house extension foundations in London showing concrete with steel rebar reinforcement

From rafts and trenches to piles and ground screws, this article delves into house extension foundations. We’ll explore both traditional and modern approaches to foundation design.

When embarking on a new house extension, it’s crucial to design the correct foundations from the outset. Rectifying any issues later can be costly, disruptive, and time-consuming.

It’s essential to remember that a chartered structural engineer should always be involved in designing your house extension foundations. At Architecture for London, we provide both architectural and engineering services to our clients.

What are house extension foundations?

House foundations are structural elements designed to distribute the load of an extension evenly across the soil, ensuring stability. It’s the ground that supports the weight, with the foundations ensuring even distribution.

Selecting the appropriate house extension foundations involves considering several factors: the type and grading of the soil, seasonal groundwater levels, potential soil contaminants and chemicals, and any naturally occurring gases like radon.


Dartmouth Park House Extension, NW5

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Dartmouth Park two storey rear extension design with stone cladding and barbecue

Types of house extension foundations

Foundations must satisfy building regulation requirements. There are numerous types and configurations of house extension foundations. Selecting the appropriate design is crucial as it must suit site-specific conditions, soil type, any constraints present on-site, and the structural design of the new extension. For a thorough assessment of the site conditions, it’s always recommended to obtain a geotechnical report.

A good structural engineer can focus on using the least amount of material. This approach is cost-effective and it also reduces embodied energy, therefore being more sustainable too. In the past, the embodied carbon in foundations often went overlooked, leading to over-specification. Today, it’s entirely feasible to design a sustainable foundation without any compromises.

Traditionally, most house extension foundations have been made from concrete. This preference arises from concrete’s adaptability; it can be tailored to specific resilience needs without requiring additional coatings or protections and can be moulded efficiently with minimal waste.

However, the cement content in concrete makes it less than ideal from a sustainability standpoint. To address this, depending on the performance requirements, concrete replacements and admixtures can be employed to reduce the cement ratio in the mix, effectively lowering the embodied carbon. Moreover, if steel reinforcement is called for, opting for recycled steel can further curtail the carbon footprint. Leveraging these eco-friendly building materials can lead to a substantial reduction in embodied carbon.

For more information about working with Architecture for London, please contact us on 020 3637 4236 to discuss your project, or visit our architecture project portfolio to see a selection of our previous architectural and engineering projects.

Some of the most popular foundation types are described below.

Bearing foundations

Bearing foundations refer to cubic or box-shaped concrete blocks, or footings, that rest upon and are supported by the ground. The foundation’s depth or level should be determined by where the ground offers sufficient bearing capacity for the structure.

In the London area, where buildings and extensions are concerned, heave protection is often incorporated at the foundation level. This addition combats the swelling and shrinking effects induced by London Clay, ensuring the structure’s longevity and stability.

Owing to their simplicity in construction, these footings are a popular choice for supporting house extensions when site conditions are favourable.


Grade II listed villa Stone House, Barnsbury, N1

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rear extension listed building

Concrete pad and strip foundations

Concrete pad and strip foundations can be constructed using just concrete, or they can be fortified with steel reinforcements. When steel bars are integrated into these foundations, they enhance the strength and robustness of the footings. This reinforcement can often permit the use of shallower foundations, particularly advantageous in areas where deeper excavations might pose challenges.

Trench-filled foundations

Trench-filled foundations can vary in depth, sometimes exceeding 1.0m, depending on soil conditions and the self-weight of the new extension or building. These foundations typically consist of trenches filled with concrete and generally do not require reinforcement, making them one of the simplest foundation types to construct.

While deeper excavations increase the risk of surrounding soil collapsing into the trench, if the trench depth remains under 1.2m, there’s typically no need for temporary propping of the excavated walls.

Piles, pile caps and ground beams

Piles, pile caps, and ground beams are typically reserved for larger construction sites due to their economic viability for such projects. However, when considering house extension foundations, small diameter or ‘mini piles’ can be utilised, often paired with reinforced concrete ground beams.

