With various soil types and ground conditions influencing foundation design, it’s essential to consider factors like soil composition, depth, and nearby structures. In this digest, we’ll provide insights into the key considerations for foundation planning, the challenges presented by different soil types, and the necessary precautions to take to avoid potential issues.
Whether you’re a homeowner planning an extension or a contractor working on a new build, this guide aims to equip you with the knowledge needed for a successful and secure foundation.
Did you know?
The foundations of any new building or extension are crucial to get right as they’ll help spread the weight of the structure and prevent uneven settlement, subsidence, and structural damage to your home. There are several factors that your designer and contractor must consider when determining the depth of your foundations and the type of foundation to choose. (Building Control cannot design your foundations for you.)
Soil type
Soils react differently to the presence of water, depending on their type and the extent to which they retain water. Soil also needs to be able to bear the load of your extension, and this ability differs in different types of soil. Your architect or designer will establish the type of soil your building or extension will sit on (you may need to have a trial hole dug), and they may consult with a structural engineer or geotechnical specialist to verify the type and ground conditions and decide that you need specialist or engineered foundations.
This is hugely important if the structure is to remain stable and not jeopardise the main structure if it’s an extension.
Clay is prone to shrinkage and heave, so if the foundations aren’t deep enough (avoiding the less stable layer that can change depending on the weather conditions and the presence or removal of trees) and aren’t of the right type, then your structure could become unstable. Generally, foundations in clay soils should be at least 1m deep, and in some cases, up to 3m deep. You may want to consider engineered or piled foundations to reduce time and potentially lower costs.
Sand and gravel soils can typically support heavy foundation loads; however, when the groundwater level is high or the sand or gravel is very loose, additional measures may be necessary, and consulting a structural engineer may be advisable.
Rock can be an extremely stable base to build on, and the foundation may only need to be laid directly on top of it, but if it’s rock that’s impervious to water, a drainage system may need to be put in place.
Chalk is susceptible to frost, and the top layer may become quite soft as a result. Therefore, foundations will need to be built at a depth that avoids this layer.
Peat is extremely challenging to build on as it is often waterlogged and isn’t able to bear heavy loads. Where possible, the peat layer has to be excavated, and the foundations built on more stable ground.
Filled ground is ground that has been previously dug out and filled in with rocks, debris, or even landfill material. This generally requires digging out to a depth below the infill layer to find a stable base, and frequently, piled foundations are more suitable. For filled ground, you should seek the expert advice of a structural engineer, as you will generally need an engineered solution like ground beams, a raft foundation, or piled foundations. You may need to take mitigation measures for any contaminants or gases that are present.
In some areas of the country, mining can impact the foundations, and in such cases, a specialist engineer will need to provide guidance.
Trees near your proposed structure can potentially cause problems if your foundations aren’t deep enough. Still, LABC Warranty’s Foundation Depth Calculator can help you or your designer decide how deep to go. You’ll need to identify the species, the potential height of the tree and the distance from any structure.
If there’s a nearby building, you’ll generally need to dig down to at least the level of that building. Your contractor will need to ensure that any nearby buildings are adequately supported when excavating the foundations.
To prevent the weight of your new structure from weighing down onto existing drains and sewers and damaging them, you’ll need to dig to at least the same depth as the bottom of the drain or sewer, known as the invert level. If a drain passes through a foundation, you will have to provide lintels to support the loading from the foundation and walls above and prevent the drain from fracturing.
Your building inspector will have expert local knowledge on ground conditions and solutions, but can’t design your foundation for you. They will, however, want to inspect the foundations before concrete is poured.
Always make sure your budget includes a contingency for unforeseen additional expenses.
What if my existing foundations have failed?
Sometimes, underpinning is recommended by a loss adjuster or structural engineer if the house has suffered from subsidence. Building regulations approval is always required for underpinning because underpinning is a structural alteration. If it isn’t carried out correctly by a competent contractor, it could result in serious structural problems or even the collapse of your home.
Underpinning is a technique used to stabilise your home if it’s found that the foundations have moved (because of subsidence or a change in the soil conditions beneath your home, for example), or it can be used to support the walls above if you are adding an extra storey or a loft conversion. It’s also used in the construction of cellars and basements beneath existing buildings.
A structural engineer will be able to tell you if underpinning is required and how the work should be carried out. Their recommendations will have to consider nearby drains and sewers. Expert advice should always be sought for this type of work.
The most common underpinning method involves excavating sections of ground underneath the existing foundations and injecting each one with concrete, typically with reinforcement. This is known as mass concrete underpinning. You will need to request that each section be inspected by a Registered Building Inspector before it’s concreted in. Mini piling is also used if access permits (concrete piles with a narrow diameter).
N.B. If the work is to be done adjacent to or near another building, you will also have to comply with the Party Wall etc. Act 1996.
The NHIC would like to thank the WG2 Consumer Voice members for their contributions to this guide.
Please note that this guidance document, produced by the NHIC, was correct when published. However, information and regulations may have changed since this document was published. Therefore, it is always advisable to seek the most up-to-date information and expert advice before making any decisions related to improving your home.
Copyright National Home Improvement Council 2025.
