Foundations are priced from the ground, not from the drawing
Every other element on a building can be measured from a plan. Foundations cannot, because what matters is under the ground and nobody drew it.
That is not a reason to estimate blindly. It is a reason to estimate in a specific order: ground investigation, then foundation type, then excavation, then the concrete and the rest. Skip the first step and every number after it is a guess dressed as a calculation.
This guide covers the order, the quantities, and the allowances that separate a foundation estimate that holds from one that turns into a variation before the concrete arrives.
Ground investigation before quantities
Three levels of investigation, in increasing cost and usefulness:
A desk study: geology maps, previous site use, and any record of mine workings, made ground or contamination. Cheap, and it rules out the worst surprises.
Trial holes: dug by machine or by hand to expose the sub soil at the depth of the proposed foundation. A morning's work for three or four holes, and it tells you the bearing stratum and the water table.
Boreholes and testing: for larger buildings, poor ground or where a raft or piles might be needed. A real cost, and a real saving when it changes the foundation type.
For a domestic extension, trial holes are almost always proportionate and are frequently skipped. That skip is how a strip foundation at 1.0 m depth becomes a 2.4 m trench through made ground, with a concrete volume that nobody priced.
Foundation types and how each is measured
Type
Measured by
When used
|---|---|---|
Strip, traditional
Length x width x depth, plus concrete and brickwork up to DPC
Good ground, domestic walls
Trench fill
Length x width x depth, concrete to near ground level
Good ground, faster than strip
Pad
Count x dimensions
Columns, steel frames
Raft
Area x thickness, plus edge thickening
Poor ground, or where load spreads
Piles
Count x depth, plus pile caps and beams
Deep bearing stratum
The choice is the engineer's, and it changes the quantity by a large factor. A trench fill foundation for a house wall might use 0.6 cubic metres of concrete per linear metre. A raft for the same wall on soft ground uses more concrete and far more reinforcement, and it needs the whole floor area prepared.
Excavation: the three numbers people forget
Length comes off the drawing. Width is the foundation width plus working space, typically 300 mm each side for a strip. Depth comes from the trial hole, not from a default.
Then three allowances:
Working space. An excavator bucket needs room. A 600 mm foundation in a 600 mm trench cannot be blinded, reinforced or inspected properly.
Over excavation. Machines do not cut a flat bottom. Allow 50 to 100 mm and make it up with blinding concrete.
Bulking. Excavated soil expands by 20 to 30 percent, so the volume you dig is not the volume you remove. A 20 cubic metre excavation is 24 to 26 cubic metres in a lorry or a skip.
Worked example. A rear extension, 6.0 m by 4.5 m, strip foundations 600 mm wide at 1.0 m deep, perimeter run of 21 m plus an internal spine of 4.5 m:
Total foundation length = 25.5 m
Trench width = 0.6 + 0.6 working space = 1.2 m
Excavation volume = 25.5 x 1.2 x 1.0 = 30.6 m³
Add 75 mm over excavation = 2.3 m³, so 32.9 m³
Bulked for removal at 1.25 = 41 m³ of spoil, which is six to eight lorry loads or a lot of skips
Blind concrete at 50 mm across 30.6 m² = 1.5 m³
Foundation concrete: 25.5 x 0.6 x 0.25 = 3.8 m³, plus 5 percent = 4.0 m³
Reinforcement: A142 mesh across the foundation width, 25.5 x 0.6 = 15.3 m², plus laps = 17 m², so two sheets
Damp proof course at the wall line: 25.5 m, plus laps
The excavation is eight times the volume of the concrete. That ratio is normal, and it is the reason groundwork is where a domestic extension budget usually breaks.
Concrete, and the waste that actually happens
Concrete is ordered in cubic metres, and the waste is not a percentage of the order, it is the difference between what you order and what you use. Over order and you pay for a part load to go back. Under order and you have a cold joint and a second delivery charge.
The practical allowance is 5 percent on calculated volume, plus a decision on whether to order a small surplus deliberately. On a 4 m³ foundation, ordering 4.2 m³ is sensible. On a 0.5 m³ footing, ordering a minimum load of 1 m³ is what actually happens, and the price per cubic metre rises sharply for small loads.
Where concrete is mixed on site, the arithmetic changes: cement, sand and aggregate by weight, plus a mixer, water and labour. On small domestic jobs this is often cheaper and slower, and it is worth pricing both.
Reinforcement in foundations
Foundation reinforcement is usually mesh, measured by area with laps of one full mesh pitch. For a strip foundation the mesh runs along the length at the foundation width, and it needs chairs or spacers to hold it at the correct cover.
For a raft or a pad, the reinforcement may be a designed cage, and then it is a bar schedule, measured by weight as described in the reinforcement guide. The estimating trap is pricing a raft as if it were mesh.
Laps on mesh are 10 to 20 percent of the area depending on the sheet size relative to the foundation width, and forgetting them is the most common reinforcement shortfall on groundworks.
