Basements and The Party Wall etc Act 1996
There are many reasons why people are requesting basement developments, particularly in Towns and cities where land may be scarce and expensive. Some may have an existing basement but the height is too low for using it as a habitable room, and would require enlargement. Others see basements as an opportunity to create additional living space, perhaps for a playroom/games room, gymnasium, swimming pool, or to enable supplementary accommodation to an existing property.
There are risks associated with providing basements to existing properties, so care needs to be taken to avoid problems. If you are a neighbour or adjoining owner, you may well have concerns about major construction works next door affecting your property, quite apart from the noise, dust and general disruption to the road. If you are a Building owner and have commissioned an architect or surveyor to design you a basement, you may well need to consider implications of the works to your neighbours.
The Party Wall etc Act 1996 is legislation that may affect both you and your neighbours, even if you don’t actually carry out work to a party/shared wall. Section 6 of the Act is there to protect adjoining owners if excavations are carried out nearby to their property. Your professional adviser should be fully aware of this, so that the correct notices can be served on a neighbour adjoining owner, in good time.
Nick Huband, forensic structural and building engineer, for William J Marshall and Partners, explains to Neil Thind of Arun Associates, Chartered Building Surveyors, that in constructing basements to an existing property, one is effectively cutting a large hole beneath the footprint of the building. This also involves removing soil near the foundations of a property. In some cases, where not properly planned in advance, this could undermine the support to the foundation, with serious consequences.
A designed solution will be necessary with the assistance of a structural engineer. In some cases, the foundation to a wall or walls may need underpinning with concrete, to bring the foundation down to a lower level than before. Underpinning is covered in the fifth video presentation.
Temporary works or shoring supports are needed to support the underpinning and excavation forming the basement, to avoid structural movement. Movement can manifest itself as cracking to walls and floors, tilting of foundations and walls, and tapering cracks, and in the worst case, subsidence due to loss of support.
Where older foundations have projecting brickwork at foundation level, called ‘corbels’, they will need trimming back to allow the underpinning works to proceed.
The corbels were there to spread the load over a wider area. In the detail shown above, this is only illustrative, and steel reinforcement may be required in the underpinning foundation to cope with the stresses imposed. If the wall were a party wall/shared wall, then notices would need to be served on the adjoining owners for the special foundations, and where the foundation had to be lowered. Solutions using cantilevered, or overhanging foundations can also be used to avoid loading the existing foundation.
The above detail may incorporate a blockwork wall to protect a basement damp proof membrane. Where space is at a premium, the designer may opt for a retaining wall solution to be constructed and underpinned, below an existing masonry or concrete wall. This is more complex and costly, and would most likely need to be reinforced with steel, and be classed as ‘special foundations’ under the Party Wall Act, where it was supporting a Party Structure.
However, it can enable the footprint of the rooms and basement areas to be kept to the maximum, and may even enhance the value of certain properties, due to minimal intrusion. Effectively the old wall is underpinned, via the new reinforced concrete retaining wall which can then be combined structurally, with the new basement floor structure.
The basement design should allow for safely transferring the loads to the subsoil. The process of underpinning or forming new foundations to the perimeter of the basement is complex and requires advanced structural calculations and pre-planning. Mass concrete can be used to lower the foundations in some cases, but often reinforced foundations are required, and if that affects your neighbours to a party wall, that is notifiable under the Party Wall Act. This is because it is referred to as ‘Special Foundations’.
Support struts will be needed to support both the underpinning and retaining the soil at the perimeter of the excavation.
Forming the structural basement around the perimeter of the room or area is a challenge. There will be extensive use of concrete, often reinforced with steel. When one casts concrete foundations/walling to the perimeter or to an existing wall, it may well be reinforced with a steel cage. One will not be able to see the far side of the concrete as it is against the subsoil.
Therefore, one needs to take extra care in the quality control of the concrete mix to ensure it is compacted and vibrated efficiently, to prevent air pockets and weaknesses in the concrete. Weaknesses may not show themselves for several years when failure could result and structural movement occurs. Testing of the batches of concrete is essential at the time of pouring it, and records are kept.
Planners will have a view on the construction of basements, as it is a form of development that will have an impact on the surrounding area during the construction phase, and longer-term, such as increasing accommodation and the need for extravehicular parking. Some London-based local planning authorities are now very strict on allowing basement developments in their areas.
