I’ve lost count of the times I’ve been handed a site appraisal with a single line for “abnormals”. Usually it’s a round number, usually it’s far too small, and usually it was put in before anyone had dug a single trial pit.
Then the ground investigation comes back, and suddenly the scheme that worked on paper doesn’t work at all.
I’m a civil engineer, and I’ve spent a lot of my career on the receiving end of that moment. So this is the article I wish more land buyers, promoters and developers read before they exchange contracts. It covers the ten groundworks abnormals I see most often, what each one actually means on site, and why each one can quietly take a large slice out of what a site is worth.
What are groundworks abnormals?
Abnormals are construction costs over and above what you’d expect to spend developing a straightforward, flat, uncontaminated greenfield site. In groundworks terms, they’re the costs that come from the ground itself, the drainage and the site’s constraints, rather than from the buildings you’re putting on it.
They matter for value because of how land is priced. Most development land is valued using a residual approach: the value of the finished scheme, minus build costs, fees, finance and profit, leaves what you can afford to pay for the land. Every pound of abnormal cost comes straight off that residual. Planning practice guidance on viability works the same way and expects abnormal costs to be reflected in the land value, not absorbed by the scheme.
Put simply, every abnormal you miss at appraisal is money you’ve paid the landowner that you’ll never get back.
The 10 abnormals at a glance
| Abnormal | Typical consequence | How you find it | |
| 1 | Poor bearing ground | Piled or deep foundations | Intrusive ground investigation |
| 2 | Shrinkable clay and trees | Deeper foundations, heave protection | Soil testing and tree survey |
| 3 | Made ground and contamination | Remediation, expensive muck-away | Desk study, then intrusive sampling |
| 4 | High groundwater | Dewatering, flotation design, failed soakaways | Monitoring wells over time |
| 5 | No infiltration | Attenuation tanks, controlled discharge | BRE 365 soakaway tests |
| 6 | No outfall or sewer capacity | Requisitions, pumping stations, off-site works | Water company pre-development enquiry |
| 7 | Mining and natural cavities | Drilling, grouting, special foundations | Coal Authority data, specialist GI |
| 8 | Sulphates and aggressive ground | Higher-spec concrete, protective measures | Laboratory chemical testing |
| 9 | Difficult levels | Retaining walls, cut and fill imbalance | Topographical survey and early levels design |
| 10 | Utilities and diversions | Diversions, easements, long lead times | Utility searches and DNO enquiries |
1. Poor bearing ground
This is the big one, and the one that changes a scheme’s economics fastest.
Most houses and many commercial buildings in the UK are designed around simple shallow foundations: strip or trench-fill footings poured into a trench a metre or so deep. That works when the ground at that depth is firm enough to carry the load. When it isn’t, because of soft alluvium, peat, loose fill or weak clay, you’re into piling, vibro stone columns or deep engineered foundations.
NHBC guidance treats foundations deeper than 2.5m as needing an engineer’s design, and that’s roughly where costs start climbing steeply. A piled solution needs a piling rig, a working platform, ground beams instead of simple footings, and usually a suspended ground floor rather than a ground-bearing slab. It isn’t one extra cost. It’s a whole different substructure.
My view: if there’s any hint of soft ground, such as a site near a river, a former pond, a low-lying area or historic maps showing a watercourse, don’t put a number in your appraisal until you’ve got boreholes or at least a decent set of trial pits. Guessing foundations is the single most expensive guess in development.
2. Shrinkable clay and trees
Clay soils swell when they get wet and shrink when they dry out. Trees suck moisture out of the ground, and their roots can reach much further than people expect. Put the two together and you’ve got a recipe for foundation movement unless you design for it.
NHBC Standards Chapter 4.2 sets out how foundation depths increase near trees, depending on how shrinkable the clay is, the species of tree, its mature height and its distance from the building. The baseline minimum depths are 0.75m for low volume change potential clay, 0.9m for medium and 1.0m for high. Near trees, depths can go well beyond that, and you’ll also need heave protection: compressible material against foundations and void formers under slabs to cope with the ground swelling after trees are removed.