Damp proofing, tanking and the layer between
Damp proof course in the wall at 150 mm above ground level, measured in linear metres
Damp proof membrane under a ground bearing slab, by area with laps
Tanking for a basement or a room below external ground level, by area with all the detail items
Radon or gas membrane where the area requires it, by area, with sealing and a sump
Each of these is a thin layer with a large consequence and a small price per square metre. The detail items, meaning laps to walls, pipe penetrations and terminations, are where the cost and the failure both live.
Backfill, hardcore and compaction
Backfill around the foundation returns material to the trench, but not the same material. Excavated clay is difficult to compact and should not be used under a floor slab. Granular fill, hardcore or type 1 is imported and compacted in layers.
Volumes are measured after compaction, and the loose volume needed is 20 to 30 percent more. Compaction is a plant cost: a plate or a roller, and time.
Under a floor slab, hardcore is typically 100 to 150 mm, blinded with sand, with a membrane over it. That is another set of quantities that belongs in the groundworks section rather than in the floor.
Spoil, contamination and the cost of a surprise
Removing spoil is a real cost, and the price depends on what it is. Clean sub soil is cheap to tip. Clay is heavier and sticks. Made ground may be contaminated, and contaminated soil is a different category entirely, with testing, a licensed destination and a much higher rate.
Two practical safeguards. Keep a waste classification in the estimate as a stated assumption rather than an absolute, and check whether any of the spoil can be used on site for landscaping, which avoids both the transport and the tipping fee.
On a tight urban site, the cost of removing spoil can exceed the cost of the concrete. It is not a rounding error.
Water, weather and the working conditions
Groundwater changes everything. A trench that fills with water needs pumping, and a foundation poured into standing water is a foundation that fails.
Allow for a pump, for a sump, and for the time to keep a trench dry. On sites with a high water table, the excavation may need sheeting to stop the sides collapsing, which is a temporary works cost and a safety requirement.
Weather also matters in a way that is specific to the season. Winter earthworks in clay are slower, the spoil is heavier, and backfill is harder to compact. A programme that places groundworks in February in a wet climate is a programme that will slip.
Productivity and plant
A mini excavator with one operator digs 20 to 40 cubic metres a day in good ground, less in clay or where access is tight. Hand excavation is 2 to 4 cubic metres a day, which is why hand dug foundations are only used where a machine cannot reach.
A two person gang places and compacts 10 to 20 cubic metres of concrete a day for strip foundations, faster with a pump and slower where the trench is deep and narrow.
The plant hire line is a fixed daily cost and it does not care how much you dig. That is why a foundation estimate should state the number of days on site, not just the volume.
Services, and the trench that gets dug twice
Groundworks is where every service crosses. Water, drainage, electrics, gas, telecoms and often a rainwater connection all pass through the same zone as the foundations, and each one needs a trench or a duct.
The cost problem is sequencing. If services are laid after the foundation is backfilled, part of that work is re-excavation. If they are laid first, they are exposed to damage from the machine digging the foundation.
The standard solution is ducts and sleeves cast into the foundation or laid in the same trench, in the right positions, before the concrete. That means the service positions are decided early, which means the design is settled early.
Estimating this properly needs a services plan rather than a guess. Where there is no plan, an allowance for ducts and two or three crossings is a fair provision, and it is far cheaper than a drainage connection dug out of fresh concrete.
Piling, and the point where the estimate changes shape
Piles become the foundation solution when the bearing stratum is too deep for economic excavation, or where vibration and ground movement must be avoided.
A piled solution is priced by pile count, diameter and depth, plus the pile caps, the ground beams between them, and the testing. The testing item is important: a percentage of piles is load tested or integrity tested, and that is a cost with a report attached.
For the estimator, the practical point is that a piling decision changes the programme as well as the price. Piles need a piling rig with its own access requirements, a platform to work from, and a gap of days or weeks before the caps can be built. A foundation estimated as strip and built as piled is a different project, not an adjusted one.
Common errors
No trial hole, so the depth is assumed rather than known
Working space not included, so the trench is too narrow to build in
Over excavation and blinding missed
Bulking not applied, so the spoil removal is understated
Mesh laps forgotten
Backfill taken as equal to excavation volume
No allowance for groundwater or pumping
Spoil classification assumed to be clean
Frequently asked questions
How deep do strip foundations need to be? To a bearing stratum and below the frost line, typically 0.8 to 1.2 m in temperate climates. The trial hole decides, not a rule.
How much concrete for a house extension foundation? For a 25.5 m run of 600 x 250 mm strip, about 4 cubic metres, plus blinding and any trench fill.
How much spoil will I have to remove? The excavation volume plus 20 to 30 percent for bulking.
Do I need reinforcement in a strip foundation? Often just mesh, and sometimes none. It depends on the engineer's design and the ground.
What is trench fill? Concrete taken up close to ground level instead of building blockwork in the trench. Faster, and it uses more concrete.
Can I use excavated soil as backfill? For landscaping yes, for under a floor slab no. It will not compact reliably.
Estimate the groundworks in FRELUX
The FRELUX foundation calculator takes trench dimensions, foundation type and the spoil classification, then returns excavation, blinding, concrete, mesh, damp proofing and removal with costs from your market's price book.