Do I need a party wall agreement for an extension?

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The answer to this question primarily relies upon the type of property you own, the works you are undertaking and the neighbour’s chosen response to a notice. If you are carrying out works to a flat, terraced house or semi-detached house the probability of needing a party wall agreement becomes far more likely as your property will typically share a party structure with your neighbours’ property. You may also be excavating within 3 or 6 metres of an adjacent property, therefore even detached properties can fall within the Party Wall etc. Act.



Rear extensions or side extensions would typically involve excavating to form new foundations to support the new structures. If these excavations are within 3 metres of a neighbour’s property and to a lower depth than the neighbour’s foundations an Adjacent Excavation Notice should be served on your neighbour. Although you are unlikely to know the depth of your neighbour’s foundations, it is generally assumed that a building pre-dating the 1980’s is likely to have a shallower foundation than the foundations to new construction. This is primarily because building regulations in recent years have required deeper foundations, typically a minimum depth of 1 metre.


If your extension’s foundations are particularly deep, perhaps at lower ground level or incorporating a basement or utilising piled foundations the 3 metre distance extends to 6 metres. In these circumstances an imaginary 45 degree line is taken from the bottom of the adjoining owners foundation towards the proposed excavation and if that line intersects the proposed excavation/foundation then a notice would be required. A general rule of thumb here is that if the depth of the excavation is longer than the horizontal distance from the neighbour’s foundation to the proposed excavation plus an allowance for the existing foundation depth, then the 45 degree line would be intersected.


Party Structure

Extensions to buildings that share party walls commonly require steelwork to be inserted to support the structure above the opening formed from the existing part of the building into the new extension. This would require a Party Structure Notice to be served. Other typical works for extensions affecting the party wall may include cutting away structures from the party wall such as walls or chimneybreasts. Other examples include cutting into the party wall to tie in new walls, cutting away corbel footings to accommodate new floor slabs or foundations, cutting into a party wall or adjoining owners wall to install lead flashings, raising, or underpinning a party wall or demolishing and rebuilding a party wall.


Building Adjacent to a Boundary

Another common element of building works that are notifiable under the Act in relation to extensions, outbuildings or new structures is where it is proposed to build a wall at the boundary. This is referred to as works to the line of junction. This relates to where a wall is built directly adjacent to the boundary, as wall wholly on building owner’s land or astride the boundary as a new party wall.




After the service of a Notice the decision over whether a party wall agreement is required rests with the adjoining owner(s). If they choose to consent to the notice, no further action is required under the Act. If they choose to dissent the parties are technically in a dispute under the Act and the dispute is resolved by an Agreed Surveyor or Two Surveyors resolving the dispute by service of a party wall agreement known as an Award.

In summary a party wall agreement would be required if you are carrying out works that are notifiable under the Act and in a response to a notice served for these works your neighbours choose to have a party wall agreement (Award).


Please feel free to make contact with our team at MSA to discuss your party wall requirements,, we would be happy to assist you.