Ready-Mix Concrete C25: The Standard Grade for UK Extension Foundations

Your builder orders 5m³ of concrete for a 4m x 6m extension foundation. The truck arrives, pours it in, and leaves. Two weeks later the building control officer asks to see the delivery ticket. Nobody kept it. The BCO asks what grade was poured. Nobody knows. Now you're in a conversation about whether the foundation is compliant, and the only way to prove it is to core-drill through your new concrete and send samples to a lab. That costs over £500 and delays your build by weeks.

This is a completely avoidable problem. The concrete grade for your extension foundation is specified by your structural engineer and checked by building control. It's almost always C25/30, designated RC 25 under BS 8500. Knowing what that means, how to order it correctly, and what paperwork to keep is the difference between a smooth foundation pour and an expensive headache.

What C25/30 concrete is

C25/30 is a strength rating. The "C25" means the concrete achieves a minimum compressive strength of 25 N/mm² (newtons per square millimetre) when tested as a cylinder after 28 days of curing. The "/30" is the same concrete tested as a cube, which gives a slightly higher reading. Same concrete, two test methods, two numbers. When people say "C25 concrete," they mean C25/30.

For domestic extension foundations in normal ground conditions, C25/30 is the standard minimum grade specified under Approved Document A (the building regulations covering structural loading). It's strong enough to support a single-storey or two-storey extension on most UK soil types. Your structural engineer specifies the exact grade on their calculations, and your BCO checks the delivery ticket against that specification.

The designation confusion (and how to cut through it)

UK concrete has multiple naming systems that overlap and contradict each other. Supplier websites, forum posts, and even some builder's merchants use different names for the same product. This catches homeowners out regularly.

Here's the mapping that matters:

The RC (Reinforced Concrete) and FND (Foundation) designations are the modern ones under BS 8500. The ST (Standardised Prescribed) mixes are older and have higher embodied carbon because they specify more cement than necessary. Ready-mix suppliers with QSRMC or BSI Kitemark certification will know exactly what RC 25 means. If a supplier looks blank when you say "RC 25," use a different supplier.

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Ground conditions change everything

RC 25 is the standard for normal ground. But if your site has sulphate-bearing soils (common in parts of the Midlands, East Anglia, and along river valleys), your structural engineer will specify a different designation:

• AC-1 (low sulphate): RC 25 with standard cement. No change.
• AC-2 (moderate sulphate): FND 2. Same C25/30 strength but uses sulphate-resisting cement.
• AC-3 (high sulphate): FND 3. Sulphate-resisting Portland cement (SRPC).
• AC-4 (very high sulphate): FND 4. Higher strength (C35/45) with specialist cement.

You won't know your sulphate class without soil investigation. Your structural engineer or BCO will tell you. Don't guess. The wrong cement type in sulphate ground degrades over decades, weakening the foundation long after the guarantee has expired.

How much you need

Foundation concrete volumes depend entirely on whether your builder is doing strip foundations or trench fill. The difference in concrete quantity is enormous.

Strip foundations use a shallow layer of concrete (minimum 150mm deep for single-storey, 225mm for two-storey) at the bottom of the trench, with blockwork built up from there. They use less concrete but more blockwork and labour.

Trench fill floods the entire trench with concrete to within 150mm of ground level. It uses three to four times more concrete than strip foundations for the same footprint, but it's faster (the whole foundation goes in during a single pour) and is now the more common method for domestic extensions.

Worked example: 4m x 6m single-storey rear extension

Assume a perimeter of approximately 20m (including any internal walls), a trench width of 600mm, and a minimum trench depth of 1m for clay soil:

Strip foundation: 20m x 0.6m x 0.225m = 2.7m³ net concrete, plus blockwork from foundation to ground level.

Trench fill: 20m x 0.6m x 0.85m (filling to 150mm below ground level) = 10.2m³ net concrete.

That's the difference. Strip foundations for a typical single-storey extension need 3-5m³ of concrete. Trench fill for the same extension needs 7-12m³. Your builder's quote should tell you which method they're using, and the concrete volume should match.

What to say when you call the supplier

When you phone to order, you need to give five pieces of information:

1. Grade: RC 25 (or C25/30). If your structural engineer has specified FND 2 or similar, say that instead.
2. Volume: In cubic metres, rounded up to the nearest 0.5m³ with 10% waste allowance.
3. Consistence class: S3 for strip foundations. S4 for trench fill. Consistence (sometimes still called "slump") describes how fluid the concrete is. S3 is stiffer and holds its shape; S4 is more fluid and flows into deep trenches. Get this wrong and trench-fill concrete that's too stiff won't fill the voids at the bottom of the trench.
4. Aggregate size: 20mm is standard for foundation work.
5. Delivery date and time: Book only after your BCO has confirmed they've inspected and approved the trenches.

Building control and the foundation pour

The foundation pour is a formal hold point in the building control inspection sequence. This means your BCO must inspect the excavation before any concrete goes in. Pour first, call later, and you have a serious problem: building control cannot retrospectively certify a foundation they didn't inspect. The worst case is a compulsory break-out.

The sequence works like this: notify your BCO at least 24 hours before you plan to pour. The BCO visits, checks the trench depth and width, inspects the soil conditions, looks for tree roots nearby (which can require deeper foundations), and signs off. Only then do you phone the concrete supplier to confirm delivery.

Don't book the concrete truck before the BCO has confirmed approval. BCOs discover unexpected conditions during their inspection (nearby trees requiring deeper trenches, soft ground needing wider footings) and these can require design changes that delay the pour by days.

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Do you need a pump?

Most rear extension pours need a concrete pump. The reason is access: a standard ready-mix truck is 8m long and needs to park within 3-4m of the trench for its chute to reach. If your extension is at the back of the house with only a side passage, the chute won't reach.

