I’m considering buying a plot of land to build a new home. Beyond planning permission, I’ve been told the ground itself is critical, as it affects both foundation design and the feasibility of a percolation area for wastewater.
In practice, how important is it to commission a soil investigation/suitability test before buying? What ground conditions should I look for as red flags (for example: peat, heavy clay, high water table, rock)? What are the typical costs if the ground turns out to be unsuitable for foundations or for a percolation system?
This is an often-overlooked consideration for many self-builders but an important one that can result in unforeseen costs and design implications if not determined early.
Firstly, it is recommended that a purchaser enters a legal contract with a vendor to purchase a site “subject to planning”. This protects you from the risks of obtaining planning permission or the vendor altering or backing out of the deal subsequently. A solicitor can help arrange this for a standard fee. You can also include “opt out” clauses pertaining to site suitability (ie ground conditions).
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You will be required as part of any planning application to submit a site characterisation report, or percolation test, which outlines the type and nature of the soil and the permeability or “soakage” capacity to support the proposed design of the wastewater treatment system. This is relatively inexpensive and consists of excavating trial holes and preparation of a report by your engineer, chartered building surveyor or architect. You could expect a cost of typically €700-€1,000 for a small excavator and driver to carry this out. You will need the landowner’s permission.
This can be carried out at the outset to inform the qualified professional you have retained about the ground conditions, propose a foundation design and identify where best to locate the percolation area. An experienced engineer, chartered building surveyor or architect should be present when excavating the trial holes and they will be able to comment generally on the suitability of the conditions upon visual inspection to give you rational advice. You should be reasonably informed at this stage as to the conditions and risk, before committing to planning application or site purchase costs.
In relation to conditions to look out for, all the issues you listed and more. Ideally you are looking for firm, well-drained sandy or gravelly soils. These soil characteristics support both standard foundation and percolation area designs. You can still work with other soil types, but they may influence design.
If the ground is less than ideal, several engineering solutions can be considered, such as raft foundations, screw piles or ground improvement methods depending on the specific site conditions. It is not possible to provide an estimated cost for each method without further context on the house type or actual soil conditions, but it is reasonable to expect anything from a few thousand to tens of thousands in additional costs for dealing with really poor conditions.
Early investigation is, therefore, a wise investment to inform your decision and budget realistically.
Craig Potter is a chartered quantity surveyor and a member of the Society of Chartered Surveyors Ireland
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