That last point catches people out. Felling trees doesn’t solve the problem. The clay underneath has been dried out for decades and will swell back up once the trees are gone. You still need to design for it.
My view: get the tree survey and the soil plasticity testing done together, early. On a clay site with mature hedgerows and tree belts, the foundation design can vary plot by plot, and a site layout that keeps houses back from the worst trees can save a surprising amount.
3. Made ground and contamination
Made ground is anything that’s been placed by people rather than formed naturally: old fill, demolition rubble, ash, industrial waste, the remains of previous buildings. On brownfield land it’s almost guaranteed, and it’s one of the commonest reasons a cheap-looking site turns out to be expensive.
Made ground causes problems in two ways. First, it’s usually poor bearing ground (see number one). Second, it may be contaminated with things like hydrocarbons, heavy metals, asbestos or ground gas. Contamination brings remediation, planning conditions requiring a remediation strategy and a verification report, and for residential schemes, often a clean cover system in gardens and soft landscaped areas.
Then there’s muck-away. Every cubic metre you dig out has to be classified as waste before it leaves site. Inert material is relatively cheap to dispose of. Material classified as hazardous costs many times more, and landfill tax applies on top. On a site with a lot of excavation in contaminated made ground, disposal alone can become one of the largest lines in the groundworks budget.
My view: start with a proper Phase 1 desk study, which looks at historic maps and records, and follow it with intrusive investigation that includes waste classification testing. The best way to reduce disposal costs is a design that digs less. Raising levels slightly, reusing suitable material on site under the CL:AIRE Definition of Waste Code of Practice, and planning cut and fill carefully all help.
4. High groundwater
Water in the ground changes everything about how you build. Excavations flood, trench sides become unstable, and you end up pumping water just to keep working. That’s dewatering, and on some sites it runs for months.
High groundwater also affects design. Underground tanks and chambers can float if the water pressure underneath exceeds their weight, so they need to be anchored or weighted. Basements need waterproofing to a much higher standard. And infiltration drainage usually won’t work, which leads straight into abnormal number five.
The trap here is timing. UK groundwater levels are usually highest in late winter and early spring. If your ground investigation was done in a dry August and nobody installed monitoring wells, you may be working from a water level that’s a metre or more lower than reality.
My view: standpipes or monitoring wells, read over several months including winter, are cheap insurance. One reading on the day of drilling isn’t enough to price a scheme on.
5. No infiltration
Surface water from a new development has to go somewhere. Under Part H of the Building Regulations and SuDS policy, the preferred option is to let it soak into the ground. Only if that isn’t reasonably practicable can you discharge to a watercourse, then to a surface water sewer, and only as a last resort to a combined sewer.
Infiltration is usually the cheapest option. When soakaway tests to BRE Digest 365 show the ground won’t drain, which is common on clays and in high groundwater, you move down the hierarchy. That means attenuation: storing water on site in tanks, oversized pipes, ponds or basins, and releasing it slowly at a restricted rate, often close to what the greenfield site would have produced naturally.
Attenuation tanks take up space, need excavation and backfill, often need muck-away, and must be designed against flotation if groundwater is high. Ponds and basins take up land that could have had houses on it. Either way, it’s a real cost and sometimes a loss of developable area.
My view: run the infiltration tests before you buy. They’re quick and cheap, and they tell you more about your drainage cost than almost anything else. Also make sure the tests are done at the depth and in the location where you’d actually put soakaways, not just wherever was convenient.
6. No outfall or sewer capacity
Every site needs somewhere to send its foul water, and usually its surface water too. When there’s a public sewer of the right type, with capacity, close to the site boundary and lower than the site, life is easy. When there isn’t, it gets expensive quickly.