A line pump (a ground-level pump with a flexible hose) costs £275–800 and handles most domestic pours. The pump company coordinates with the concrete supplier so the truck and pump arrive together. A typical 5-8m³ pour takes under an hour with a pump.

Without a pump, you're looking at wheelbarrows. A cubic metre of concrete weighs approximately 2,400kg. That's roughly 30 barrow loads per cubic metre, each weighing 80kg. For a 5m³ strip foundation pour, that's 150 barrow runs. For a 10m³ trench fill, it's 300. The concrete starts setting within 90 minutes of mixing. You can see the problem.

The pump hire adds to the cost, but for any pour over about 3m³ or any site with restricted access, it's the only sensible option.

Cost and where to buy

C25/30 ready-mix is ordered directly from concrete suppliers, not from builders' merchants. The four national suppliers (Tarmac, Hanson, CEMEX, and Breedon) between them have over 500 batching plants across the UK. Independent regional suppliers are typically 8-12% cheaper.

Orders below 4m³ attract a short-load surcharge of £40–150 because the truck runs below capacity. For small strip foundation pours where you need 3-4m³, a volumetric mixer is worth considering. Volumetric trucks mix concrete on site from raw materials, charge only for what you use, and don't apply short-load surcharges. They're certified to BS 8500 and accepted by building control.

What a typical extension foundation costs in concrete

For a 4m x 6m single-storey rear extension:

• Strip foundation: 3-5m³ at £90–130/m³ per m³ = £270-650 for concrete alone
• Trench fill: 7-12m³ at £90–130/m³ per m³ = £630-1,560 for concrete alone
• Add pump hire if needed: £275–800
• Add short-load surcharge if ordering under 4m³: £40–150

Trench fill costs roughly three times more in concrete than strip foundations. But trench fill is faster to pour (one day vs several days of blockwork) and many builders prefer it because it reduces their on-site time. The extra concrete cost is often offset by lower labour costs.

Weekend and out-of-hours deliveries attract a premium of approximately 20% or £30-80 extra. Plan for a weekday morning delivery.

Ready-mix vs hand-mixing: don't even think about it

For foundation work, ready-mix or volumetric concrete is the only acceptable option. Hand-mixing with a cement mixer cannot demonstrate compliance with BS 8500 to building control. You can't prove the mix ratio, the water content, or the cement quality. BCOs reject hand-mixed structural concrete. Structural engineers won't certify it.

The community consensus on this is unanimous. Forum threads across BuildHub and DIYnot consistently advise against hand-mixing for foundations. One experienced member summed it up: hand-mixing relies on labourers being accurate with "volumes and measurements and quality control," and the evidence of mix strength simply isn't there.

Ready-mix with QSRMC or BSI Kitemark certification comes with a delivery ticket that proves the grade, batch, and compliance. That delivery ticket is your evidence for building control. It's also your insurance if anything goes wrong later.

Curing: the bit everyone forgets

Concrete doesn't dry. It cures. The chemical reaction between cement and water (hydration) generates heat and builds strength over time. C25/30 reaches its rated strength at 28 days, but it needs at least 7 days before any load is placed on it.

Temperature matters. Below 5°C, the curing reaction slows dramatically. Below 2°C, it can stop entirely. If ice crystals form in fresh concrete, they can cause up to 50% strength loss. Official LABC guidance is clear: don't pour on frozen ground, don't let fresh concrete drop below 5°C, and protect it with insulated blankets or polythene sheeting in cold weather. Major ready-mix suppliers offer heated-water mixes for cold-weather pours, but the simplest approach is to plan your foundation pour for a mild week.

Alternatives

C20/25 (GEN 3): Lower strength and lower cost. Suitable for non-structural uses like blinding (a thin layer at the bottom of the trench to give a clean working surface) and unreinforced paths or bases. Not recommended for load-bearing foundations. Some older guidance quotes C20 as acceptable, but current practice and most BCOs expect C25/30 as the minimum for extension foundations. Don't save £10/m³ and risk a building control dispute.

C30/37 (RC 30): Higher strength. Required where the structural engineer specifies reinforced concrete (with rebar or mesh), or for foundations near trees where deeper, narrower piles are used. Costs approximately £5-10/m³ more than C25/30. Only use if your engineer's calculations specify it; there's no benefit to over-specifying.

Common mistakes

Ordering the wrong designation. If you phone a supplier and ask for "ST2 concrete for foundations," you'll get a weak mix (roughly C8/10) that isn't suitable for structural foundations. The correct order is RC 25 or C25/30. Check the delivery ticket when the truck arrives.

Calculating volume without waste allowance. Foundation trenches are never perfectly uniform. The walls bulge, the base is uneven, and some concrete splashes or overflows. Add 10% to your calculated volume and round up to the nearest 0.5m³. The extra half-cubic-metre costs £50-65. Running short costs thousands.

Pouring before the BCO inspects. This is the single most expensive mistake on this page. A foundation poured without building control inspection cannot be retrospectively approved without destructive testing. Notify your BCO 24 hours before and wait for their sign-off before booking the concrete truck.

Splitting the pour across two days. Concrete foundations must be poured in a single continuous operation. A cold joint between two pours creates a structural weakness. If you run short, you need rebar dowels across the joint and BCO involvement. Order enough concrete to finish in one go.

Ignoring the consistence class. Ordering S2 (stiff) concrete for a trench-fill foundation means the concrete won't flow into the bottom of deep, narrow trenches. Ordering S5 (very fluid) risks segregation, where the aggregate separates from the cement paste. S3 for strip, S4 for trench fill. Tell the supplier which foundation type you're doing and they'll get it right.

Where you'll need this

• Foundations and Footings - the primary use for C25/30 concrete in any extension project