and-pour delivery. This matters for residential concrete delivery because concrete starts to set from the moment it’s mixed, so any delay caused by access problems eats into your working time on site. Getting a proper look at your access before the pour date, rather than on the day itself, saves a lot of stress.
Why Concrete Pumping Suits Tight Residential Sites
This is exactly the kind of problem solved by a concrete pump. For home extensions, this means that, rather than relying on the lorry itself reaching the foundation, a trailer or line pump can sit on the road or driveway and pump the concrete through a hose, getting it precisely where it needs to go even if that’s down a narrow side return, over a fence, or into a back garden with no vehicle access at all.
It also means concrete for house foundations gets placed steadily and continuously, which matters for raft foundations especially, rather than being carried by hand in wheelbarrows, which slows the pour down and increases the risk of the mix starting to go off before it’s fully placed. For most tight residential extensions, concrete pumping isn’t a luxury, it’s simply the practical way to get the job done properly.
Ready-Mix Concrete vs Site-Mixed: What’s Right for an Extension
Ready-mix concrete for home extensions is the sensible choice over mixing on site for most projects. A ready-mix lorry delivers concrete that’s been batched precisely to the specified mix at a plant, so you know exactly what strength you’re getting and there’s no risk of a labourer getting the ratios wrong with a site mixer.
Site-mixing can work for very small pours, such as a single fence post base, but for anything the size of a foundation trench or raft slab it quickly becomes slow, inconsistent and genuinely hard to keep up with once you factor in mixing time against setting time. The only real question when considering concrete for house foundations at this scale is how that ready-mix concrete gets from the lorry to the pour, which brings you back to access and delivery method.
Questions to Ask Your Builder or Concrete Supplier Before Work Starts
Before any concrete for house foundations arrives, it’s worth running through a short list of questions with whoever’s supplying or placing it. This is a conversation we have with customers every day, and it heads off most problems before they happen.
Questions to ask include:
- What mix and strength is specified for my job? Confirm this matches your engineer’s drawings, whether it’s a strip foundation concrete mix or a raft specification, before the order goes in.
- How will the concrete reach the foundation? Ask whether the site suits a standard lorry or whether pumping will be needed for residential concrete delivery on a restricted plot.
- What’s the volume being ordered, and is there a margin built in? A small buffer avoids the cost of a second delivery.
- Who’s responsible if access turns out tighter than expected on the day? Get this agreed upfront, not once the lorry can’t get near the trench.
Get Your Extension’s Concrete Sorted With 2 Brothers Concrete & Pumping
Getting concrete right for a home extension comes down to the right foundation, an accurate specification, and a delivery method that suits your site. If your driveway’s narrow or your garden’s hard to reach, that’s where we come in, with years of experience getting concrete cleanly into awkward sites using our trailer, line and boom pumps.
To get started, give 2 Brothers Concrete & Pumping a call on 01489 552737 or fill in our contact form, and we’ll make sure your next order goes off without a hitch.