Planning an extension under the Party Wall Act

Our specialist Party Wall team of chartered surveyors at Southampton-based Harrison Clarke helps clients with Party Wall matters every day.

In this article, we briefly explain what the Party Wall Act is, why it is needed, how it affects you when you are planning an extension, what your responsibilities are and why it is important that you take those responsibilities seriously. 

What is the Party Wall Act?

Party Wall legislation dates back to 1666, when the Great Fire of London devastated the city and changed how the design and construction of buildings in London was regulated. Out of this disaster came the London Rebuilding Act 1667, which aimed to prevent the spread of fire between adjacent buildings. 

Several amendments came later, through the London Building Acts which are still in action under current legislation. 

Previous legislation was predominantly restricted to work within London, providing a limited framework for resolving disputes in relation to construction adjacent to neighbouring properties. This resulted in expensive and drawn-out litigation to resolve disputes, often involving relatively straightforward matters. As such, the Party Wall etc. Act 1996 was introduced, superseding local acts.

The aim of the Party Wall Act is to enable building owners in England and Wales to proceed with their proposed works, to provide a framework to resolve disputes between parties and to protect adjoining owners. 

Often, we find that building owners who want to construct extensions to their property see the Party Wall Act as an unnecessary cost and just another legislative hurdle to jump through, causing time delays to their project. 

However, it is important to understand that the Act is designed to enable and facilitate building owners to proceed with their works and not to prevent them from carrying them out. So, whilst many people feel negatively about the Act, it is actually a very positive piece of legislation designed to help you proceed with your work. 


How does the Party Wall Act relate to my extension?

When you are building an extension, the notifiable elements of work tend to be in relation to building a new wall on the line of junction – the boundary between you and your neighbour, and the excavations for the construction of new foundations in close proximity to your neighbour’s building. 

We do also see instances where your extension may affect a Party Wall itself. However, this is more common with work such as a loft conversion, where you may need to cut into the Party Wall to insert steel beams. 

If you have a semi-detached house or a terraced house and you are planning a rear extension, you will almost definitely need to serve a Party Wall Notice on your neighbour. 

However, if you have a detached house that is set back from your neighbour’s property, and your extension is not built up to the line of junction, you may not need to serve a Party Wall Notice at all. 

Whether your works are notifiable largely depends on how close your excavations are to your neighbour’s building, and how deep your foundations will be. 

This is why it is extremely important to seek the advice of an experienced Party Wall surveyor to review your proposals. They will be able to tell you whether your works are notifiable or not. 


I live in a detached property, why do I need to serve Party Wall Notices?

A common misconception about the Party Wall Act is that it only affects properties with a shared Party Wall. However, this is not the case. 

When constructing extensions, it is often not about shared walls, but instead about the foundations and the potential damage that can be caused to a neighbour’s building when you are excavating below the depth of a neighbouring building’s foundations.

At Harrison Clarke, we often recommend that, when you are carrying out notifiable works, you should consider instructing a Photographic Schedule of Condition on a neighbour’s property even if they consent to your Party Wall Notices. 

By documenting the condition of their property prior to construction works, this will provide evidence if your neighbour believes that damage has been caused to their property by your works. This protects your neighbour and builds trust in your relationship, at the same time as protecting you from spurious claims of damage. 


What if I want to construct my extension on the boundary line?

If you are planning to make the most of the space available to you by constructing your extension on the boundary line, you will need to serve a Party Wall Notice on your neighbour. 

Importantly, if you want to build the new wall of your extension astride the boundary line, this new wall will be a Party Wall and you can only do this with your neighbour’s written consent. This is because you have no right to build astride the boundary. 

If your neighbour refuses to give their consent, you will need to modify your design proposals. 

This potential scenario highlights the importance of seeking the advice of an experienced Party Wall surveyor as early as you can in your planning process. 

We also recommend that you open a line of communication with your neighbour before you serve your Party Wall Notices, to try and find out if they would have any objections to you building astride the boundary, as this may prevent time delays and additional costs in modifying your design. 

It is worth speaking to your architect to determine whether they would charge additional fees in modifying the position of your extension, should your neighbour object to building astride their boundary. 

If you plan to build the new wall wholly on your land, but up to the boundary line, you must still serve a Party Wall Notice, but your neighbour will not be able to stop this proposal. 


Common issues with building on the line of junction

Whilst you have the right to build your extension up to the boundary line and, if your neighbour agrees, astride the boundary, this does not necessarily mean that you have the right to build your foundations on their land. 

The Party Wall Act allows you to construct your foundations for a new wall on your neighbour’s land, but only where necessary. We find that, in almost all cases, eccentric foundations can be designed to ensure that your foundations do not encroach on your neighbour’s land.  

Another point to consider is that you have no right to allow your gutters, soffits or fascias to overhang the boundary. This could be considered trespass under common law and is not recommended. 

If you do overhang the boundary and your neighbour decides to build their own extension in the future, the Party Wall Act gives them the right to cut off any projections that are preventing them from constructing their own extension, which could potentially cause you issues and disputes further down the line. 


How Harrison Clarke can help

If you are planning to build an extension and you need expert advice on how the Party Wall Act relates to your proposals, our experienced team of Party Wall surveyors is here to help you. We look forward to speaking with you about options for your new extension that will be compliant with the Party Wall Act. 

For further guidance and useful tips and information on the range of surveying services we provide, you can read our blogs and watch our videos. These are available on our website and our YouTube channel


How you can contact Harrison Clarke

Call our friendly, expert and highly qualified surveyors on 023 8155 0051, or email us at

We would welcome the opportunity to help with any queries or needs you may have.

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Faye Williams, party wall and building surveyor at Harrison Clarke chartered surveyors.

About the author

Faye Williams,
BSc (Hons) MFPWS Senior

Surveyor & Winner of Young Property Person of the Year 2023

Faye joined Harrison Clarke in 2018. Faye found an interest in Party Wall surveying, and became a Member of the Faculty of Party Wall Surveyors in 2022.

Since then, Faye has set out a revolutionary approach to party wall instructions, by focusing on people and relationships, backed up by expert knowledge. Faye’s approach has saved building owners £1,000s in unnecessary party wall fees.