Ground investigation is an investment, not a cost
A geotechnical investigation represents a modest sum against the stakes, in the order of a few hundred to a little over a thousand euros for a house project depending on the extent of testing. That sum compares with the tens of thousands of euros poor ground can cost.
That sum bears no relation to the gap between shallow and deep foundations.
Clay, rock, heterogeneous ground or the presence of water may require deep footings, a raft or piles. Each of those solutions represents an additional cost running into tens of thousands of euros on a house, and considerably more on a collective scheme.
Carrying out the investigation after buying the land amounts to discovering the price after the purchase. A conditional clause is the instrument that reverses that order.
What a slope triggers
A sloping site is not merely harder; it changes the nature of several items. Earthworks, foundations and site access are all affected.
Earthworks increase in volume and sometimes in complexity, with spoil removal or reuse.
Retaining structures appear, retaining walls or stabilised banks, which do not exist on a flat site.
Foundations step in levels, with offsets complicating execution.
Access and connections lengthen and grow more complex, particularly for gravity drainage, which may require pumping.
Against that, a well-exploited slope sometimes allows a semi-basement level to be gained, improving the ratio of cost to area. It is a design decision, not a misfortune.
Access determines the methods
An item often absent from estimates and nonetheless real. It is site access and lifting equipment.
Road width, the possibility of standing a crane, delivery distance and permitted hours determine site methods. Constrained access requires smaller plant, more handling and more time.
On a narrow urban plot the access question alone can represent an item comparable to a services trade. Permits to occupy the public domain and suspension of parking are added.
Party walls are an item and a risk
In Belgium, where terraced and semi-detached fabric dominates in many municipalities, party walls are the rule rather than the exception. It is therefore treated as a project given rather than an exception.
They carry four economic consequences.
Contradictory condition surveys with neighbours before work starts, which are a study cost but above all a protection.
Underpinning where neighbouring foundations sit shallower than those proposed.
Protection and propping during the works.
The legal exposure of damage next door, whose prevention cost is always lower than its resolution cost.
| Site constraint | What it triggers | When to resolve it |
|---|---|---|
| Ground conditions | Type and depth of foundations, any underpinning | Geotechnical study, before signing |
| Slope | Earthworks, retaining walls, site access | Topographic survey, before the sketch |
| Site access | Lifting equipment, lorry movements, storage | Site visit, before tendering |
| Party walls | Schedule of condition, protection, insurance, periods | Formal record, before starting |
| Siting rules | Setback, envelope, footprint, height | Municipal planning information |
Siting depends on rules at three levels
In Belgium, what may be built on a plot depends on the region, the municipality and sometimes the district. The check is therefore made with the municipality before any study.
Setbacks, massing, footprint and height are fixed by regional codes and refined by municipal or zoned regulations. In Brussels, habitability standards are added that may govern a brief's feasibility more than massing itself.
A plot may therefore be developable without permitting the brief envisaged. That check precedes the cost study, not the reverse.
What this means for an estimate
Four rules.
Commission the ground investigation before acquisition where possible, or make the purchase conditional on it.
Visit the site before pricing. Access, slope and neighbours cannot be read from a cadastral plan.
Provide specifically for foundation risk while the investigation is outstanding, rather than burying it in a general contingency.
Check siting rules before the brief, because they may render it unachievable.
This article reflects professional practice and regulation at the date of checking and serves as orientation. It does not constitute legal advice and does not replace assessment of the individual case.