Building Adjacent to Neighbouring Property and The Party Wall Act 1996

Building Owners may wish to carry out works on, or adjacent to party walls, and this may affect neighbouring properties, termed Adjoining Owners. This is taken from The Party Wall etc Act 1996.

One situation discussed in the Video, ( Video 7 in the series), is where a neighbour wishes to build up against the boundary and adjoining property. Neil Thind of Arun Associates, Chartered Building Surveyors, draws a sketch on a whiteboard, to illustrate this. Often, the neighbouring wall’s foundation will project slightly onto the land next door, ie where it encroaches the boundary line between the two properties.

The existing wall is usually, (but not always), concentrically loaded, sometimes termed as axially loaded, where the forces go through the centre of wall and foundation. The neighbour, termed the Building Owner, wants to build their extension or whatever, alongside to gain the maximum amount of space. This means that when the new wall goes down below ground level it might impact the adjoining neighbour’s foundation. There is a legal right under The Party Wall etc Act 1996 for the Building Owner to trim and cut away the projecting part of the neighbour’s foundation, but must ensure it is not made unstable in doing so.

Nick Huband, forensic building and structural engineer, of William J Marshall & Partners, explains some of the issues when a new wall’s foundation would otherwise clash with an existing, older foundation near a boundary line. Nick advises that if the projecting footing is carefully cut away where it projects over the boundary line, then the remaining neighbour’s foundation would become eccentrically loaded, ie not down the middle line as before. That has the effect of altering the stresses in the foundation and the soil underneath and increases the pressure on the soil.

The Building Owner’s new foundation may be 1 metre,1200mm, or similar depth below ground level, i.e. similar to the neighbour, (subject to local authority building control). As both the foundations would become eccentrically loaded, a solution might be to underpin the neighbour’s foundation with concrete, and to extend that underpinning across to the building owner’s side, to form a new larger concentrically loaded foundation. That would even up the stresses considerably, and avoid high point loads under the soil.

The sequence of work would be (a) Excavate down (b) underpin Adjoining Owner’s foundation, and extend a concrete ‘toe’ across to the Building Owner’s side (c) Build the new wall for Building Owner which then bears evenly on the extended concrete foundation bridging across from the Adjoining Owner’s side. It is good practice to provide a piece of flexible material between the sides of the two foundations, such as ‘Corkpak’.

An alternative solution is to provide a ‘Cantilever slab foundation’ to transfer the new loads to the soil, positioned away from the neighbouring existing foundation. This does mean the floor slab will need to be steel-reinforced top and bottom of the slab. This solution avoids cutting away the Adjoining Owner’s foundation in most situations.

Nick Huband draws an illustration of the foundation and cantilevered slab in the video. The aim is to not extend the new foundation below the level of the Adjoining Owner’s foundation and avoid underpinning, which can be expensive.

The new wall sits close to the edge of the new cantilevered slab and should be perfectly stable if the design has been properly calculated and constructed. Where the new slab is close physically to the older foundation, they should be separated with some easily compressible, flexible material. Note that some plastic materials are not very compressible, and can transfer loads before compressing, which is to be avoided to prevent structural movement. One may need the advice from a professional on this point.

Selecting which solution is appropriate may simply come down to the projected cost of the works, and time is taken in constructing them. Underpinning always takes time, as the work has to be done in carefully controlled bays, of about one metre wide. The professional advisers would take a view on which option was appropriate to the site. Some Adjoining Owners are not happy about having part of their property underpinned, so that can sometimes give favour to the cantilevered slab solution.

In some cases, it may be possible to cut back the Adjoining Owner’s foundation and leave it trimmed, but this will be dependent upon the loads carried from above, and the condition and type of soil underneath the foundations. Load tests might be necessary. This would not be common, and that underpinning would be a more common solution to the Adjoining Owner’s foundation. Neil Thind comments that if a Building Owner’s design is submitted, there are limitations on the Adjoining Owner’s surveyor requiring changes to that design.

In most cases, there will be mutual discussion and observations made on the design, but the Building Owner’s surveyor will have the financial cost and timescale as a prime consideration in the final works selected.

The moment that a Building Owner’s foundation extends below the Adjoining Owner’s foundation, in most situations there would be a requirement to underpin with concrete that neighbour’s foundation to ensure the wall is stable.

One advantage of having completely independent foundations on each side of the boundary, is that it makes it simpler to carry out other structural works at a later date, perhaps when owners change. The moment a foundation is combined, and future loading such as another storey added at roof level, would have structural implications for both neighbours/owners.

Now consider the situation at existing roof level on the Adjoining Owner’s side: There could be an existing overhanging roof eaves, gutter, soffit and fascia boards which extend slightly over the boundary. The Party Wall etc Act 1996 allows a Building Owner to cut back those items to build his/her wall near the boundary and make alternative provision for diversion of rainwater goods. One needs to be careful with solutions here, as in some cases the remaining gap between new and existing walls can be so small as to collect debris, build-up and result in dampness to the walls lower down. Vermin might be encouraged to use the remaining space, which may be difficult to access.

These items may be considered as non-structural. However, the detailing needs to be considered carefully, to avoid future problems. Sometimes dampness issues can manifest themselves many months or years later. Party Wall surveyors are used to dealing with checking drawings and details for flashings, weatherproofing, and protection to avoid dampness problems, or maintenance issues for access.

The build-up of mortar droppings or ‘snots’ as the new wall is built can prove a headache for neighbours, later on, if not checked during construction. The Adjoining Owner’s Party Wall surveyor will not be on site all the time, but will be conscious of the important issues to be raised with the Building Owner’s Surveyor.

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 to build immediately next to your neighbour’s property or have any issues or concerns regarding your neighbour’s impending building works or have received Adjacent Excavation and/or Party Wall Notice you are welcome to contact us to discuss further 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 ‘Building Adjacent to Neighbouring Property 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.