The right concrete for a foundation depends on what you are building and, just as much, on what is under your garden. Brighton's ground ranges from solid chalk to soft valley deposits and pockets of clay, and each one changes the mix, the depth and the bill. Here is what to expect before you order.
For a typical house extension, porch or garden room, the foundation is unreinforced mass concrete poured into a trench. The two standard designated mixes are GEN1 and GEN3. GEN1 (roughly C8/10 strength) is accepted for simple strip and trench fill footings in non-aggressive ground. Many local builders and engineers specify GEN3 (roughly C16/20) instead, because the extra cost is small and it gives a comfortable margin.
If the foundation contains steel, such as a raft, a reinforced strip over poor ground or pads under a steel frame, you need a reinforced concrete mix, as stated on your structural engineer's drawings. Where the soil contains sulfates, an FND mix is used.
Most of Brighton and Hove sits on chalk, which is a good foundation material. It bears load well and does not shrink and swell with the seasons. On firm chalk, building control will often accept trenches around 600mm to 1m deep, provided the bottom is solid rather than weathered and crumbly.
It is not chalk everywhere. Valley bottoms such as the London Road and Lewes Road corridors hold softer coombe deposits, some hilltops are capped with clay-with-flints, and parts of Hove and Portslade have pockets of clay and brickearth. In clay, depths of 1m or more are normal, and considerably deeper near trees. Clay is also where sulfates are more likely. Older parts of the city can throw up made ground, old cellars and forgotten wells, so expect to keep digging until you reach sound natural ground.
Whatever the drawings say, a building control inspector, from Brighton & Hove City Council or a private approver, must see the open trench before any concrete goes in. Depth and mix are confirmed then, so arrange the inspection ahead of your delivery slot.
Volume is trench length times width times depth. A 4m by 3m rear extension has about 10m of trench on three sides. At 600mm wide and 1m deep, filled to within 150mm of ground level, that is roughly 5 cubic metres. Add about 10 per cent, because trench sides dug through flinty chalk are rarely neat.
Ready-mixed concrete delivered in the Brighton area typically runs somewhere around £120 to £170 per cubic metre, depending on mix, supplier and timing, with FND and RC mixes at the upper end. Loads smaller than a full lorry (usually 6 to 8 cubic metres) often carry a part-load charge, and you pay waiting time if the lorry is kept beyond its allowance, commonly about 30 minutes. For small pours, a volumetric mixer that blends on site and charges for what you use can work out better. Treat these as guide figures and get a written quote.
Access matters as much as the mix. Streets in Hanover, Seven Dials and around Preston Park are largely terraced, often steep, with permit parking and no side access. A full-size mixer lorry may not be able to stop outside, and you may need a parking bay suspension from the council, which has to be applied for in advance.
If the lorry's chute cannot reach, the choices are barrowing through the house, which is slow and hard on floors, or a concrete pump. A pump typically adds a few hundred pounds but can place 5 cubic metres in well under an hour. Tell the supplier you are pumping so they can adjust the mix to flow through the line. Never let anyone add water on site to loosen it, as that weakens the concrete.
Avoid pouring in frost or into waterlogged trenches. Bricklaying can usually start after a few days, though concrete takes about 28 days to reach its design strength.
For a garden wall or shed base, yes. For an extension, ready-mixed is the better choice because you get a consistent, certified mix placed in one pour, and building control may ask to see the delivery ticket.
On firm chalk, often around 600mm to 1m. In clay it is usually 1m or more, and deeper near trees. The final depth is agreed with the building control inspector once the trench is open.
Usually not on clean chalk, but it may be needed in clay or made ground. A soil test settles it, and your engineer or building control will then specify the FND class.
Tell us what you are planning and we will come and take a look. Free, no-pressure quotes across Brighton and Sussex.