As structural engineers, it’s our job to assess and design the most appropriate foundations for your structure. This requires an understanding of the history of your plot and the surrounding area as well as determining what lies underneath it. To do this, we specify and instruct a site investigation on your behalf.

Site investigations (SI) are a fundamental component of the construction process and can help prevent your project from being plagued with prohibitive costs & unnecessary risks by identifying design constraints and considerations as early as possible in the design process.

Site investigations can range from straightforward tasks (hand-dug inspection pits) to complicated endeavours (boreholes and gas monitoring) depending on the environment and the scope of your project. In this article, we’ll give you an overview of our role in a site investigation, and why it’s so important.


The biggest unknown when arriving on site is what’s in the ground. These unknowns need to be identified as early as possible. A site investigation helps us to identify risks to the integrity of your proposed design caused by the surrounding ground.

These risks can come from a wide range of sources, both manmade and natural. These could include mature trees, heave, a high water table, a steeply sloping site, made ground and the presence of harmful gases to name a few.

Every site investigation starts with a desktop study. This study looks at a combination of geological plans and historical records to help us understand how extensive the site investigation needs to be.

A site investigation looks at the history of your site for potential contaminants in the ground. To give an example, a petrol station keeps its fuel underground, which can contaminate the surrounding soil and make it unsuitable for building if not properly removed.

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If a building that contained asbestos has previously been removed from the site, it may have contaminated the ground. If the site has previously been infilled or built up in made ground this may need to be removed or increase your foundation depth and may also impact your preferred ground floor construction method.

Not all potential risks are artificial. Clay, mudstone, and tree roots all work to create cohesive soil. This soil will move and shape depending on its water content, leaving any structure above unstable. A structural engineer must have an understanding of the natural composition of the site as well as its manmade history.

Soil heave can also destabilise your structure by pushing the soil upwards causing costly damage. We perform calculations to predict the likelihood of these events occurring.

Monitoring for the presence of harmful gases is another crucial step in a site investigation, serving both safety and environmental compliance purposes. If gas mitigation is required, we hire a specialist consultant to advise on gas protection and venting measures. Planning authorities will stipulate strict gas testing on-site which requires multiple visits and can add cost and impact your project program.


Planning for a basement can add additional challenges and costs to your project. Proximity to neighbouring buildings needs to be considered, de-watering in high-water sites becomes more difficult and the site investigation will typically require boreholes to determine ground conditions at greater depth.  The geology and overall makeup of the surrounding soil can prohibit certain construction methods and the basement’s depth and size.

This is another reason why a thorough investigation must be completed early in the planning stage, as excavating certain materials, such as bedrock, can be more costly and time-consuming than others.

All structural engineers will agree that the biggest threat to your basement is water. A high water table may cause issues when it comes to excavating the ground for the basement and also add additional loading to your basement walls. Buoyancy will also need to be considered for lightweight structures.


Site investigations help structural engineers and architects gain a comprehensive understanding of the physical and geological characteristics of the site. This knowledge is crucial for making more informed decisions during the design and construction phases.

Whether identified or not, site risks will always cause problems if not properly addressed. Identifying potential risks early in the process enables better risk management. This includes addressing geological hazards, ensuring proper foundations, and taking preventive measures against issues that could arise during or after construction.

A site investigation can save you time in the construction process and minimise the risk of unexpected delays and costs once the builders begin their groundwork.

Getting the investigation done as early as possible will make the process smoother and quicker, informing costing exercises and reducing the likelihood of design changes.


The cost of your site investigation is completely dependent on the project’s scale and site-specific considerations.

Standard, unspecialised site investigations can cost between £1000 – 2000 all in but may provide inefficient reports depending on the condition of the site and are generally only suitable for small-scale extensions on low-risk plots.

Hiring specialists who deliver detailed reports will generally cost more. In our experience, a full-site investigation with requirements such as lab testing and heavy excavators begins at around £3,500-£6,000. Additional costs may be incurred for remote or difficulty-accessed sites.

As a self-builder, you must always be prepared for extra costs, and site investigations are no exception. If gas and water monitoring are required, don’t be surprised if the cost rises to around £10,000. Your engineer may offer an early assessment of your plot to assist in evaluating likely costs.

The timeframe of a site investigation is also dependent on the project scale. Creating the initial site report can take 2 to 3 weeks and must be completed before the investigation can be completed as it will inform the recommended scope of the intrusive investigation.

If ongoing monitoring is required, this will add extra time to the process. Typically, 3 to 6 months is required to allow for multiple visits.

A site investigation may have variable costs, but they consistently add value to your overall build and work to shorten and streamline the process.  A thorough investigation leads to a structurally sound finished product with reduced risk of remediation.

As specialists in residential properties, our engineers and technicians work to provide a service that prioritises cost-saving for our self-build clients. We are constantly working with our sister company AC Architects to deliver projects that are backed by sufficient information to ensure long-term satisfaction.


 A site investigation’s scope and speciality is dependent on the location, characteristics and history of the chosen site.

As structural engineers specialising in self-build, we will always recommend site investigations as we understand the importance of managing cost and program confidence to self-build clients.

Hiring a structural engineer is not a legal requirement, however, some local authorities may require a structural engineer and a site investigation as part of the approval process or for insurance coverage.

In Scotland, an SER Certifying Engineer will normally review and certify the structural design of your project.



In some situations, your ground investigation may require boreholes.

This is a narrow shaft drilled into the ground to assess and sample the ground conditions. Boreholes give you a more extensive report to a greater depth and allow ongoing monitoring.

Boreholes will be professionally drilled as part of your intrusive investigation and will vary in cost depending on the depth required, site access and number of borehole locations required (3-4 locations can normally be undertaken in a single day).



Trial pits are hand-dug or machine-excavated holes to determine anticipated ground-bearing strata. They typically range from 2 to 3 metres in depth. The maximum depth of a trial pit may be determined by site conditions depending on safety considerations as assessed by the contractor. Poor natural ground or uncompacted made ground will increase the risk of collapse of the sides of the excavation, limiting the achievable depth.

Using a trial pit can be a quick and cost-effective way of assessing your ground but is completely dependent on ground conditions. Trial pits are typically only suitable for smaller-scale projects where suitable bearing strata are anticipated at shallow depths (less than 1.5m).

If there are existing foundations on site, we’ll use a trail pit to find their depth, dimensions, and condition. With this information, our engineers can assess the capacity of the existing foundation and determine any strengthening works (widening of existing foundations for example).

Trial pits can also be used for percolation testing, which is the process by which water moves downward through the soil under the earth’s gravity. This test is required to determine the suitability and design requirements of soakaways.

As mentioned before, a small-scale project may only require a trial pit rather than a full site investigation. We would still recommend consulting a professional in this situation.

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The role of the structural engineer in a site investigation is paramount in ensuring the success, safety, and longevity of your project. Our involvement should begin in the early design stages to provide valuable insight to your architect and contractors on the conditions of the site.

By assessing your site in detail, we can help you make informed decisions, minimise risk, and save you money in the long term.

Looking to carry out a site investigation? Our team of engineers are happy to help. Get in contact with one of our team to find out more.