Ground beams, a critical component of house extension foundations, are generally reinforced and measure around 600mm in width, though the dimensions can vary based on the project’s scale. Once piles are installed, their tops can be cast and integrated into the ground beams.

In standard piling undertakings, the installation process involves setting all the required piles across the site first. Only then are the concrete ground beams and other ground-level structures poured. Preparations often need to be made to the ground to accommodate the piling rig, ensuring its stability and the vertical alignment of the piles.

When using ground beams as part of the foundational structure, several ground floor options are available. One could opt for a reinforced concrete slab to overlay the beams. Alternatively, the beams could support a pre-fabricated flooring system, such as a concrete hollow-core floor or a conventional timber joist floor, equipped with necessary waterproofing and insulation.

Mini piles and ground beams must be tailored to the specific conditions and constraints of the site. A structural engineer or a design-and-build piling contractor would usually oversee this design. Once in place, these components provide a sturdy and reliable foundation for the extension or building.

Mini piling

Mini piling can be an optimal choice when considering house extension foundations for certain projects. However, its success hinges on thorough planning. Factors like getting the piling rig to the exact location and ensuring there’s adequate space and headroom for it to function are crucial.

When executed correctly, mini piles establish a robust foundation for new structures, seamlessly integrating with the existing building fabric.


Primrose Hill House Extension, NW3

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architects in primrose hill

Reinforced concrete piled raft

A reinforced concrete piled raft is akin to a concrete floor slab. It is constructed atop a grid of piles previously installed in the ground. For house extension foundations, this type of foundation can be an apt choice. This is especially true if the top layers of soil lack the necessary stability or are unsuitable for bearing loads.

Additionally, it’s a viable option if site constraints prevent the construction of strip or trench-filled foundations. Proper construction planning and sequencing are paramount for piled raft foundations. This is crucial in areas with limited site access or restrictions.

Piles tend to be tailored to specific sites. Their design typically draws from in-depth site investigations. This includes borehole data and soil analysis. The design and specification of the reinforced concrete raft are usually overseen by a structural engineer or a design-and-build piling contractor.

At the ground level, the reinforced concrete raft can double as a ground floor slab. Once the raft’s top is insulated, screeded, and finished according to the requirements, there’s typically no need for an additional floor structure. This feature is beneficial when a lower floor level is desired.

This foundation type is known for its robustness. However, it might prove to be costlier than other house extension foundations. This is due to its potentially higher demands on both labour and materials.

Ground-bearing slab

A ground-bearing slab isn’t strictly a foundation. It’s a lightly reinforced concrete floor slab resting on the ground. This slab directly transfers loads from its top into the ground.

A slip membrane is essential between the slab and the ground. This allows for slab movement under certain conditions. It also prevents uncontrolled cracking, which might look unsightly if the slab remains exposed without additional finishes.

Often, the ground-bearing slab pairs with pad, strip, or trench-filled foundations. The ground beneath the slab must be properly specified and prepared. This ensures the entire foundation scheme operates as intended.

Much like the raft, this slab is usually insulated, screeded, and finished. There’s no need for an added floor structure on top. Due to its simplicity and structural efficiency, it’s a favoured choice for smaller-scale new house extension foundations.

Retaining structures

Buildings and extensions might be founded on retaining structures. This is especially true if they’re within landscaped areas, sloping grounds, or sites with many underground services.

Retaining structures can be sensitive to changing loads. This includes shifts in seasonal groundwater or significant load variations from the soil. Groundwater pressures behind these structures can be mitigated. Designing weep holes at the base helps, but thorough geotechnical investigations are vital beforehand.

Fixing retaining structures can be costly and challenging. Thus, they’re often seen as key structures in design. When constructing them, it’s vital to focus on excellent workmanship and detailing.


