Basement Construction in London: A Deep Dive into Their Increasing Demand
Basement construction in London has become a top choice due to severe planning limitations on outward and upward expansion in desirable, historic neighbourhoods. Strict regulations protect the original look of Victorian, Edwardian, and Georgian homes, making subterranean development the only way to increase usable space without altering the exterior or footprint of the property.
Given London’s extremely high property values and limited land, basement extensions are a highly attractive investment. They allow homeowners and developers to maximize property area without the expense of moving, often adding luxury amenities (like gyms, cinemas, or offices) that significantly boost property value. In prime areas, this can command a premium of over 100% compared to properties without a subterranean extension.
Basement construction is also rooted in history, where they served as functional, cool areas to used as kitchens or storage. Furthermore, the combination of surrounding soil and high-thermal-mass concrete gives basements naturally stable temperatures, resulting in excellent resistance to fast changes in heat or cold.
In densely built, architecturally protected parts of London, subterranean development is a key solution: it’s a practical, economic, and legal method for creating extra space without requiring a land purchase, stamp duty, or the extensive costs of relocating.
Basement Construction Under Existing Properties
Consequently, retrofitting a basement costs considerably more than integrating one into a new build, primarily due to the necessity of underpinning the existing house’s weight and the challenge of highly limited access for machinery and earth removal. This difficulty is further magnified on narrow plots—a very common situation, since homeowners with larger surrounding gardens would often opt for less expensive, more straightforward surface extensions instead of complex subterranean excavation.
If the property is terraced or semi-detached the design will need to be incorporated into a Party Wall Agreement that will need to be negotiated with your neighbour(s) solicitors and Structural Engineer. This is always a time consuming and expensive process.
Using ICF for basements under an Existing House
Choosing Insulated Concrete Formwork (ICF) for your basement construction allows us to consider two different approaches for safely digging out and supporting the existing house.
Typical existing foundations are shallow (only 400–1,000mm deep). Building the new basement requires excavating to about 3,000mm below ground level, immediately next to the original foundations. Consequently, these existing perimeter walls, along with any load-bearing internal walls, must be fully supported before this deep excavation can safely begin. There are 2 ways to do this: mass concrete underpinning or reinforced concrete underpinning.
Mass Concrete Underpinning
This traditional technique involves excavation down to the selected founding level in short lengths of 1 – 1.5 metres, underneath the existing wall footings.
1. External Access
If there is space only outside the building, then this can be carried out by a small digger with the bucket reversed to allow excavation underneath the existing footing.
After the excavation is complete a shutter is placed across its width so that the concrete is restricted to the same width as the old foundation above it. The concrete is then poured leaving a gap of approximately 75mm between the top and the underpinning concrete on the underside of the original foundation. Dry pack (mix of sand and cement) is then rammed into this space.
This is carried out around the building by excavating and concreting a few sections at a time so that there is at least a 2m gap between each excavation.
After the underpinning is complete the ground floor is removed, and the internal excavation can be carried out. As the excavation proceeds it is absolutely essential to prop the underpinning internally to stabilise it against loading from external earth pressure.
2. Internal Access
Alternatively, if the required external space is not available the excavation will need to be carried out by hand from inside the building. Normally the entire ground floor is removed and a small digger used to excavate down to the underside of the perimeter wall foundation level.
Then pits need to be excavated, using the same bay length as for external underpinning. However, they will need the sides of the excavation to be shored up, as it proceeds, to ensure that the sides do not collapse.
All the excavation has to be carried out by hand and wheelbarrowed out, unless it is possible to install a conveyor belt. The whole process will take a lot longer and be far more expensive than external underpinning.
3. Inner Reinforced Concrete Retaining Wall
After the underpinning is complete an inner reinforced concrete retaining wall is installed that will form the walls to the basement living space and most importantly prop the underpinned perimeter walls from lateral earth pressure.
The temporary propping can only be removed after the new wall has gained sufficient strength.
These walls can be built very economically using ICF.
4. Party Wall Act
The Party Wall Act allows a building owner to install mass concrete underpinning underneath the party wall without the consent of the adjoining owner. Therefore, a design that incorporates this and an inner RC retaining wall or box avoids the need for special foundations consent.
Reinforced Concrete Underpinning
This is a modern approach to the traditional mass concrete underpinning.
It is only carried out when all the underpinning has to be carried out internally. The sequence of construction is similar to that for mass concrete underpinning but instead of mass concrete the underpinning is carried out by the construction of a reinforced concrete wall in single bay lengths with the same 75 mm top gap that is then filled with drypack. Each bay will need to be connected to the adjacent bays with suitable shear connectors.
After the entire perimeter has been completed a reinforced concrete ground slab is installed that connects all the sections together. It will be necessary to prop the individual reinforced concrete wall segments until they are all connected together by the ground slab and the basement ceiling slab to form a rigid reinforced concrete box.
On the face of it this appears to be structurally efficient because one structural element is carrying both the vertical and horizontal loading, but there are several disadvantages:
- Waterproofing is very difficult because of all the joints
- The temporary propping needs to be constantly changed as construction proceeds, and the risk of a mistake is high
- Building a high quality reinforced concrete wall in this manner needs very efficient quality control systems
- Reinforced concrete underpinning, under the Party Wall Act, is called a ‘Special Foundation’ that needs the adjoining owner’s agreement.
However ICF can be used for the wall bays and will save time in a very laborious process.
Critical Considerations for your Basement Project
1. Secure an Experienced Structural Engineer
This is the most crucial first step. If you plan to use Insulated Concrete Forms (ICF) for the construction, it’s essential to hire a structural engineer who possesses years of specific experience in both ICF technology and deep basement construction. Their expertise will be vital for a successful and safe build.
2. Managing Excavation Waste (Spoil)
Excavation generates a massive volume of spoil (the dug-out earth), and removing it is often one of the largest, unbudgeted costs in a basement project. For instance, a 100m² basement can easily produce several hundred tonnes of waste. You must have a clear plan, budget, and logistical route for the efficient disposal of this material from the site.
3. Party Wall Agreement
A Party Wall Agreement is a legal necessity under the Party Wall Act 1996 if your new basement walls or foundations are planned:
- Within a specific distance (usually 3m – 6m) of a neighbour’s property.
- Deeper than your neighbour’s existing foundations.
This agreement protects your neighbour’s property and establishes a formal procedure in case of damage. Because this legal process can take months to complete, it is critical to initiate it very early in the project timeline.
Final Thoughts
Building a basement beneath an existing property is an excellent strategy for expanding your living space, especially in areas like London where land for horizontal expansion is limited. While undertaking a basement construction project in London under an existing structure can be complex and costly if mistakes are made, it is fully manageable with the right expertise. That’s why using Insulated Concrete Forms (ICF) is a brilliant system for both new and existing basement construction. Regardless of whether you are in London or elsewhere, any successful basement project demands experienced engineering design and meticulous planning, specialised services which we are fully equipped to provide at Polysteel.
Get in touch with us today to discuss your basement project with one of our engineers.