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Tips on Side Extensions and Party Walls

Your neighbour can be your biggest ally in delivering the project you want!

If you are looking to building a side extension (or side return) to your property, the party wall matters should be at the top of your priorities. In most cases, you will be effectively building a new wall that your neighbours will be looking at for a long time, and creating a lot of disruption during the works. It is crucial to keep them informed of what is happening at the earliest possible stages. Furthermore, your neighbour might be able to collaborate with you, helping you get the project you really want.

Typically, if there is a fence between the two gardens, the new extension flank wall will be built up to the fence (let’s call you Number 25). In some cases, the owner of the adjoining plot (let’s call them Number 27) may agree to the wall being constructed astride the boundary in exchange for the right to use the wall at a later stage for their own extension. In this case, No.25 would pay for the wall, foundations and gutter. The slight variation to this – which is particularly pertinent in London – is where there is an existing party fence wall between the gardens. In which case the Party Wall etc Act 1996 enables/requires this wall to be raised or rebuilt astride the boundary line in the same position. This scenario also lets no 27 incorporate the wall into their extension at a future date.

Peckham Glass box by Unagru Architecture Urbanism

Advantages of building astride of the boundary

By building astride the boundary, your neighbour (Number 27) has acquired the right to use the wall. The advantages are multiple:

  1. When Number 27 decides to use the wall, they will pay 50% of the wall and foundations' cost back to you when they make use of the wall. In other words, the two neighbours share the cost of the wall rather than paying twice for the same construction element.

  2. Furthermore, both properties will benefit from reducing the thickness of the construction along the boundary. A single party wall will measure about 350mm in thickness, which is approximately 130mm wider than the typical brick boundary wall that usually separates properties. In the region of 750mm thickness, a double party wall will reduce the width of both properties by 180mm on each side.

  3. A party wall astride of the boundary will be in continuity with the existing wall separating the two properties, and will be more consistent with the construction of the house. This will result in a simpler structure and more streamlined interiors.

  4. It is possible to have more say over the height and shape of the wall. The planning department mostly decides on this matter, but two neighbours who agree on the design of their extension can submit a joint application, which will allow more freedom on the height and shape of the party wall.

  5. A single party wall is much easier to build. This is particularly useful to the neighbour that is not yet extending their house. Once the extension flank wall is built inside the building owner's plot, the adjoining owner will have to agree and solve a few construction issues when erecting their own wall. For example:

    1. Sectional hit and miss bay foundations may be required.

    2. A weatherproofing flashing detail at roof level.

    3. Some form of weatherproofing seal at the junction of the two walls

    4. The adjoining owner will likely need to install a slip membrane between the building owner's existing foundations and their proposed foundations.


  1. In the short term, the adjoining owner will lose about 100mm of garden space and allow foundations to run even further inside the plot.

  2. A few design considerations will be needed for both properties. The adjoining owner will be encouraged to think about their extension, and this may take some time.

  3. The building owner will need to consider more items and factor in more time when planning the extension. The design in the first stages becomes a bit more complex.

Of course both parties should consider both options carefully, but my advise is usually to work in collaboration with your neighbour to try and figure out a way of sharing this little bit of responsibility and design, to gain considerable advantages in the long run for you both.

If you need to discuss party wall matters, we recommend you get in touch with CWC Party Walls, who have helped us on several projects in the past five years.

Note: This post was updated in May 2022 to pick up a few of CWC's comments and suggestions.


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