A house extension may look straightforward on the architectural drawings. The new walls align with the existing house, the layout works, and the proposed foundations appear relatively simple.
But what happens if the ground beneath the extension cannot safely support the intended foundation?
That question needs answering before construction, not after the contractor has excavated the trenches.
Ground investigation helps establish the conditions beneath a site so the structural engineer can design foundations appropriate to the building and the soil supporting it.
For some house extensions, the ground conditions are well understood and the foundation design is relatively straightforward. Other sites need more detailed investigation because of weak ground, shrinkable clay, made ground, groundwater or nearby structures.
The important point is that foundation design should respond to the actual ground conditions, rather than relying on a standard detail that may not be suitable.
What Is a Ground Investigation?
A ground investigation is a process used to understand the soil, rock, groundwater and other conditions beneath a proposed development.
It helps the design team establish whether the ground is suitable for the proposed structure and what engineering measures may be necessary.
A ground investigation can involve reviewing existing information about the site, examining its development history and carrying out physical investigations.
The scope depends on the project.
A small residential extension may need a more limited assessment than a large new-build house with a basement, but neither should assume that ground conditions are suitable without adequate evidence.
The NHBC guidance on land quality and ground conditions identifies the importance of assessing geotechnical hazards and using appropriate design precautions where problematic ground is present.
The purpose is not simply to collect soil samples. It is to obtain information that can inform a safe and practical foundation design.
Is Ground Investigation the Same as Soil Testing?
The terms are often used interchangeably, but they are not quite the same.
Ground investigation describes the wider process of understanding a site’s conditions.
Soil testing refers to particular tests used to establish the properties of soil or other ground materials.
For example, a ground investigation may include reviewing geological information, examining previous site uses, excavating trial pits, drilling boreholes and undertaking laboratory testing.
The resulting information can help establish soil strength, ground composition, groundwater conditions and the potential for movement.
Not every project needs every investigation method.
A geotechnical specialist should recommend a proportionate scope based on the building, site history and identified risks.
When Do You Need Soil Testing for a House Extension?
There is no universal rule requiring every house extension in England to have the same soil-testing package.
The need for investigation depends on the site and the information already available.
An engineer may require additional evidence where the ground conditions are uncertain or where there are indications that conventional foundations may not be appropriate.
For example, a proposed extension might be located on land that has previously been filled, close to an existing basement, on shrinkable clay or where groundwater could affect excavation.
The ground may also vary across a relatively small plot.
One part of the site might contain suitable natural soil while another contains made ground or disturbed material.
This matters because a foundation that performs adequately in one location may not perform in the same way elsewhere.
For a homeowner, the sensible approach is to establish the foundation requirements during technical design and obtain any necessary investigation before the construction method is finalised.
What Ground Conditions Can Affect Foundation Design?
Shrinkable clay soils
Some clay soils change volume as their moisture content changes.
During dry periods, shrinkable clay can lose moisture and contract. When moisture returns, it can swell.
These changes may affect foundations, particularly where the design does not adequately account for the site’s conditions.
The British Geological Survey’s shrink–swell guidance explains how clay mineral content, rainfall, drainage and vegetation can influence ground movement.
A site with shrinkable clay does not automatically require piled foundations. It requires a foundation design that appropriately addresses the identified soil conditions and influences.
Made ground and previously disturbed land
Made ground is material that has been placed or altered by human activity.
It can include historical fill, demolition material or soil used to change site levels.
Its composition and engineering properties may be inconsistent.
This can create uncertainty about bearing capacity and settlement.
A foundation design should not assume that made ground has the same characteristics as undisturbed natural ground.
Where the history or condition of the material is uncertain, further investigation may be necessary.
Weak or compressible ground
Some soils can compress significantly when additional loads are applied.
If the foundation does not distribute loads appropriately, this can result in excessive settlement.
The engineer needs to establish whether the proposed foundation can transfer loads to suitable ground without unacceptable movement.
Depending on the investigation findings, the design may require changes to the foundation type, dimensions or founding level.
