Groundworks is the term used by builders for the substructural works: the foundations on which the house will sit. In this article, I look at excavations and foundation works only: ground floors and drainage are sometimes included in the blanket definition of groundworks, but they will be discussed later in the series. Like so many areas in construction, groundworks is a hotchpotch of different activities and to understand, and price, groundworks you have to break it down into its component parts and then apply prices to quantities.
The initial work is to clear the site and take off the topsoil from under the house footprint. This can be simple and take just an hour or two, or, especially if sloping ground is involved, it can be a long and expensive process, involving moving muck away in 20 ton lorries.
To estimate the amount of work involved, you need to know the volume of earth to be removed. The area will be the footprint of the house and garage with at least a 2m working space around the footprint, plus any areas of hard standing or driveways. The depth will depend on the lay of the land. On a flat site, you would anticipate removing a minimum of 150mm of spoil, though the top 50mm you may well be able to keep back, if it’s good topsoil. If the site slopes, then you have to clear it so that it is level. The cost for clearing and removing the oversite is usually around £15/m³, made up of around £6/m³: for the JCB, around £6/m³ for the lorry hire and tipping fees and around £3/m³ paid to the tip in Landfill Tax (charged at £2/ton for inert material). On a flat site, with a detached house with a 120m³ footprint plus garage and driveways, this would amount to around 30m³ of excavated material, costing £450-£500 to remove.
If the site slopes, and has to be levelled, then the cost escalates. Levelling a 5° (or 1 in 10) slope could involve removing as much as 200m³ of additional spoil, adding £3,000 to the excavation costs. Estimating the volume is often an imprecise process, but you should be able to work out the rough cross-sectional area by pacing and judging ground by your own height.
A full 120m² basement would result in around 300-400m³ to be cleared: expect it to cost £5,000 upwards, just to excavate and cart away the earth.
Stage two is to set out the foundation trenches — a skilled job involving basic surveying skills and close reference to the plans. Every load-bearing wall in the house has to have a foundation under it.
There is very little material cost involved: just a few pieces of timber and string plus the cost of hiring a good level. However, it takes time to do this job well, and it’s important that it’s done well and carefully because mistakes are always expensive. Most detached houses will require two people for a day to set out.
On a simple site, allow £200-£300 for the setting out of the foundations.
The foundation trenches are then excavated to a minimum depth of 1m below ground level. The trench bottoms are viewed by the building inspector, who may insist on further excavation or other work to minimise the risk of later foundation failure. In extreme situations, you may have to provide an entirely different, engineer-designed foundation system.
The excavation costs are essentially the same as for site clearance. Allow around £15/m³ for earth taken out of the ground and disposed of.
Our 120m² detached house and garage have 93m of foundations, 60m around the perimeter and 33m of load-bearing internal walls. You can work this figure out on your own house with nothing more than a ruler and a calculator. For conventional house shapes, the length of the foundations (in metres) is likely to be around 60-70% of the footprint (in m²). The width of the foundation trenches is normally 600mm, or the nearest digger bucket size, and the depth can be estimated to be just under 1m (although it can be more if deemed necessary). That means that for every 1m run of foundation trenches, you will be pulling out 0.6m³ of spoil. Multiply this figure by the length of foundation trenches (93 x 0.6) and you have 55m³, the minimum you will be required to excavate and cart away.
Excavating and muck away for 55m³ of spoil @ £15/m³ – £825 or nearly £10/m run.
For each 100mm extra you have to dig down, add £1/m run.
Once the trenches have been inspected and passed by the building control officer, ready-mixed concrete needs to be poured as quickly as possible — usually on the same day as the inspection. The concrete levels in the trenches need to be set out (usually done with stakes) and pouring normally happens in one operation over a few hours.
Ready-mixed concrete varies in price across the country from as little as £40/m³ up to £70/m³. It depends partly on how far you are from a plant, and partly on land costs — the south east of England, not surprisingly, has the most expensive ready-mixed concrete. There are several different grades of ready-mix specified – the floor mix is usually stronger than the foundation trench mix – but this doesn’t have such a big impact on price. You can generally work to a budget of around £60/m³. Most builders now choose to fill their trenches with concrete (known as trenchfill), which reduces the amount of footings blockwork. Thus the amount of ready-mixed concrete required is only slightly less than the amount of spoil just excavated from the ground: in our 120m² detached house this equates to about 45m³ of concrete.
The labour for preparing foundations and pouring ready-mix is going to be around three to four days on most sites.
- 45m³ of concrete @ £60/m³ – £2,700.
- Four days labour preparing trenches and pouring ready-mix – £600.
- Each 100mm extra depth of ready-mix will cost around £4/m run of foundations.
The ‘footings’ are the term given to the brickwork and/or blockwork between the foundation concrete and the damp proof course (DPC) level, placed just (150mm) above ground. They are normally laid immediately after the foundation concrete has cured — sometimes the next day after pouring. It is necessary to build the footings before the ground floor is installed. Footings can be built as cavity walls (normal if they are supporting cavity work above), as single skin blockwork (normal if they are supporting room partition walls built up through a slab) or as blocks laid on their side (this is standard with beam and block floors).
The costs of footings vary depending on how they are constructed, but overall the cost of building up footings is remarkably similar to pouring ready-mixed concrete into a trench: footings are cheaper on materials but more expensive on labour. The average cost of footings is around £4/m run of foundation per 100mm of height. Footings are normally set out at multiples of block heights, e.g. 225mm, 450mm or 675mm. Our house has 93m of foundations, and the footings are 450mm high (top of concrete to DPC level). The anticipated cost will be around £1,700 (£4/m of 100mm height blockwork x 4.5 x 93m of foundations). In addition to this, a damp proof layer is installed on top of the footings: this is not an expensive detail, costing around £1/m run, but it is important and needs to be inspected by the building control officer.
On a simple site, the foundation costs can be surprisingly low.
- Site clearance:?£500 or £4/m² footprint
- Setting out:?£300 or £2.50/m² footprint
- Foundation excavation:?£800 or £7/m²
- Trenchfill:?£3,300 or £28/m²
- Footings (short):?£1,700 or £15/m²
- Combined total:?£6,600 or 5% of total
About These Articles
This series is based on a typically constructed, 4/5 bedroom house, with an internal floor area of 200m² (2,150 sq.ft.) plus an integral garage. Its raw build cost (that is only labour and materials) is £135,000. Professional fees, contractors’ overheads and profit, insurances and warranties would all be in addition to this. The raw build cost presumes a straightforward job finished to a fairly basic standard; the sort of finish you would expect from a professional housebuilder. In this series we will be looking at how the costs break down for all the various component parts of a house, and looking at the cost implications of choosing alternatives. This house is also featured in Mark Brinkley’s latest book, The Housebuilder’s Bible.
All costs and prices correct as of April 2005