Water, dampness and basements seem to go together, but need to be separated in construction work, and managed to avoid dampness and soil erosion underground. The basement will need to be waterproofed or ‘tanked’ with a membrane, to protect the walls and floor.
Nick Huband explains in the video that the Victorians often dug their basements down just above layers of gravel in London, and allowed the watercourse below to run just above the underlying clay subsoil. This kept the basement relatively damp-free. The new modern owner then acquires the property and decides the basement is too small in height and needs to enlarge the height. This means the proposed excavation will penetrate deep into the clay subsoil, and this may alter the watercourse or stream that runs underground.
Water pressure then builds up against the taller / deeper walls and new floor, and this means the damp-proofing to the basement needs to be continuous and perfect, as any weakness will result in a very damp basement. Sometimes there can be two or more levels of watercourses that have to be dealt with. Trial pits and boreholes before work starts will usually identify this beforehand.
Specialist damp proofing tanking firms will often be subcontractors in basement developments.
The bays to progress this work must be relatively short in width, about a metre long, so there will be many bays to cover the length of the wall. The steel reinforcement and timber shuttering or moulds will be joined to the adjacent bays in a specific sequence to limit settlement of the ground. Reinforcement needs to be overlapped and tied together to ensure the wall acts as one when the concrete has set and hardened.
Monitoring movement in larger schemes will be advisable to ensure that the building is not under distress, and the same may apply to neighbouring properties in some cases. This can be achieved with steel pins across old cracks, ‘tell-tales’ which are graduated plastic gauges, or dynamic monitoring using computers on larger schemes.
When asked by Neil Thind about the need for reinforced concrete or special foundations to develop basements, Nick Huband made the point that mass concrete can be used in many situations, as the footprint of the wall is being taken down lower. However, what adds complexity is the additional sideways horizontal pressures and forces on the wall and foundation, which may necessitate some reinforcing of foundations and retaining walls, for structural stability.
Many excavations can be seen in towns and cities, and there will be visible large props and struts, sometimes of enormous physical size. These are necessary to provide continuous support to the excavations and basements while the construction work is done. Without them, the subsoil will move, often with devastating effect, as there will be structural forces to walls and perimeter which act horizontally, and need to be resisted. Nick Huband emphasises the need to provide the appropriate amount of temporary works, which will include shoring and support to underpinning, walls and excavations.
The temporary supports will be removed eventually, to enable the rest of the work internally to proceed, but engineering advice on the timing and sequence of removing shoring and props is advisable.
The role of the Party Wall Surveyor on such a project is to liaise with the other parties’ surveyor and the engineer. Sometimes, a specialist engineer is appointed to provide a further opinion on a drawing or structural proposal. The Party Wall Surveyor is there to protect ‘the wall’ and does not have clients, but only Building Owners and Adjoining Owners. This is a difficult concept for an outsider or layperson to accept but is how the Party Wall etc Act 1996 is written. This approach helps to make the Party Wall Surveyor impartial in making observations and decisions.
In basement excavation developments, it is always advisable to appoint a party wall surveyor, where the works to the Act trigger a Party Wall Act situation. This is due to the complex nature of the works, and every situation may be different. A party wall surveyor may also consider issues such as working hours to access the party wall, the noise and disruption, scaffolding, and matters arising.
Before any Award (Agreement) is signed off by the Party Wall surveyors, they will have to satisfy themselves that the basement development proposal has been carefully planned and has a properly engineered solution, which will not put the properties, or persons at risk. The design needs to be sufficiently robust for long-term stability.
Note that any building works may require statutory consents and approvals, such as Building Control, Planning, Listed Building Consent where appropriate, and Party wall notices served in good time in accordance with the Party Wall etc., Act 1996.
If you are planning a basement project or have any issues or concerns regarding your neighbour’s impending project or have received Party Wall Notice you are welcome to contact us on 020 8381 3910 or message us at Arun Associates by clicking here.
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This text is provided to amplify the video ‘Basements and the Party Wall Etc., Act 1996’ and was produced on behalf of Arun Associates and William J Marshall & Partners LLP. Whilst every care has been taken in the information provided, no liability can be accepted by parties relying on the content in respect of loss, damage, consequential loss.