Groundwater
Groundwater can influence both the permanent design and the construction process.
Water entering an excavation may affect soil stability, temporary support requirements and the practical method of construction.
It may also influence basement waterproofing and drainage design.
The Health and Safety Executive’s excavation guidance identifies groundwater, excavation collapse and the risk of undermining nearby structures as matters that need to be considered when planning excavation work.
Existing foundations and neighbouring buildings
An extension is rarely being built in isolation.
Its foundations may sit close to those of the existing house, a neighbouring property or a boundary wall.
The depth and condition of existing foundations can therefore influence the new design.
Where excavation is close to neighbouring structures, temporary stability and potential Party Wall requirements may also need consideration.
These are separate issues from soil testing, but they often need to be coordinated during the same technical design stage.
How Is a Ground Investigation Carried Out?
A proportionate investigation commonly starts with a desk study.
This involves reviewing available information about the site, such as geological records, historical land use, nearby development and any existing ground reports.
The desk study helps identify what is already known and where uncertainty remains.
If physical investigation is needed, the geotechnical specialist can then specify suitable methods.
These may include trial pits to expose shallow ground conditions or existing foundations, boreholes to investigate deeper deposits, and sampling for laboratory testing.
Tests may be used to assess properties such as soil classification, plasticity, strength or other characteristics relevant to the proposed foundation.
Groundwater observations or additional investigations may also be required.
The findings should be documented in a report that explains the investigation, identifies relevant ground conditions and provides recommendations appropriate to the project.
Those recommendations can then be used by the structural engineer.
What Does the Structural Engineer Do With the Results?
A ground investigation report is not automatically a completed foundation design.
The structural engineer still needs to consider the proposed building, its loads and the way those loads will be transferred into the ground.
For a house extension, that may involve the weight of masonry walls, floors, roof construction and any new steelwork.
The engineer uses the available ground information to develop an appropriate structural solution.
AC Design Solution’s structural engineering services cover foundation design and structural alterations, with structural drawings and calculations developed around the requirements of the individual project.
The resulting foundation design should reflect the identified ground conditions, rather than assuming that a foundation used successfully on another property will necessarily be suitable.
What Types of Foundations Might Be Considered?
Different ground conditions can lead to different engineering solutions.
Conventional strip or trench-fill foundations may be suitable where the ground can safely support the proposed loads and other relevant conditions are satisfied.
Where suitable bearing ground is deeper, the engineer may need to consider a different founding level or foundation arrangement.
For more challenging sites, alternatives such as engineered raft foundations, piles and ground beams may be considered.
The choice depends on the investigation findings, structural loads, movement requirements, construction access and other site constraints.
A piled solution should not automatically be described as better than a conventional foundation.
Equally, a deeper trench is not necessarily the correct answer to every ground problem.
The appropriate foundation is the one justified by the actual engineering assessment.
Real Project: Marsden Manor, Prestbury
AC Design Solution’s work at Marsden Manor, 54 Macclesfield Road, Prestbury, provides a practical example of why ground investigation matters.
The project involved a five-bedroom Georgian-style replacement residence on a mature, tree-lined plot.
AC Design Solution was appointed after planning permission had already been secured, with responsibility for detailed architectural design, structural engineering, Building Regulation drawings, drainage, M&E coordination and planning-condition discharge.
Site investigation identified poor ground conditions that made conventional shallow foundations unsuitable for the principal building.
The engineering response involved sheet piling to support the excavation, with the permanent substructure and principal frame developed using reinforced concrete, including basement walls, columns and capping beams.
The detached garage had different structural requirements and was designed separately as a steel-framed structure.
The important distinction is that sheet piling was used to support excavation; it should not be confused with the permanent foundation system.
The project also had protected trees and associated planning constraints, but the available project information does not establish that the trees caused the poor ground conditions.
Those were separate matters requiring coordination.
The ground-related engineering is also documented in AC Design Solution’s Marsden Manor project discussion.
What this project demonstrates is that the foundation strategy cannot always be finalised from architectural plans alone.
The conditions beneath the site can materially influence how the building is engineered and constructed.