Here are the problems I see most often:
- No gravity connection. If the nearest sewer is uphill, or too shallow, you need a pumping station. That means the pumping station itself, rising mains, power supply, and an adoption process with the water company that can drag on.
- No capacity. The water company may say the local network can’t take more flow without reinforcement. Off-site reinforcement or a sewer requisition under the Water Industry Act can bring both cost and long delays.
- Third-party land. If the route to the outfall crosses land you don’t own, you need an easement or the owner’s agreement. A strip of land between your site and the sewer can turn out to be a ransom strip, and its owner knows exactly how much it’s worth to you.
My view: send a pre-development enquiry to the water company before you commit to a site, and check land ownership along every possible drainage route. A site without a clean outfall isn’t necessarily undevelopable, but it is definitely worth less than one with one.
7. Mining and natural cavities
Large parts of England, Wales and Scotland sit over historic coal mining, and plenty of other areas have mining for ironstone, limestone, salt or other minerals. Shallow workings and old mine entries can collapse, and building over them without treatment is asking for trouble.
In England and Wales, the Coal Authority defines Development High Risk Areas. A planning application in one of those areas normally needs a Coal Mining Risk Assessment. If investigation confirms shallow workings, the usual treatment is drilling and grouting to fill the voids before construction. That can be costly, and the investigation itself, with rotary boreholes probing for voids, costs more than a standard ground investigation.
Mining isn’t the only cause of voids. Chalk can contain solution features, and some gypsum-bearing rocks can dissolve and create cavities. They need specialist investigation and design too.
My view: check the Coal Authority’s data and the geology before you even visit a site. It costs almost nothing and takes minutes. If you’re in a high-risk area, budget for proper investigation, not just a desktop report.
8. Sulphates and aggressive ground
Some soils and groundwater contain chemicals that attack concrete, mainly sulphates and, where pyrite is present, the sulphates formed when it oxidises. Certain clays and mudstones are well known for it.
Concrete in the ground is designed to BRE Special Digest 1, which classifies the ground’s aggressiveness and sets the concrete specification accordingly. On benign ground, standard concrete is fine. On aggressive ground, you’ll need higher cement content, special cement types, extra cover to reinforcement, and sometimes additional protective measures such as coatings or membranes. It affects foundations, piles, manholes, chambers and anything else made of concrete that sits in the ground.
It’s rarely as dramatic as piling or mining, but it adds cost to every element of substructure, and it’s easy to overlook if nobody runs the chemical tests.
My view: make sure your ground investigation includes the right laboratory testing for sulphates and pyrite, and that your designer actually uses the results. I’ve seen schemes where the testing was done and then nobody carried the classification through into the concrete specification.
9. Difficult levels
A sloping site is rarely a problem on its own. Badly handled levels are.
Levels drive a lot of costs. Estate roads must meet maximum gradients for adoption, and access to new homes needs to meet Part M requirements for level or gently sloping approaches. Drainage needs falls. Gardens need to be usable. On a steep or uneven site, satisfying all of that usually means retaining walls, significant earthworks, or both.
Retaining walls are expensive, especially once they’re tall enough to need structural design, and they bring long-term maintenance questions. Earthworks are expensive if the cut and fill don’t balance. Exporting surplus material costs money in haulage and disposal, and importing fill costs money too.
My view: do a proper topographical survey and get an engineer to draw an early levels strategy before you fix the site layout or the price. A layout that works with the slope instead of against it can save more than almost any other design decision. Ideally, design a cut and fill balance on site and nothing leaves in a lorry.
10. Utilities and diversions
Overhead power lines crossing the middle of a site. A high-pressure gas main along the frontage. A trunk water main or fibre route right where your access road needs to go. These are common, and they’re expensive.
Diverting or undergrounding services usually has to be done by the utility company or its approved contractors, on its timescale and at its price. Lead times can run to many months. Some services simply can’t be moved economically, so you end up with an easement strip you can’t build on, which means lost plots.
