Building a house extension on a neighbour’s boundary

House extensions in London are often built on, or very close to, a neighbour’s boundary. This article offers advice about how to negotiate the process.

When building a house extension on a neighbour’s boundary, we’d always recommend speaking to your neighbour as early as possible. Ideally, this should happen once sketch plans have been produced, but before any application for consents has been submitted.

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If the neighbour finds out about your project through a planning consultation letter, they are likely to be more adversarial than if they had been involved in an early design discussion. Inviting a neighbour to contribute to the design before a planning application allows them a sense of ownership and participation. This may reduce the likelihood of objections during the planning process.

Early neighbour involvement is also useful for the party wall process, particularly if you wish to build on (astride) the boundary. You can usually build up to your boundary without a neighbour’s consent, but building on the boundary itself does require their consent. Building on the boundary might effectively gain you an extra 150mm or 200mm of width internally in a rear extension. This could be valuable extra space, particularly in a terraced townhouse with a plot that may be only 5 or 6 metres wide.

Successful neighbour collaboration at Grade II listed house extension

 At our ‘Stone House’ project in Islington, we refurbished a Grade II listed townhouse and extended it to the rear. Early neighbour consultation was carried out, and the neighbour in fact decided to propose their own house extension at the same time. Architecture for London was also appointed as the architect for the neighbour’s project, and this made the whole process much smoother! There were no planning objections, and it made building astride the boundary the default choice.

The previously dilapidated garden wall was also able to be rebuilt in brick, up to the height of the rear extensions, without any objection. The costs for doing this were shared between the neighbours. If a project can be carried out jointly with a neighbour, there may be cost efficiencies for both neighbours. There could be reduced architect’s fees and cost efficiencies during construction if a single contractor can be used.

Consider potential issues near the boundary of your neighbour’s land

Be aware of tree root protection areas. Even if you don’t have trees on your own land, your neighbours may have protected trees. This could affect the position and type of foundations that can be used for your project. An early appointment of a good arboriculturist and structural engineer is recommended.

 There may be restrictive covenants in relation to building close to a neighbour’s land. Any such covenants should be investigated with a solicitor. Cost effective insurance is sometimes available for proposed breaches where the covenants are historic and unlikely to be enforced.

There is a possibility of Japanese Knotweed or contamination on a neighbouring property, even if not on your own land. These issues may affect your project and would need specialist investigation.

House extensions on a boundary: planning and heritage considerations

In a heritage context, a building project’s impact on neighbouring buildings becomes more important during the planning and listed building consent process. An assessment will be made of the wider planning and heritage context, streetscapes, and views.

If your proposals include underpinning works to an existing heritage boundary wall, there is a chance that a heritage officer will determine it to be detrimental to the historic fabric. For this reason, it may not be supported, even though the underpinning work itself is below ground and not visible.

View Architecture for London’s residential architecture projects. If you would like to discuss your project, please contact us.