Ground Investigation and Building Regulations
Ground investigation is not the same as Building Regulations approval.
However, the information obtained can be important in demonstrating that the proposed foundation design is appropriate.
In England, Approved Document A: Structure provides guidance on structural requirements, including foundations and ground movement.
Approved Document C addresses matters including site preparation, unsuitable materials, contaminants and sub-soil drainage.
These requirements need to be considered according to the particular development.
AC Design Solution’s Building Regulation Drawings can incorporate the appropriate foundation and structural information into the wider technical design package.
A planning approval does not establish that the ground can safely support the proposed foundations.
That is a separate technical matter.
Who Carries Out Ground Investigation?
Ground investigation and geotechnical testing should be undertaken by appropriately competent specialists.
A geotechnical engineer or suitable ground investigation consultant may be appointed to establish the conditions and provide recommendations.
The structural engineer then uses relevant findings to inform the building’s structural design.
These roles should be coordinated, but they should not be confused.
AC Design Solution provides structural engineering and technical design services. Specialist soil testing and geotechnical investigation are separate services that may need to be commissioned from an appropriate provider.
This distinction matters because a visual structural inspection cannot establish every characteristic of the soil beneath a proposed building.
Should You Investigate the Ground Before Planning Permission?
Not every planning application needs a detailed ground investigation.
For a straightforward extension, some technical investigations may reasonably take place after the planning design has been established.
However, where there are known ground risks, significant excavation, a basement or an unusual foundation requirement, early investigation can be valuable.
The information may affect the feasibility, layout or cost of the proposed development.
Waiting until construction begins can leave the project exposed to redesign, programme changes or unexpected engineering requirements.
The timing should therefore be proportionate to the identified risks.
How Much Does Ground Investigation Cost?
There is no reliable fixed price that applies to every site.
The cost depends on the investigation method, number and depth of exploratory locations, access, testing requirements and the complexity of the ground conditions.
A limited investigation for a domestic extension will not necessarily have the same scope as a detailed investigation for a substantial new-build property.
The quotation should clearly identify what is included, what reporting will be provided and whether further testing may be recommended after the initial findings.
It is also important to distinguish the cost of ground investigation from the separate cost of structural foundation design.
Frequently Asked Questions
Do all house extensions need soil testing?
No. There is no universal requirement for every extension to undergo the same soil-testing process. The appropriate investigation depends on the ground conditions, existing information, proposed structure and level of risk.
Can a builder determine the foundation depth without a ground investigation?
Foundation requirements need to be based on appropriate technical information. A contractor’s experience can be useful, but uncertain or challenging ground conditions may require specialist investigation and structural engineering input.
Is clay soil unsuitable for house extensions?
Not necessarily. Many buildings are constructed successfully on clay soils. The foundation design needs to account for the relevant soil characteristics, including shrink–swell behaviour where applicable.
What happens if poor ground is discovered during excavation?
The affected work should be reviewed before proceeding on an unsuitable assumption. The structural engineer may need further ground information and may revise the foundation design. Excavation stability must also be maintained.
Can soil testing tell me exactly which foundations to use?
Testing provides information used to inform design. The structural engineer still needs to assess building loads, ground conditions, movement and construction constraints before selecting the appropriate foundation arrangement.
Can I use my neighbour’s ground investigation report?
A nearby report may provide useful background information, but it does not necessarily establish the conditions beneath your own site. Its relevance and limitations should be assessed by the appropriate specialist.
Get the Foundation Design Right Before Construction
Ground investigation is about reducing uncertainty.
For some extensions, the available information will support a relatively straightforward foundation design. For other sites, investigation may identify conditions that require a different engineering approach.
Marsden Manor demonstrates why this matters. The approved architectural scheme still needed a structural response to the actual ground conditions before the building could be constructed.
AC Design Solution can develop the structural drawings and calculations and coordinate the foundation design with the wider architectural and Building Regulations information, using specialist ground investigation findings where required.
If you are planning an extension or new-build project and need advice on the structural design, contact AC Design Solution.