Then there’s the cost of bringing new services in. If the local electricity network needs reinforcement to supply the site, the connection cost can be significant, and the timescales are a growing problem in many parts of the UK.
My view: get utility searches and early enquiries to the network operators done before you commit. Walk the site and look up as well as down. And treat any diversion quote with a healthy dose of scepticism about the programme until you’ve got it confirmed in writing.
Honourable mentions
These didn’t make the top ten because they’re more site-specific, but they’ve caught out plenty of developers:
- Japanese knotweed and other invasive plants. Treatment or excavation and controlled disposal can be costly, and you can’t just dig it up and move it around the site.
- Unexploded ordnance. In cities and areas bombed in the Second World War, or near former military land, an assessment is often needed and sometimes specialist clearance during excavation.
- Archaeology. Planning conditions requiring trial trenching, watching briefs or full excavation can add cost and time.
- Ecology. Protected species, mitigation and biodiversity net gain can limit where and when you can work and how much of the site you can develop.
How to protect yourself before you exchange
Here’s the approach I’d take with any site where the price depends on the groundworks being straightforward:
- Do the desk work first. Historic maps, geology, Coal Authority data, flood risk, utility searches and a water company pre-development enquiry cost very little and rule out a lot.
- Get an intrusive investigation before exchange, or make the price conditional on one. Trial pits, boreholes, soakaway tests, contamination sampling and groundwater monitoring wells.
- Ask a groundworks contractor to look at it. An experienced contractor who’s built on similar ground will spot things a desktop report won’t, and can give a realistic view on cost and buildability.
- Put abnormals in your appraisal as specific items, not one round number. If you can’t name what the abnormal allowance covers, you don’t really have one.
- Renegotiate when the facts change. If the investigation finds significant abnormals, the land price should reflect them. That’s exactly what the residual method and planning viability guidance expect.
Frequently asked questions
What counts as an abnormal cost in development?
An abnormal cost is any cost above what would be needed to develop a straightforward, level, uncontaminated greenfield site. In groundworks, typical examples are piling, remediation, dewatering, attenuation, pumping stations, mining treatment, retaining walls and utility diversions.
How do abnormals affect land value?
Development land is usually valued as a residual: scheme value minus build costs, fees, finance and profit. Abnormal costs reduce that residual pound for pound, so they should reduce the price paid for the land.
When should a ground investigation be done?
Ideally before you exchange contracts, or with the purchase made conditional on its results. Groundwater monitoring should run over several months, including winter, to give a reliable picture.
Which abnormal is usually the most expensive?
It varies by site, but poor bearing ground needing piling, contamination with large volumes of hazardous waste, and the lack of a drainage outfall are often among the largest.
Can abnormals make a site unviable?
Yes. If the combined abnormal costs are larger than the margin in the scheme, the site may not be viable at the agreed price. That’s why identifying them early, and reflecting them in the price, matters so much.
Groundworks Case Study: A UK groundworks contractor’s services and experience
MAC Group Ltd is a groundworks and civil engineering contractor delivering projects for developers and main contractors across the UK. Its work spans groundworks, earthworks, foundations, deep and surface drainage, SuDS and attenuation, pumping stations, S278 highway works, roads and sewers, and paving, on schemes ranging from residential developments to large logistics parks and distribution centres.
Because MAC Group delivers the full groundworks package itself, from enabling works and earthworks through to drainage, foundations and surfacing, the team regularly deals with the abnormals described in this article: soft and variable ground, sites with no infiltration, complex outfalls, difficult levels and constrained access. That experience means MAC Group can contribute to the conversation early, helping clients understand the practical implications of ground investigation findings and building sensible, realistic groundworks costs into an appraisal before decisions become expensive to change.
MAC Group operates its own plant fleet and has in-house paving capability, so it can programme and deliver work without depending on multiple separate subcontractors. That gives clients one point of responsibility for everything below finished ground level. For more on the cost side, see this guide on how much groundworks cost in the UK.