De Beauvoir House Extension, N1

de beauvoir architect house extension

Damp proofing and insulation for house extension foundations

A Damp Proof Membrane (DPM) is often placed between the ground and the foundations or ground-level structures. Insulation can be detailed either below or above the ground slab. Additionally, insulation might be placed around shallow foundations, or even around the top of piles.

Underpinning existing foundations for house extensions

Underpinning plays a pivotal role in strengthening and stabilising the base of existing structures. The choice between traditional methods and modern techniques often boils down to the project’s specific requirements, cost considerations, and potential disruptions.

Traditional underpinning

Traditional underpinning is a tried-and-true method, where sections beneath existing foundations are systematically excavated and filled with concrete. The step-by-step nature, allowing each section to cure before moving on, can make this method lengthy. Undertaking this indoors, especially under integral structural parts, can prove quite disruptive.

However, its saving grace is often in its cost-effectiveness. With careful planning, the required materials and equipment are affordable and accessible. But remember, proprietary systems might introduce extra costs, especially if the aim is to hasten the process or navigate site limitations.

Resin grout injection

A more contemporary approach to underpinning, resin grout injection, offers ground remediation with minimal disruptions. This technique, though on the expensive side, involves injecting grout into soils through drilled holes in the ground-level structure. Continuous monitoring throughout the procedure ensures effective results.

While this method might not offer the precision of traditional underpinning or mini piling, its success often depends on the contractor’s expertise. The added benefit? Future potential issues might be addressed with additional injections, backed by warranties.

In the evolving landscape of house extension foundations, underpinning remains crucial, with options to cater to varied project demands.

For more information about working with Architecture for London, please contact us on 020 3637 4236 to discuss your project, or visit our architecture project portfolio to see a selection of our previous architectural and engineering projects.

London architecture firms extensionHouse extension foundation considerations

Age of property

The age of the house can give clues about the type of existing foundations in place. This knowledge impacts the design of the extension’s new foundations.

Trees and vegetation

It’s crucial to consider nearby tree types and vegetation during foundation design. Some might have tree preservation orders attached.


Existing drainage systems, including manholes and inspection chambers, might affect new foundations. Any conflicts between current drainage and new foundations should be resolved in the design phase. For certain changes, such as sewer diversions, you might need water authority approval.

Utility and services

The early stages of the project should account for utility and service connections to the property. This data informs and integrates into the design of the extension or new building.

Previous foundations

Hidden buried remnants from prior buildings may exist. These could either be removed, disposed of, or incorporated into the new foundation scheme.

Water levels

Proximity to water bodies or a high water table affects foundation design. Soils like London Clay are sensitive to moisture levels. Rapid moisture changes could destabilize extensions if not considered.

History of site

Knowing a site’s history, especially in areas like London impacted by WWII bombings, is beneficial. Bomb-affected regions might require deep foundations, as the residual ground conditions could be unsuitable for shallow footings.

Adjacent buildings

The foundations of neighbouring properties can influence the design, especially if they share a party wall or if the new foundation could potentially undermine the neighbour’s structure.

Geotechnical report

Depending on the project size and the amount of information required for the project, a geotechnical report would provide the most reliable, site-specific information for designing the optimum foundations for a new extension or building.

To obtain the relevant geotechnical report for the project, the structural engineer would need to review the architectural drawings and provide an initial structural and foundation scheme.  The chosen soil investigator would then conduct a set of investigations and tests against the structural scheme, and issue the findings in the form of a report.


What building regulations are there for foundations?

Building regulations specific to foundations can be found in Section 2E of The Building Regulations Approved Document A, focusing on plain, unreinforced concrete foundations. If any queries arise regarding these regulations, consulting a structural engineer is recommended.

Do I need a structural engineer for house extension foundations?

Yes, engaging a structural engineer is highly advisable when planning house extension foundations. Their expertise, combined with that of a contractor, can provide invaluable insights, ensuring your foundation is both sturdy and compliant.

For more information about working with Architecture for London, please contact us on 020 3637 4236 to discuss your project, or visit our architecture project portfolio to see a selection of our previous architectural and engineering projects.